Factors Influencing Epidemiology and Management of Blackberry Rust in Cultivated Rubus Laciniatus

Home , Phragmidium, Phragmidium violaceum, Rust (fungus), Urediniospore

Factors Influencing Epidemiology and Management of Blackberry Rust in Cultivated Rubus laciniatus

K. B. Johnson, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; and W. F. Mahaffee, United States Department of Agriculture–Agricultural Research Service Horticultural Crops Re- search Laboratory, Corvallis, OR

susceptibility to infection, with the most ABSTRACT susceptible foliage being the youngest, Johnson, K. B., and Mahaffee, W. F. 2010. Factors influencing epidemiology and management fully expanded leaves near the tips of of blackberry rust in cultivated Rubus laciniatus. Plant Dis. 94:581-588. canes (6). In late summer and through fall, uredinia differentiate to telia that produce The blackberry rust pathogen Phragmidium violaceum was first observed in Oregon in spring overwintering teliospores. Physiologic 2005 on both commercially cultivated Rubus laciniatus (Evergreen blackberry) and naturalized races of P. violaceum have been reported in R. armeniacus (Himalayan blackberry). Several commercial plantings suffered severe economic regions where it has been established for losses. In 2006 to 2008, all five spore stages of this autoecious, macrocyclic rust pathogen were longer periods of time (7). In detached leaf observed annually, and asexual perennation of the pathogen on old leaves or in leaf buds was not evident in the disease cycle. In field experiments, teliospore germination and infection by assays, this pathogen has been shown to be basidiospores occurred mostly during April. On potted “trap” plants exposed for periods of 1 sensitive to the fungicides that are com- week under dense collections of dead leaves bearing teliospores, basidiospore infection was monly used to control rust diseases in associated with wetness durations of >16 h with mean temperatures >8°C. Trap plants placed other crops (21); however, there is little under the bundles of collected leaves frequently developed spermagonia, whereas only 1 of 630 information of fungicide performance in trap plants placed in a production field of R. laciniatus became diseased, an indication that the the field. effective dispersal distance of basidiospores may be limited. In growth chambers programmed The purpose of this study was to study for constant temperatures of 5, 10, 15, 20, 25, and 30°C, a minimum of six continuous hours of infection and suppression of P. violaceum leaf wetness was required for infection by urediniospores, with >9 h required for moderate infec- on R. laciniatus under western Oregon tion (>4 pustules/cm2) at 15 and 20°C. With diurnal temperature regimes averaging 5, 10, 15, 20, conditions. We examined the phenology of or 25°C, urediniospore germination and infection was highest in the range of 5 to 15°C; simi- the disease cycle, indentified environ- larly, in the diurnal environment, >9 h of leaf wetness was required to attain moderate infection. mental conditions that promote infection, In the field, lime sulfur applied as a delayed dormant treatment significantly suppressed telio- and developed an effective chemical sup- spore germination and basidiospore infection. Over two seasons, one application of myclobu- pression strategy to prevent epidemic de- tanil, a demethylation-inhibitor fungicide, applied in early May near the time of spermagonial appearance provided effective suppression of the summer epidemic. velopment. MATERIALS AND METHODS

Teliospore germination. Germination The rust fungus Phragmidium vio- fields of the commercial R. laciniatus of teliospores of P. violaceum was studied laceum was first observed in spring 2005 Willd. cvs. Thornless Evergreen and Ever- during 2006 and 2008 at two locations in on naturalized Himalayan blackberry thornless became infested with the patho- western Oregon: Oregon State University’s plants (Rubus armeniacus Focke) along gen, with several fields incurring severe North Willamette Research and Extension Oregon’s southern coast (17) and, within a economic losses. The impacted fields Center (NWREC), located near Aurora, and few months, was found in many counties showed reduced fruit yield and quality, and the Department of Botany and Plant Pathol- in western Oregon, western Washington, plant vigor in the following season (K. B. ogy Field Laboratory, located near Corvallis. and northern California, including those in Johnson, personal observation). A 0.12-ha planting of R. laciniatus cv. the Oregon’s Willamette Valley, where Incursion of blackberry rust into the Thornless Evergreen, located at NWREC, commercial blackberry production is con- United States represents a new production was used as the source of all teliospores and centrated (1,17; W. F. Mahaffee, unpub- constraint for the Pacific Northwest’s $36 for fungicide trials described below. Telio- lished data). In addition to infecting un- million blackberry industry. Although the spores either overwintered in the planting or, wanted, naturalized blackberry, nearly all autecology of P. violaceum and epidemiol- for Corvallis experiments, leaves bearing ogy of blackberry rust has been investi- telia were collected in fall and stored dry in gated with regard to its use as a biological an unheated screenhouse before being Corresponding author: K. B. Johnson control agent of naturalized blackberry in placed outdoors in early March. At this time, E-mail: [email protected] Australia (4,6–8,10,18), little is known leaves bearing telia were spread uniformly about the management of this disease in between 1-by-1-m sheets of plastic, poultry- The use of trade, firm, or corporation names in this publication is for the information and convenience susceptible blackberry plantings (21). P. mesh fencing (18-mm grid; Tenax Corp., of the reader. Such use does not constitute an offi- violaceum is an autoecious, macrocyclic Baltimore, MD) that was suspended on a cial endorsement or approval by the United States rust fungus, which causes a defoliating horizontal plane 1 m above the ground in an Department of Agriculture or the Agricultural disease on leaves and flower buds (15,22). open area. Each suspended mesh platform Research Service of any product or service to the Each spring, P. violaceum reportedly un- held 400 to 600 leaves and was replicated exclusion of others that may be suitable. dergoes an obligate sexual cycle (6,10,15), three times. Accepted for publication 18 January 2010. with asexual perennation of this pathogen Teliospores, collected weekly during in buds or on overwintering foliage con- spring from Aurora and Corvallis, were sidered insignificant in the disease cycle subjected to a germination test in the labo- doi:10.1094/ PDIS-94-5-0581 (22). Urediniospores infect through sto- ratory (6). The spores were scraped from This article is in the public domain and not copy- mata on undersides of leaves and produce leaves into a drop of distilled water and rightable. It may be freely reprinted with custom- ary crediting of the source. The American Phyto- a new generation of spores every 8 to 12 drops were spread onto microscope slides pathological Society, 2010. days (4,6). Leaf age greatly influences covered with a film of water agar (15

Plant Disease / May 2010 581 g/liter). Slides with teliospores were incu- not occur. Potted-plant rotations were re- tower at an intermediate height to capture bated in the dark at 20°C for 48 h inside of peated through May at both Aurora (2006 spore aggregates for the first 20 s after closed plastic boxes lined on the bottom and 2007) and Corvallis (2006 to 2008). At release. After shield removal, spores were with moist paper towels. Percent germina- Aurora, groups of 10 plants were placed at allowed to settle onto the water agar plates tion (teliospores with an emerging promy- three areas within the blackberry planting for 90 s. The dishes were then divided celium) was determined for three sets of and, in Corvallis, 10 plants were placed randomly and placed in growth chambers 100 teliospores sampled from each loca- under each of the three platforms on which programmed with diurnal temperature tion. As the season progressed, the per- the teliospore-bearing leaves had been regimes that averaged 5, 10, 15, 20, or centage of previously germinated telio- suspended. After incubation, exposed and 25°C (described below) over 24 h. Percent spores (empty teliospore shells) also was control plants were assessed for sperma- spore germination, defined as a germ tube recorded. The phenology of spore germi- gonia by recording the leaf node (counted length twice the spore diameter, was as- nation in Corvallis in western Oregon was down from tip) and total number of sper- sessed by counting the number of germi- compared with that observed in Montpel- magonia on all diseased leaves. After dis- nated spores out of 200 spores examined lier in southern France (6). For this com- ease assessment, canes were cut back to per plate. parison, an “expected” period of teliospore the soil line and plants regrown for use in Urediniospore infection experiments germination for the cooler, western Oregon subsequent seasons. were arranged in split-plot designs with site was developed by shifting the spring Weather data were monitored at the temperature as the main plot and leaf wet- germination period observed in southern Corvallis site with an Adcon A730 weather ness duration as subplots. Two types of France (6) by the amount that a 13-year station equipped with standard tempera- temperature treatments were used: cham- record of mean daily temperature differed ture, rainfall, and leaf wetness sensors bers set to a constant 5, 10 15, 20, 25, or between the two locations (data for Mont- (Adcon International Inc., Davis, CA.). At 30°C and chambers programmed to fluctu- pellier and Corvallis were obtained from Aurora, temperature, rainfall, and leaf ate diurnally with average daily tempera- Weather Underground Inc., Ann Arbor, wetness data were obtained from the tures of 5, 10, 15, 20, or 25°C. The diurnal MI). Agrimet Weather Network (U.S. Bureau of temperature regimes were developed from In 2008, the effect of lime sulfur on Reclamation, Pacific Northwest Region, hourly data recorded by the Agrimet sta- teliospore germination also was evaluated. Boise, ID). As suggested previously (2,13; tion at NWREC. Leaf wetness duration In Corvallis, three subsamples of detached J. Pscheidt personal communication), a treatments were created by sealing plants leaves were sprayed to near runoff with continuous period of leaf wetness began in humidity chambers for 0, 2, 4, 6, 8, 12, 10% (vol/vol) calcium polysulfide (Rex with rain and ended after 8 h of “dry” was 18, and 24 h after inoculation. Three repli- Lime Sulfur Solution 28%; OR-CAL Inc., recorded by the leaf wetness sensor. Con- cations in time served as the blocks, with Junction City, OR) mixed in water; then, sequently, adjacent wetness periods with temperatures assigned to growth chambers after drying, they were placed in a 0.15- breaks of <8 h were joined into a single randomly at the beginning of each replica- by-0.15-m plastic mesh bag that was posi- event. Hourly records of temperature were tion; two plants composed an experimental tioned at the edge of the same suspended used to obtain a mean temperature for each unit. The experiments were repeated twice; platforms that held the nontreated leaves. wet period. When a dry break occurred for the diurnal temperature experiment, At Aurora, leaves bearing teliospores were during a wetness event, only hourly tem- different sets of derived temperature re- sampled from plots that had received de- peratures that coincided with wetness were gimes were used for each experiment. layed dormant sprays of lime sulfur on 26 included in the mean. For 10 weekly expo- For inoculation, a 1:1 mixture of uredin- March (10%) and 16 April (2.5%) (this sure periods with multiple wetness events iospores of isolates HIM1 and ET1 was treatment is described below under but no infection, all wetness periods were suspended in a 0.05% (wt/vol) TWEEN 20 “Chemical control”). considered in data interpretation. For three solution and adjusted to 2 × 104 uredinio- Basidiospore infection. A rotational weekly exposure periods with infection spores/ml with the aid of hemacytometer. “trap” plant study was conducted to iden- and more than one wetness event, only the The abaxial surface of the top two fully tify environmental conditions that favored most favorable period (longest and warm- expanded leaves on each plant was misted basidiospore infection. Plants were ob- est) was considered in data interpretation. to near runoff with a handheld atomizer tained by rooting two-node, greenwood Conditions for germination and infec- (Nalgene, Rochester, NY). Immediately cuttings of cv. Thornless Evergreen on a tion by urediniospores. Environmental after inoculation, plants were sealed indi- mist bench. Cuttings were then planted in conditions giving rise to germination and vidually in 3-liter humidity chambers 3.8-liter pots filled with Sunshine mix no. infection from urediniospores were quanti- (spaghetti jars; Snapware, Mira Loma, 1 (SunGro Horticulture, Bellevue, WA) fied in growth chambers (I-30VL; Percival CA) that had been misted on the inside and grown in a greenhouse (15 to 26°C, Scientific Inc., Perry, IA). Plants of cv. with distilled water. The temperature and 16-h photoperiod). Plants were fertilized as Thornless Evergreen were produced as humidity within one jar in each growth needed (Champion 17-17-17 with micro- described above and grown in a green- chamber was recorded every 5 min by nutrients’ McConkey’s, Portland, OR) at a house in 250-ml pots. Urediniospores of suspending a HOBO U10 Temp/RH Data nitrogen concentration of 200 ppm. Prior two local isolates of P. violaceum (HIM1 Logger (Onset Computer Corp., Bourne, to use, plants were trimmed to three to four and ET1) were produced on blackberry MA) on monofilament just below the lid. actively growing canes, with individual plants maintained in separate, isolated Leaves remained wet if humidity was canes ranging from 20 to 80 cm in length. growth chambers (18°C with a 16-h pho- above 95%. At the end of wetness duration Beginning in mid-March, sets of the toperiod). Urediniospores were collected periods, lids were opened to allow the potted blackberry plants were exposed in from blackberry leaves using a cyclone leaves to dry. After opening jars from the the field for 7 days, after which a new set spore sampler (19). longest wetness duration (24 h), all plants of plants was rotated into the field and the The effect of temperature on uredinio- were removed from their jars and placed in previously exposed plants incubated in a spore germination was evaluated by releas- a single growth chamber (18°C) for 14 greenhouse (at approximately 18°C) for 21 ing a 1:1 mixture of isolates HIM1 and days. Lesion density (pustules per square days. As a control, on the day of each rota- ET1 inside a settling tower at a height of centimeter of leaf area) was assessed on tion, five symptomless plants were trans- 30 cm above 20 petri dishes that contained the inoculated leaves. Noninoculated ferred from the propagation greenhouse to water agar. Approximately 0.002 g of plants were also placed in this growth incubate with the plants that had returned spores was released by blowing air at 68 chamber to serve as a control, and all from the field. Plants in the greenhouse kPa through a cyclone sampler for 10 s. plants were bottom-watered to prevent leaf were watered such that leaf wetness did Initially, a shield was positioned in the wetness during incubation. Urediniospore

582 Plant Disease / Vol. 94 No. 5 infection was modeled statistically as a An exception to the above cane man- magonia (4 May 2008 and 30 April 2008), function of temperature and leaf wetness agement occurred in the first season with the second application made just prior duration, with the actual mean temperature (2005), after blackberry rust had become to bloom (4 June 2007 and 11 June 2008). recorded in each chamber used as the in- severe by July. As a consequence, to de- Foliar fungicide subplots had a total of dependent variable. Temperatures in the velop clean plants for fungicide screening, four treatments: myclobutanil in early chambers were within 0.5°C of the pro- half the rows were cut off near ground May, early June, or at both timings (2008 grammed temperature, except for the one level on 3 August 2005, which caused a only); azoxystrobin in early May followed of the 5°C diurnal temperature regimes, flush of 10 to 20 new primocanes to by myclobutanil in early June (2007 only); which averaged 7°C. Regression analysis emerge from each plant by early Septem- and a water-treated control. All materials was conducted with Proc NLIN in SAS ber. The new primocanes grew through were applied with hand-pumped, 12-liter (version 9.2; SAS Institute, Cary, NC). September, achieving a length of 1 to 1.3 backpack sprayers equipped with hand Response surface regression analyses were m before growth stopped in early October. held spray wands (Model 475; Solo, New- conducted with Sigma Plot 10.0 (SYSTAT Fungicide treatments were made on 2 and port News, VA) at a volume of 0.6 li- Software, Inc., San Jose, CA). 29 September and included the following ter/plant (equivalent to 935 liters/ha). Seasonal phenology of suitability of materials: myclobutanil (0.37 g a.i./liter, Measurements of rust development were western Oregon climate for infection. Rally 40 W; Dow AgroSciences, Indian- made on 29 April, 11 July, and 8 August Infection responses of urediniospores to apolis, IN), azoxystrobin (0.30 g a.i./liter, 2007 and on 27 May, 3 and 17 June, and 3 temperature in the growth chambers were Abound 2SC; Syngenta, Greensboro, NC ), July 2008. The first assessment in each used to develop an expectation of the sea- copper hydroxide (4.8. g a.i./liter, Kocide season assessed density of spermagonia, sonal suitability of the western Oregon 2000; DuPont, Wilmington, DE), and a whereas the densities of uredinia were climate to rapid development of blackberry water-treated control. Treatment suspen- assessed on the later dates. Rust was rust. With growth chamber data at 12 h of sions were applied to near runoff (0.2 li- measured by laying a 1-m2 frame onto the wetness, lesion density was first made ter/plant) with a 2-liter CO2 pressurized canopy and examining the leaves within relative to a maximum of 1.0 and then backpack sprayer at 275 KPa (Model GS; the frame for incidence of disease and regressed on mean incubation temperature R & R Sprayers, Opelousas, LA). The number of pustules per leaf. For the first for both the constant and diurnal tempera- experiment was arranged in a randomized assessment, the leaves in the 5th through ture regimes. These equations represented block design with four single-plant replica- 10th nodal positions from the tip of the optimum curves with which the relative tions. The plot area was adjacent to four cane were examined whereas, in the later seasonal favorability with respect to tem- rows of severely rusted blackberry, which assessments, the leaves in the 1st to 5th perature could be estimated by inserting a provided a source of urediniospores. Pre- nodes below the fruit cluster were exam- value for mean daily temperature; data for liminary assessment of control treatments ined. Assessments were made on both this purpose were obtained from the Na- determined that nearly all disease was sides of the canopy with 350 to 450 leaves tional Climatic Data Center (U.S. Depart- located at the 9th through 13th nodal posi- examined in each plot (mean of 382 leaves ment of Commerce, Ashville, NC) and tions from the tip of the cane. Tags were per 2 m2). Because disease severity re- were composed of the 30-year average of affixed to leaves located at these positions mained at low to moderate levels, and mean daily temperature for the midpoint of on each of four randomly selected canes of because pustules were aggregated, inci- each month at Salem, OR. Similarly, to each plant. Disease severity (percent leaf dence of diseased leaves was the most estimate the seasonal favorability for leaf area with symptoms) on the upper surface useful variable for evaluating development wetness, a surrogate variable was created of each tagged leaf was assessed on 10 and of the epidemics and effects of fungicides. from 71-year averages of days per month 24 October and 16 November with the aid For assessments of spermagonia (29 April with >2.5 mm precipitation for Salem, OR of standard area diagrams. Rust severity 2007 and 25 May 2008), the whole-plot (data from National Climatic Data Center) assessments were summarized as area under data were analyzed with a paired t test. For and these values were divided by days in a the disease progress curve (AUDPC) using the July assessment dates, the effects of month to estimate probability of precipita- midpoint rule integration between each subplot treatments on incidence of leaves tion for the midpoint of each month. The assessment date. AUDPC values were sub- with rust was analyzed by ANOVA; product of the mid-month temperature jected to analysis of variance (ANOVA; Fisher’s protected least significant differ- favorability index and mid-month prob- SAS Proc ANOVA); Fisher’s protected least ence test at P = 0.05 was used for separa- ability of precipitation was termed the significant difference test at P = 0.05 was tion of treatment means. “rust favorability index,” which was plot- used for separation of treatment means. ted as a function of month of year. Spring fungicide trials conducted in RESULTS Chemical control. Fungicide experi- 2007 and 2008 were arranged as split-plot Observations on early blackberry rust ments were conducted in a 4-year-old designs with lime sulfur treatments in development in western Oregon. In 2006 planting of Thornless Evergreen blackberry whole plots and timings of myclobutanil to 2008, blackberry rust was observed each located at NWREC near Aurora, OR. The (and, in 2007, azoxystrobin followed by year on both naturalized R. armeniacus 0.12-ha planting consisted of eight rows (43 myclobutanil) as subplots. Whole plots and cultivated R. laciniatus growing in m) with 3-by-1.8-m plant spacing. Plants consisted of a half row (11 plants) random- western Oregon. Spermagonia were ob- were trained to a wire trellis with plot ized in four blocks of two treatments: lime served first on R. armeniacus, perhaps maintenance (weed control, irrigation, sulfur or nontreated. Subplots consisted of because this host initiated leaf growth 2 and fertilization) following best manage- two adjacent plants separated by a buffer weeks earlier than R. laciniatus. In 2008, ment practices. The planting was man- plant. Calcium polysulfide (Rex Lime April was abnormally cool and bud break aged for “alternate year” fruit production Sulfur Solution 28%) mixed 10% (vol/vol) in R. laciniatus occurred during the last where, in each season, half the rows were in water was applied to dormant canes on week of the month; correspondingly, in fruiting and the other half produced only 24 March 2007 and 26 March 2008 and, this season, spermagonia were not ob- new primocanes. In fall, rows with old because of the unusual lateness of spring served until near the middle of May, which fruiting canes (and primocanes associated 2008, 2.5% (vol/vol) calcium polysulfide was approximately 3 weeks later than in with these plants) were cut off near (Rex Lime Sulfur Solution 28%) was ap- 2006 and 2007. Over 3 years of observa- ground level whereas, in the nonfruiting plied on 16 April (a week prior to bud tion, symptoms or signs that would have rows, the primocanes 2 to 3 m in length break). The first treatment of myclobutanil been consistent with asexual perennation were tied to the wires to fruit the follow- (or azoxystrobin) was timed to coincide (diseased buds or development of uredinia ing summer. with expected time of appearance of sper- prior to spermagonia) were not observed.

Plant Disease / May 2010 583 Teliospore germination. Mean seasonal leaves inside of mesh bags), teliospores duration periods (53 and 77 h) with an temperatures for Corvallis in early spring that received this treatment had greatly overall mean temperature <8°C; however, lagged behind those of Montpellier, France reduced germination relative to those that within the period was a subset period with by approximately 4 weeks; consequently, did not (Fig. 1). Over five sampling dates a mean temperature ≥8°C for more than 16 based on the period of teliospore germina- in April, nontreated spores averaged 66.2 ± h (Table 2). tion measured in Montpellier (6), we ex- 4.6% (standard deviation [s.d.]) and 58.9 ± Conditions for germination and infec- pected that teliospore germination in west- 18.9% germination at Aurora and Corval- tion by urediniospores. Infection oc- ern Oregon would occur during a 9-week lis, respectively, whereas teliospores curred at all constant temperatures regimes period extending from late March to early treated with lime sulfur averaged 11.5 ± except 30°C, with the most infection oc- June, with a peak of germination occurring 19.4 and 4.7 ± 5.3% germination at these curring at either 15 or 20°C depending on in early May. For the observed data, the respective locations. wetness duration (Fig. 2A, C, and E). With periods of teliospore germination ranged Infection by basidiospores. At Aurora, diurnal temperature regimes, the greatest from 8 to 10 weeks (Fig. 1; only 2008 data where potted plants of R. laciniatus were infection occurred at 10 to 20°C depending is shown); however, the periods of high exposed in a blackberry planting for two on wetness duration (Fig. 2B, D, and F). spore germination began in early April and spring seasons, only 1 of 21 weekly plant Under the diurnal regime, the amount of remained consistently high into early May, exposures resulted in an infection event. infection at 5°C after 24 h of wetness was and then declined steadily during the latter For that event, four spermagonia were more than twice the amount of the constant half of May. By early June, most collected observed on one leaf of one plant (Table 5°C regime whereas, at 20°C, the amount teliospores (>90%) were shells that had 1). In contrast, at Corvallis, where plants of infection under the diurnal regime was germinated previously. were exposed under platforms holding only about a fifth of that observed under In 2008, regardless of type of lime sul- telia-bearing leaves, 12 of 34 weekly expo- the constant regime. Under both regimes, a fur treatment (sprayed in field or onto sures over 3 years resulted in basidiospore minimum of six continuous hours of leaf infection. For weeks with infection, the wetness was required for infection, with incidence of diseased plants averaged 49 ± >9 h of wetness required for moderate 34% (s.d.) with a mean of 2.5 ± 1.4 leaves infection (>0.4 pustules/cm2) at 15 or 20°C with spermagonia per symptomatic plant, under the constant regime and at 10 or and an average of 6.7 ± 7.0 spermagonia 15°C under the diurnal regime. per diseased leaf (Table 1). Of the 12 Cor- Under the diurnal temperature regime, a vallis exposures with basidiospore infec- significant curvilinear relationship was tion, 9 occurred during April, whereas 2 observed (P = 0.02) between the percent- coincided with exposure in May and 1 age of spores germinated (g) and average 2 from exposure in June; no symptoms de- temperature (Td): g = –0.158 × Td + 2.821 2 veloped on plants exposed in March. × Td + 39.03 (R = 0.94). As with the infec- For Corvallis, in total, 43 wetness peri- tion response, the optimal temperature for ods initiated by rain were observed over germination was 10°C, with 51 ± 13% the 3 years of weekly plant exposures; (s.d.) germination observed, followed these wetness periods were confined to 23 closely by 5 and 15°C, which averaged 48 of the 34 exposure weeks. A wetness pe- ± 9% germination. Only 26 ± 19 and 12 ± riod was required for infection to occur 5% germination were observed at average Fig. 1. Germination of teliospores of Phragmi- but, because of high variability in the daily temperatures of 20 and 25°C, respec- dium violaceum sampled from leaves of Rubus number of infections and in wetness dura- tively. laciniatus cv. Thornless Evergreen from a field planting located near Aurora, OR (solid circle) tion per event, the fitting of a quantitative Seasonal suitability of western Ore- and from bundles of detached blackberry leaves response surface model to the data was not gon climate for infection. Estimated sea- suspended on elevated platforms near Corvallis, successful (data not shown). Nonetheless, sonal favorability for rapid development of OR (solid square) in 2008. Leaves at Corvallis categorizing the 36 wetness events by a blackberry rust based on temperature and were produced at the Aurora location. On 26 duration of <16 or ≥16 h and by a mean probability of precipitation yielded a bi- March 2008 (solid arrow), at Corvallis, a sub- temperature during the wetness period of modal response (Fig. 3). The two periods sample of leaves was sprayed to runoff with <8°C or ≥8°C placed 8 of 12 weeks with of highest favorability coincided with mid- 10% lime sulfur (open square); at Aurora, half infection events into the category that ex- to late spring and early fall. Winter was of the plot area was sprayed with 10% lime ≥ sulfur (open circle). The Aurora field plots re- perienced a wetness period 16 h with a predicted to be unfavorable owing to cold ceived an additional spray of lime sulfur (2.5%) mean temperature ≥8°C (Table 2). Infec- temperatures; July and August also were on 17 April 2008 (dashed arrow). Each point tion events outside of this category either predicted to be unfavorable owing to a represents a sample of 300 teliospores. were very light or had long wetness- combination of higher than optimal daily

Table 1. Summary of weekly exposures of potted Rubus laciniatus cv. Thornless Evergreen to teliospore inoculum of Phragmidium violaceum in western Oregon during spring 2006 to spring 2008 Within weeks with infectiona Overall No. of Total no. No. of weeks Incidence of Mean no. of Mean Mean nodal exposure weeks of of plants infection was diseased diseased leaves spermagonia position of Year Locationb period exposure exposed observed plants (%) per plant per leaf diseased leaves 2006 Corvallis 15 March to 6 June 12 360 5 63 2.8 10.6 5.1 Aurora … 12 360 1 3 1.0 4.0 7.0 2007 Corvallis 20 March to 23 May 9 270 2 38 1.8 3.3 6.7 Aurora … 9 270 0 0 – – – 2008 Corvallis 19 March to 3 June 13 390 5 39 1.8 3.9 7.0 a After exposure, the potted plants were incubated in a greenhouse for 3 weeks after which time disease was assessed; – indicates no infection occurred at this location in this season. b At each location, 30 greenhouse-grown plants were exposed each week to teliospores of P. violaceum. Near Corvallis, plants were exposed under elevated platforms on which detached telia-bearing leaves were suspended.. At Aurora, potted plants were placed in an experimental plot of R. laciniatus.

584 Plant Disease / Vol. 94 No. 5 mean temperature and a low probability of dences of diseased leaves ranged from early October, with plants treated with rainfall. 17% (2007) to 30% (2008) of leaves in the myclobutanil showing <2% rust severity, Disease dynamics. At Aurora, measure- first five nodal positions below the fruit whereas the average severity on all other able epidemics of blackberry rust where cluster (Fig. 5). Disease severity, however, treatments was >7% (Fig. 4). Disease se- observed in the planting of R. laciniatus in remained relatively low, with symptomatic verity increased through mid-November, 2005, 2007, and 2008. In 2005, on primo- leaves averaging 4.2 and 4.9 pustules per with the difference between myclobutanil canes that emerged in August after the leaf on 11 July 2007 and 3 July 2008, re- and other treatments maintained over the plants had been cut, no rust was observed spectively. In 2007, an assessment on 14 period of observation. Based on ANOVA on 9 September but older leaves began to August showed little change in the inci- of AUDPC, rust suppression with my- show symptoms by 20 September. On 29 dence of diseased leaves relative to the clobutanil was significantly (P ≤ 0.05) September, for water-treated plants, leaves July assessment; pustules per leaf averaged greater than with the other fungicides; in positions 9 to 13 from the tip of the cane 6.6 on this assessment date. Interestingly, azoxystrobin provided an intermediate (and near the base) averaged 62 to 120 rust in 2006, a spring rust epidemic did not degree of protection that was significantly pustules per leaf but infections were not develop in the plot area in spite of a large (P ≤ 0.05) better than copper hydroxide or observed at younger positions. By the final number of teliospores carrying over from the water-treated control. assessment in November, rust severity the epidemic in the previous season. Lime sulfur applied in early spring of averaged 30% on leaves in positions 9 to Chemical control. In 2005, the effects 2007 and 2008 significantly (for both 13 (Fig. 4); only an occasional pustule was of fungicide treatments on rust develop- years; P ≤ 0.1 or 0.05 for two- and one- observed at younger leaf positions. ment on primocanes became apparent in tailed paired t test, respectively) sup- Spring epidemics observed in 2007 and 2008 began with spermagonia, which then transitioned to the aecial and uredinial phases of the disease. Spermagonia were first detected on 29 April 2007 and 20 May 2008. In both years, disease increased through June such that, by early July, inci-

Table 2. Frequency of defined leaf wetness and temperature events and their association with spermagonia of Phragmidium violaceum on plants of Rubus laciniatus cv. Thornless Ever- green exposed weekly to teliospores of this pathogen near Corvallis, OR during spring 2006 to spring 2008a Length of wetness period Mean temperatureb <16 h ≥16 h <8°C No. of events 6 10 Events with infection 1c 2d ≥8°C No. of events 11 10 Events with infection 1e 8f a Leaf wetness events required initiation by rainfall. Adjacent wetness periods with breaks of<8 h were joined into a single event (2,13). If a dry break occurred, only hourly temperatures that coincided with wetness were included in the mean. Each week, thirty 1-year- old greenhouse-grown plants were exposed under elevated platforms on which detached blackberry leaves bearing telia of P. violaceum were suspended. b Mean temperature during period of leaf wetness. c In total, 11 spermagonia were observed from this event. d These events had wetness periods of 53 and 77 h where, within each period, occurred a 16-h period with a mean temperature ≥8°C. Totals of 23 and 37 spermagonia coincided with these two events. e In total, eight spermagonia were observed from this event. f Total numbers of spermagonia observed for these Fig. 2. Density of Phragmidium violaceum pustules on Rubus laciniatus cv. Thornless Evergreen as eight events were 27, 41, 225, 265, 305, 440, 935, influenced by post-inoculation leaf wetness duration and A, C, and E, constant or B, D, and F, diurnal and 2,840. For three of these events, an additional temperature regimes. The best fit regression models were based on the A and B, logistic or C and D, six leaf wetness periods were recorded during the Gaussian functions (equations not shown). E and F, A two-factor Gaussian model was suitable for weekly exposure period. Four of these wetness describing the relationship among the pustule density (Pd), leaf wetness (LW), and constant (Tc) or 2 periods were in the category of <8°C with wet- diurnal (Td) temperature regimes: Pd = 1.78 × exp(–0.5 × {[(Tc – 15.43)/5.60] + [(LW – 2 2 2 ness <16 h and two were in the category of ≥8°C 19.97)/6.14] }) (adjusted R = 0.87) and Pd = –1.11 × exp(–0.5 × {[(Td – 10.54)/–6.13] + [(LW – with wetness <16 h. 20.39)/6.97]2}) (adjusted R2 = 0.85). The models and all parameters were significant at P < 0.0001.

Plant Disease / May 2010 585 pressed the development of spermagonia importance to the caneberry industry is mination experiment (Fig. 1) and the fun- by 57 to 78% (Fig. 6). For myclobutanil, partly as “insurance” to the widely grown gicide field experiment (Fig. 6), treatment the degree of suppression of the uredinial and high quality cv. Marion, which is with lime sulfur suppressed the measured phase depended on the time of application. prone to winter injury but not susceptible dependent variable, either teliospore ger- Based on disease assessments made in to P. violaceum (21). Thus, if R. laciniatus mination or spermagonial formation. early July, one application of myclobutanil is to continue as a cultivated blackberry, Growers traditionally apply lime sulfur in late April (2008) or early May (2007) growers need to view management of annually to dormant floricanes to suppress provided significant (P ≤ 0.05) control, blackberry rust as economical within the fungal leaf and cane spot pathogens. Our whereas a single application of this mate- context of a crop with its value partially results indicate that timing of the 10% lime rial made in early June did not (Fig. 7). dependent on the yield performance of sulfur treatment should be made 2 to 3 The highest suppression of rust was ob- another cultivar. weeks before budbreak, when teliospores tained by two treatments with myclobu- Our results show a high likelihood for are ready to germinate. In addition, in tanil (30 April and 11 June 2008); in con- management of blackberry rust in R. 2008, we applied a 2.5% lime sulfur treat- trast, in 2007, an early-May application of laciniatus at low cost. The primary strat- ment just before budbreak, which did not azoxystrobin followed by myclobutanil in egy for management is chemically induce a phytotoxic response; teliospore early June provided an intermediate level achieved sanitation that targets the initia- germination data before and after this of disease control. The interaction between tion of the disease cycle which, from a treatment (Fig. 1) suggest that this second delayed dormant lime sulfur (whole plot) theoretical perspective, is unconventional application was potentially efficacious. and in-season foliar fungicides (subplots) for a foliar, polycyclic pathogen (20). Be- Similarly, myclobutanil treatments were was not significant (P > 0.10). cause asexual perennation of P. violaceum appears absent or rare (22) in the disease DISCUSSION cycle, this pathogen undergoes an apparent Since its first report in 2005, P. obligate sexual cycle each spring, which is violaceum has caused economic damage to unlike other Phragmidium spp. that cause cultivated R. laciniatus cvs. Thornless rusts on rose (11). We observed that initia- Evergreen and Everthornless and, to a tion of the disease cycle from overwinter- lesser extent, the other common species of ing teliospores appears to require long European origin, naturalized Himalayan wetness periods (≥16 h) at mean tempera- blackberry (R. armeniacus). Cultivars of R. tures that are relatively warm (>8°C) for laciniatus represent approximately 10% of western Oregon during April (Table 2). As Oregon’s commercial blackberry acreage a consequence of these constraints, we and are characterized as high yielding and observed only two to five periods of infec- very late to mature but with only interme- tion by basidiospores each spring (Table diate fruit quality. Consequently, their 1). Importantly, in both the teliospore ger-

Fig. 4. Severity of blackberry rust on leaves of Rubus laciniatus cv. Thornless Evergreen in fall 2005 as influenced by treatment with various fungicides: myclobutanil (open triangle), azoxystrobin (solid diamond), copper hydroxide (open circle), and water control (solid square). Data points (± standard error) are the average rating from four plots in an experimental planting grown near Aurora, OR. Fungicide treatments were made on 9 and 29 September.

Fig. 5. Incidence of blackberry rust on leaves of

Rubus laciniatus cv. Thornless Evergreen from Fig. 3. A, Daily average temperature (solid square), probability of precipitation (open diamond), and late spring to early summer 2007 (diamond) and rainfall (open square) for Salem, OR. B, Expectation of seasonal favorability for rapid increase in 2008 (square). Data points (± standard error) are blackberry rust in western Oregon based on the product of relative favorability of temperature multi- the average rating from eight plots in an ex- plied by the probability of precipitation at the mid-point of each month. Relative favorability of tem- perimental planting grown near Aurora, OR. perature was based the data shown in Figure 2: constant temperature regime (solid triangle) and diur- Data shown are the means from plots treated nal temperature regime (open triangle). with water and with lime sulfur only.

586 Plant Disease / Vol. 94 No. 5 more effective in late April or early May Once the disease cycle is initiated, disper- The severity of the blackberry rust epi- than in early June (Fig. 7) which, in both sal of urediniospores from other plantings demic in Oregon in 2005 drew attention to seasons, was before the appearance of the or nearby naturalized Himalayan black- this pathogen both in terms of its potential uredinial phase. Although we lack data to berry likely occurs; however, given the as a constraint to the blackberry industry explain why an early myclobutanil treat- short period of leaf and inflorescence pro- and as a potential biocontrol agent of natu- ment was a superior treatment, we specu- duction, epidemics initiated by uredinio- ralized Himalayan blackberry, an impor- late that this locally systemic material spores produced exogenous to the planting tant noxious weed in the Pacific Northwest suppressed maturation of or spore forma- may not have sufficient time to complete (5). Based on the results of the rotational tion from either the spermagonial or aecial several cycles of infection and, therefore, trap plant study and growth chamber ex- stages of the pathogen (14) and, thereby, attain an economically damaging level of periments, we have attempted to dissect also reduced initial inoculum. disease. Third, in addition to when leaf why the 2005 epidemic was more severe In addition to the obligate telial phase, tissue is susceptible to infection, our than in subsequent seasons. Budbreak of there are several reasons why targeting analysis of the seasonal favorability of the blackberry (and many other crops) in 2005 initial inoculum is apparently an effective western Oregon climate based on growth was several weeks earlier than typical (W. strategy for management of blackberry chamber response curves for infection by F. Mahaffee, personal observation). Be- rust. First, in blackberry, ontogenic resis- P. violaceum produced a bimodal curve ginning in mid-March, weather records tance (9) of leaf tissue to the pathogen with late spring to early summer and late from the Aurora Agrimet station showed develops quickly: individual leaves at 2 summer to early fall being the periods nine potential basidiospore infection weeks of age are a quarter to a third as most conducive to rust development. Al- events through mid-May (i.e., periods with susceptible as new, fully expanded leaves though this analysis was simplistic, these >16 h of leaf wetness and mean tempera- and, by 3 weeks, leaves are difficult to periods coincided with increases in black- ture >8°C) compared with an average of infect (6). For blackberry growing in Ore- berry rust observed in the field (Figs. 4 and 3.8 periods for 2000 to 2008 exclusive of gon, nearly all new leaf growth occurs 5). Because of higher-than-optimum tem- 2005. Similarly, for May and June 2005, from late April to early June, with a secon- peratures and infrequent precipitation, the 15 potential periods of moderate uredinio- dary flush of growth in September, after analysis also predicted that the rate of spore infection were observed (Fig. 2, the fruit matures. Floral bracts are also increase of blackberry rust will be slow defined as the conditions giving rise to highly susceptible but are only produced during summer, which is what we ob- >0.4 pustules/cm2) compared with an aver- during a short period in spring. Conse- served in 2007 (Fig. 5) and in 2008 (de- age of 6 periods in 2000 to 2008 exclusive quently, the spring epidemic is confined to tailed data were not taken). Little increase of 2005. Thus, it appears that an early a relatively short 6- to 8-week period in the of disease in summer also is likely affected budbreak combined with weather favorable life of the crop. Second, the effective dis- by a lack of susceptible tissue. Overhead to infection were two factors that facili- persal distance of basidiospores apparently irrigation, which is common in blackberry tated the severe disease epidemic in 2005. is limited. This was evidenced by our ob- production, partially negates the benefits Nonetheless, there may be other reasons servation of the extreme rarity of sperma- of infrequent rainfall; however, the timing for lack of a significant rust epidemic since gonia on trap plants placed in the experi- of irrigation can be managed to limit the 2005. For the cultivars of R. laciniatus, we mental planting of R. laciniatus compared length of the wetness period (12). suspect that the improved timing of the with infection of trap plants exposed under a dense bundle of telia-bearing leaves collected from the same planting (Table 1).

Fig. 6. Incidence of Rubus laciniatus cv. Thornless Evergreen leaves with symptoms of blackberry rust on A, 29 April 2007 and B, 25 May 2008 as affected by delayed dormant treatment with lime sulfur. In 2007, 10% lime sulfur was applied on 28 March; in 2008, 10% lime sulfur was applied on 26 March and, be- Fig. 7. Incidence of Rubus laciniatus cv. Thornless Evergreen leaves with symptoms of blackberry rust cause of the lateness of this season, an addi- on A, 11 July 2007 and B, 3 July 2008 as affected by delayed dormant treatment with lime sulfur and tional application of 2.5% lime sulfur was made in-season foliar fungicide treatments. Dates of application of foliar treatments are listed under the bars; on 17 April. Bars are the mean (± standard dates of application for lime sulfur were 28 March 2007 and 26 March and 17 April 2008. Bars are the error) of four plots in an experimental planting mean (± standard error) of four plots in an experimental planting located near Aurora, OR. Asterisks located near Aurora, OR. indicate treatment was significantly different (P ≤ 0.05) from the water-treated control.

Plant Disease / May 2010 587 lime sulfur treatment, which was adopted myclobutanil may be useful to suppress the 2002. Ontogenic resistance and plant disease by growers immediately, has been impor- fall epidemic. Based on observations and management: a case study of grape powdery mildew. Phytopathology 92:671-675. tant in lessening the impact of local (in- experiments subsequent to the 2005 epi- 10. Gomez, D. R., Evans, K. J., Baker, J., Harvey, field) inoculum when spring conditions demic, it likely that only lime sulfur fol- P. R., and Scott, E. S. 2008. Dynamics of in- have been favorable for teliospore germi- lowed by a myclobutanil treatment will be troduced populations of Phragmidium nation and infection by basidiospores. required for adequate control in most sea- violaceum and implications for biological con- Additionally, a 24C registration for Rally sons. trol of European blackberry in Australia. Appl. WP was obtained that doubled the rate of Environ. Microbiol. 74:5504-5510. ACKNOWLEDGMENTS 11. Horst, R. K. 1983. Rust. Pages 11-12 in: Com- myclobutanil from what had been allowed Funding was provided by United States Depart- pendium of Rose Diseases. American Phytopa- on blackberry previously, and most grow- ment of Agriculture–Agricultural Research Service thological Society Press, St. Paul, MN. ers make at least one treatment at the (USDA-ARS) Specific Cooperative Agreement 58- 12. Huber, L., and Gillespie. T. J. 1992. Modeling higher rate. For the naturalized popula- 5358-5-793, which was awarded through the leaf wetness in relation to plant disease epide- tions, it was apparent in 2005 that a minor Northwest Center for Small Fruits Research, and miology. Annu. Rev. Phytopathol. 30:553-577. USDA-ARS CRIS 5358-22000-034-00. We thank 13. Jones, A. L., and Sundin, G. W. 2006. Apple biotype of Himalayan blackberry, a facul- T. Sawyer, K. Moukalled, and T. Neill for technical scab: role of environment in pathogen and epi- tative apomict with a population compris- assistance; G. Koskela, J. DeFranceso, and the field demic development. Pages 473-489 in: The ing a limited number of biotypes (3,5,16), staff of the North Willamette Agricultural Research Epidemiology of Plant Diseases. B. M. Cooke, was highly susceptible to P. violaceum. and Extension Center for the use and maintenance B. Gareth Jones, and B. Kaye, eds. Springer, Severe rust developed on the minor bio- of the experimental blackberry planting; and J. Houten, The Netherlands. Pscheidt for providing wetness-duration data col- 14. Kemmitt, G. M, DeBoer, G., Ouimette, D., and type but only low levels of infection were lected at the Botany and Plant Pathology Field Iamauti, M. 2008. Systemic properties of my- observed on the more predominant bio- Laboratory near Corvallis. clobutanil in soybean plants, affecting control types (unpublished data). Subsequently, of Asian soybean rust (Phakopsora pachyr- observations of severe rust in naturalized LITERATURE CITED hizi). Pest Manage. Sci. 64:1285-1293. 1. Aime, M. C., and Rossman, A. Y. 2007. First 15. Laundon, G. F., and Rainbow, A. F. 1969. blackberry have become infrequent, per- report of the rust Phragmidium violaceum on Phragmidium violaceum (Schultz) Wint. In: haps due to pathogen-mediated selection Pennsylvania blackberry in California. Plant Descriptions of Pathogenic Fungi and Bacteria. for resistant biotypes. A reduced incidence Dis. 91:1517. Commonw. Mycol. Inst. Publ. No. 209. Kew, of rust in naturalized blackberry also has 2. Becker, C. M., and Burr, T. J. 1994. Discon- Surrey, England. likely reduced regional levels of inoculum. tinuous wetting and survival of conidia of Ven- 16. Nybom, H. 1988. Apomixis versus sexuality in turia inaequalis on apple leaves. Phytopathol- blackberries (Rubus subgen. Rubus, Rosaceae). In conclusion, although P. violaceum is ogy 84:372-378. Plant Syst. Evol. 160:207-218. now endemic in western Oregon, man- 3. Bijlsma, R.-J., and Haveman, R. 2007. Rubus 17. Osterbauer, N., Trippe, A., French, K., agement of this pathogen in cultivated R. canduliger, a new regional species from the McKemy, J., Bruckart, W. L., Peerbolt, T., laciniatus should be achievable at an ac- Netherlands with notes on the range structure Kaufman, D., and Aime, M. C. 2005. First re- ceptable cost. Growers are advised to ap- and the dynamics of brambles (Rubus, Rosa- port of Phragmidium violaceum infecting Hi- cae). Folia Geobot. 42:315-329. malaya and evergreen blackberries in North ply lime sulfur (10%) in late March or 4. Bruzzese, E., and Hasan, S. 1986. Host speci- America. Plant Health Prog. doi:10.1094/PHP- early April and to consider a second treat- ficity of the rust Phragmidium violaceum, a 2005-0923-01-BR. ment (2.5%) prior to bud break (mid- potential biological control agent of European 18. Pigott, J. P., Weiss, J., Evans, K. J., and Mahr, April). Near the end of April or beginning blackberry. Ann. Appl. Biol. 108:585-596. F. 2003. Predicting the effect of the rust dis- of May, they should scout for the sperma- 5. Caplan, J. S., and Yeakley, A. J. 2006. Rubus ease (Phragmidium violaceum (Schultz) Win- armeniacus (Himalayan blackberry): occur- ter) on blackberry (Rubus fruticosus L. agg.) in gonial stage and apply myclobutanil at first rence and growth in relation to soil and light Victoria (Australia). Pages 373-381 in: Plant detection. After the first spray, the weather conditions in western Oregon. Northwest Sci. Invasions: Species Ecology and Ecosystem should be monitored for frequency of addi- 80:9-17. Management. G. Brundu, J. Brock, I. tional wetness periods, and fields should 6. Evans, K. J., and Bruzzese, E. 2003. Life Camarda, L. Child, and M. Wade, eds. Back- be scouted for continued increase in dis- history of Phragmidium violaceum in relation huys Publishers, Leiden, The Netherlands. to its effectiveness as a biological control agent 19. Tervet, I. W., Rawson, A. J., Cherry, E., and ease. A second application of myclobutanil of European blackberry. Australas. Plant Saxon, R. B. 1951. A method for the collection just before flowering may be warranted if Pathol. 32:231-239. of microscopic particles. Phytopathology the rust has continued to increase in the 7. Evans, K. J., and Gomez, D. R. 2004. Genetic 41:282-285. field. Although summer is less favorable markers in rust fungi and their application to 20. Van der Plank, J. E. 1963. Plant Diseases: for disease increase, we recommend that weed biocontrol. Pages 73-96 in: Genetics, Epidemics and Control. Academic Press, NY. Evolution and Biological Control. L. E. Ehler, 21. Washington, W. S. 1987. Susceptibility of summer irrigation events be managed to R. Sforza, and T. Mateille, eds. CAB Interna- Rubus species and cultivars to blackberry rust limit the frequency and the length of wet- tional, London. (Phragmidium violaceum) and its control by ness periods. After harvest, the stems of 8. Evans, K. J, Jones, M. K., and Roush, R. T. fungicides. J. Phytopathol. 118:265-275. old floricanes can be cut to eliminate ac- 2005. Susceptibility of invasive taxa of Euro- 22. Washington, W. S. 1991. Blackberry rust. tive pustules and teliospore formation on pean blackberry to rust disease caused by the Pages 32-33 in: Compendium of Raspberry uredinial stage of Phragmidium violaceum un- and Blackberry Diseases and Insects. M. A. these tissues. If, in early September, active der field conditions in Australia. Plant Pathol. Ellis, R. H. Converse, R. N. Williams, and B. rust pustules are present on primocanes 54:275-286. Williamson, eds. American Phytopathological (next year’s floricanes), an application of 9. Ficke, A., Gadoury, D. M., and Seem, R. C. Society Press, St. Paul, MN.

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