HORTSCIENCE 43(1):260–263. 2008. Campbell, 1973). Thus, if Pythium spp. occur on blueberry, their effects may be more subtle than Phytophthora. Plants infected by Incidence of Phytophthora and Pythium may simply lack vigor, producing less growth than a noninfected plant. General Pythium Infection and the Relation declines in plant growth resulting from Pythium spp. have been documented in other to Cultural Conditions in Commercial perennial fruit crops (e.g., Hendrix et al., 1966; Mazzola et al., 2002; Spies et al., 2006). Severity of root rot by Pythium spp. was Blueberry Fields significantly reduced by applications of David R. Bryla1 and Robert G. Linderman mefenoxam, fosetyl-Al, and phosphonate fungi- U.S. Department of Agriculture, Agricultural Research Service, cides on a new planting of southern highbush blueberry (V. corymbosum hybrid ‘Millenium’) Horticultural Crops Research Unit, 3420 NW Orchard Avenue, Corvallis, in Georgia (Brannen and NeSmith, 2006). OR 97330 The objective of this study was to deter- mine the incidence of Phytophthora and Wei Q. Yang Pythium in commercial fields of blueberry Oregon State University, North Willamette Research and Extension Center, in Oregon and to identify any factors that Aurora, OR 97002 increase the probability of developing infec- tion. Oregon currently has 1780 ha of blue- Additional index words. Phytophthora cinnamomi, Pythium spp., Vaccinium corymbosum, berry, most of which is planted in the Vaccinium ashei, cultivar, plant vigor, root rot northwest part of the state, and produces Abstract. Fifty-five commercial blueberry (Vaccinium spp.) fields were sampled in north- 16,150 t of the fruit annually (U.S. Depart- west Oregon in 2001 to determine the incidence of Phytophthora and Pythium root rot ment of Agriculture, 2007a). Although root pathogens and identify cultural factors that increase the probability of developing rot is prevalent in the region, no information infection. Phytophthora was detected in 24% and Pythium was detected in 85% of the is available on the distribution and severity of fields sampled. The only species of Phytophthora identified in the study was P. cinnamomi. the disease. Root infection by P. cinnamomi was significantly related to cultivar with incidence observed more frequently than expected in ‘Duke’ and ‘Bluecrop’. Both blueberry Materials and Methods cultivars are two of the most popular grown in the region, representing 42% of the fields in this survey and ’46% of the total area planted in Oregon. Two other cultivars found Soil cores (2.5 cm diameter · 0.30 m infected by P. cinnamomi were ‘Rubel’ and ‘Briggitta Blue’, together accounting for an deep) were collected in Aug. 2001 from 55 additional 24% of the fields surveyed. Phytophthora was not detected in fields planted with commercial blueberry fields located in north- ‘Berkeley’, ‘Bluejay’, ‘Bluetta’, ‘Darrow’, ‘Earliblue’, ‘Elliott’, and ‘Powderblue’, each of west Oregon. Field size averaged 12 ha and which represented only 2% to 7% of the fields surveyed. Pythium spp. were identified to ranged from 2 to 50 ha. Each field was genus only, but one or more species of Pythium was found in all 11 cultivars included in the sampled within rows, 1 m from the base survey. Occurrence of either Phytophthora or Pythium was unrelated to soil type, planting of a plant, collecting three cores at five age, or cultural practices such as bed type, cover crop, mulch, irrigation system, fertilizer representative locations (at least 50 m apart). application, fungicide use, or the source of plant material used in the fields. Overall, most Weed and grass roots were carefully avoided fields with Phytophthora or Pythium remained largely symptomless under good soil during sampling. Each core was separated by drainage conditions and had similar levels of vigor as those without the pathogens. depth into 0- to 0.15-m and 0.15- to 0.30-m sections. Roots were removed from each section by washing, surfaced-sterilized in sodium hypochlorite (2%) for 5 min, rinsed, Root rot is a major disease in blueberry. Rand. (Caruso and Ramsdell, 1995), a wide- and placed into cups filled with 100 mL of Characteristic symptoms include discolored spread soil pathogen with a large host range dH2O. Leaf discs (5 mm diameter) of Camel- and necrotic roots, stunted growth, pale first reported in northern highbush blueberry lia sasanqua Thunb. were floated on the yellow to reddish leaves, marginal leaf necro- (Vaccinium corymbosum L.)in1961(Raniere, surface of each cup as bait for collecting sis, premature defoliation, and, in some 1961). Since then, P. cinnamomi has been zoospores (Linderman and Zeitoun, 1977). cases, plant death (Cline and Schilder, documented in many blueberry plantings After a minimum of 24 to 48 h, leaf discs (five 2006). Young plants are usually most sus- throughout the United States, including in per cup) were direct-plated onto petri dishes ceptible to root rot, although severe instances Arkansas (Sterne, 1982), Florida (Lyrene and filled with either P5ARP agar (Kannwischer can develop in mature plants located in soils Crocker, 1991), Maryland (Draper et al., and Mitchell, 1978), which is selective for with poor drainage (Sterne, 1982). The causal 1971), Mississippi (Smith, 2002), New Jersey members of the family Pythiaceae,orP5ARP organism most commonly associated with (Royle and Hickman, 1963), and North Caro- agar amended with 25 ppm hymexazol the disease is Phytophthora cinnamomi lina (Clayton and Haasis, 1964; Milholland (P5ARP + H; Tachigaren, 70% a.i.; Saankyo and Galletta, 1967). Co., Tokyo), which is selective for Phytoph- Pythium spp. also cause root rot in many thora spp. (Masago et al., 1977). The iso- Received for publication 27 Mar. 2007. Accepted plants, including members of the Ericaceae lation dishes were incubated in the dark at for publication 13 Sept. 2007. family (which includes blueberry) such as 20 °C and then examined daily for pythia- We thank A. Davis for technical assistance and azalea and rhododendron (Rhododendron ceous colony growth for at least 7 d. Pythium the Oregon Blueberry Commission for financial spp.; Coyier and Roane, 1986), but typically isolates were identified to genus only, but support. have not been associated with the disease in Phytophthora isolates were identified to spe- Mention of trademark, proprietary product, or blueberry (but see Brannen and NeSmith, cies. Species identification was done at 100 to vendor does not constitute a guarantee or warranty 2006). Like Phytophthora, Pythium spp. are 400· magnification and was based on mor- of the product by the U.S. Department of Agricul- ture and does not imply its approval to the ‘‘water molds’’ that readily infect and spread phological characteristics of sexual and asex- exclusion of other products or vendors that also in wet, poorly drained soils (Duniway, 1979); ual structures observed in the isolates may be suitable. however, they usually only infect young, (Stamps et al., 1990). Roots were oven-dried 1To whom reprint requests should be addressed; succulent feeder roots and therefore often (65 °C) after baiting and weighed to deter- e-mail [email protected]. lack the ability to kill the host (Hendrix and mine total dry biomass of each sample. 260 HORTSCIENCE VOL. 43(1) FEBRUARY 2008 SHORT COMMUNICATIONS Planting age, cultivar(s), bed type, cover There was considerable variation in the sawdust applied on the soil surface along the crop, mulching, irrigation method, fertilizer cultural characteristics of the fields surveyed, length of the planting bed, was used in 58% application, types of pesticides and fungi- including differences in plant age, cultivar, of the fields. Most fields were irrigated by cides applied, and source of planting material soil type, bed type, cover crop, mulching overhead sprinklers (96%) with only 4% in each field were noted during sampling or practices, irrigation system, fertilizer appli- irrigated by drip. Fertilizer applications obtained from the growers. Fields were also cation, fungicide use, and the source of plant ranged from 45 to 500 kgÁha–1 of nitrogen rated for vigor with a relative rating scale material. Plantings ranged in age from 1 to 50 (N; usually applied as ammonium sulfate) per of 1 to 5 in which 1 = no growth, severely years and consisted of 10 different cultivars year, although the majority of growers stunted; 2 = poor growth, low yield (less than of northern highbush blueberry, including applied between 110 and 170 (33%) or 170 5tÁha–1); 3 = moderate growth and yield (5 to ‘Berkeley’, ‘Bluecrop’, ‘Bluejay’, ‘Bluetta’, to 225 (33%) kgÁha–1 of N per year. Only 18% 10 tÁha–1); 4 = good growth, high yield (10 to ‘Briggitta Blue’, ‘Darrow’, ‘Duke’, ‘Ear- of the fields sampled reported use of fosetyl- 15 tÁha–1); or 5 = excellent growth and yield liblue’, ‘Elliott’, and ‘Rubel’, and one culti- Al (Aliette, Bayer CropScience, Research (greater than 15 tÁha–1); young fields (less var of rabbiteye blueberry (V. ashei Reade), Triangle Park, NC) or mefenoxam (Ridomil than 4 to 5 years old) were evaluated primar- ‘Powderblue’. Altogether, these 11 cultivars Gold, Syngenta Crop Protection, Greensboro, ily on growth, whereas mature fields were represent 90% of the total area of blueberry NC) fungicides for prevention and control of evaluated on production. Further details on planted in Oregon (Yang, 2002). The most root rot, whereas the remaining 82% reported soil characteristics, root distribution, and common cultivars sampled in the study were no use of any fungicide for root rot. Plants in mycorrhizal status of the fields were reported ‘Duke’, ‘Bluecrop’, and ‘Rubel’, which 89% of the fields sampled came from one of by Scagel and Yang (2005). Field soil types accounted for 56% of all the fields surveyed two commercial nurseries located in Oregon. were obtained from soil survey maps (U.S. (Table 2). Approximately 7% (n = 4) of the Phytophthora was detected in 24% of the Department of Agriculture, 2007b). fields consisted of a mix of two or more fields sampled, including in fields of ‘Blue- Weather data for the region was obtained cultivars, three fields of which the cultivars crop’, ‘Brigitta Blue’, ‘Duke’, and ‘Rubel’ from a U.S.