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HORTSCIENCE 41(7):1635–1639. 2006. been reported as being more genetically diverse on than on (Wangsomboondee et al., 2002). Epidemics occurred in 2004 Characterization of Isolates of when over 50% of the commercial crop was lost in eastern states from New York to infestans from Four Florida; 80% to 90% of early seedbeds in Florida were a complete loss from Nov. 2004 Solanaceous Hosts Growing in to Apr. 2005. Most growers reported that no available would control the rapid spread of the epidemic. Heavy losses oc- Association With Late Blight-infected curred in transit; symptoms developed on infected but symptomless fruit within 5 d of Commercial Potato Crops harvest. 1 Late blight affects both the and Kenneth L. Deahl , Richard W. Jones, and Frances M. Perez fruit of tomatoes, spreading rapidly. On Vegetable Laboratory, USDA, ARS, Beltsville, MD 20705-2350 leaves, greasy-looking, irregularly shaped gray spots appear around which a ring of David S. Shaw white may develop, especially in School of Biological Sciences, University of Wales, Bangor, U.K., and Sa´rva´ri wet weather. The spots eventually turn dry Research Trust, Siambra Gwynion, Llandygai, Bangor, LL57 4BG, U.K. and papery. Blackened areas may appear on the stems. The fruit also develop large, irreg- Louise R. Cooke ularly shaped, greasy gray spots. P. infestans Department of Applied Science, School of & Food Science, can overwinter in frost-free areas in dead Queen’s University, Newforge Lane, Belfast, BT9 5PX, U.K. tomato and diseased debris (Peterson, 1947). Because it spreads to potatoes, it also Additional index words. tomato, petunia, nightshade, tuberosum, Lycopersicon overwinters in potato tubers. For transplant esculentum, disease susceptibility tomatoes, the initial source of disease may be infected transplants. Other potential sources Abstract. The , , is a devastating pathogen of potato of inoculum are potato cull piles and volun- worldwide. Several of P. infestans are able to infect other cultivated and weed teer potatoes and tomatoes. of the family and cause symptoms similar to late blight. Changes in Solanaceous weed and ornamental spe- P. infestans populations have stimulated investigations to determine if potato strains cies may also be infected with late blight and, from new immigrant populations infect nonpotato hosts more often than those from the where they grow in proximity to potato and older population. Expansion of the effective range may be one of the mechanisms tomato crops, may act as reservoirs of in- involved in pathogenic changes in natural populations of P. infestans and to determine its oculum or aid generation of diversity if they significance, it is necessary to establish if the pathogen strains on nonpotato hosts permit contact between crop-specialized gen- represent distinct genotypes/populations or are freely exchanging with those on potato. otypes. Furthermore, late blight has also been This article reports characterization of P. infestans isolates from four solanaceous hosts reported on weed species (e.g., nightshade) in (black nightshade, hairy nightshade, petunia, and tomato) growing within and around the and where many fields of blighted potatoes in four U.S. locations and one U.K. location and their potato and tomato production areas are di- comparison with isolates collected from adjacent infected potatoes. Isolates were rectly linked geographically or by crop mar- characterized for mitochondrial DNA haplotype, mating type, metalaxyl resistance, keting. Species reported to be involved allozymes of glucose-6-phosphate isomerase and peptidase, and DNA fingerprint with the include petunia, black nightshade, and hairy RG57 probe. Analysis showed close similarity of the petunia, hairy and black nightshade nightshade. Although late blight is a highly isolates to potato isolates. However, tomatoes from New Jersey and Pennsylvania, significant, well-studied disease of potato and respectively, were infected by two distinct and previously unreported pathogen tomato, relatively little is known about this genotoypes, which had quite different fingerprints from P. infestans isolates recovered disease incited by the same organism on other from nearby infected potatoes. Potato growers should be aware that both weed and solanaceous hosts. cultivated solanaceous species can be infected with P. infestans and may serve as clandestine reservoirs of inoculum. Because some of these plants do not show conspicuous symptoms, they may escape detection and fail to be either removed or treated and so may Materials and Methods play a major role in the introduction and spread of pathogens to new locations. Collection of isolates. Samples were col- lected from solanaceous hosts with lesions similar to late blight in four locations in Late blight, incited by the oomycete path- mating type (US-8) and an A1 mating type major potato production areas in the United ogen Phytophthora infestans (Mont.) de Bary, (US-11), both resistant to the met- States and one location in Wales (see sub- has become an increasingly important prob- alaxyl, are present (Daayf et al., 2000; sequently). Blighted potato material was lem to agriculture in the United States and Dorrance et al., 1999; Gavino et al., 2000; collected from the same locations by coop- many other countries in the past decade. More Goodwin et al., 1998). The US-8 clonal erators during crop inspections of naturally aggressive fungicide-resistant and host- lineage is the most common on potatoes, infected fields. Each sample consisted of specialized isolates have appeared attacking but US-11 has been particularly troublesome infected leaves and stems from one or more potato (Solanum tuberosum L.) and tomato on tomatoes (Gavino et al., 2000). Although plants within a single crop. Information on (Lycopersicon esculentum Mill.) crops. these two pathogen genotypes predominate, site, fungicide use, potato , and blight In Canada and the United States, two the infrequent occurrence of other genotypes incidence was collected for each sample. major genotypes of the pathogen, an A2 (Goodwin et al., 1998) indicates that sexually Single lesions were incubated under high reproducing population pockets or intro- for 24 to 48 h to encourage sporu- duced pathogen genotypes pose a continuing lation, then isolates were obtained by collect- Received for publication 16 June 2006. Accepted threat. ing sporangia from infected foliage and for publication 4 Aug. 2006. Late blight has suddenly reemerged as initially maintained on detached glasshouse- 1To whom reprint requests should be addressed; a major concern in most tomato-producing grown potato leaflets or tuber slices of sus- e-mail [email protected]. areas of the United States. The pathogen has ceptible free from R-.

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Tissue pieces 3 to 4 mm2 were cut from transplants at a glasshouse in central Mary- diameters measured once average growth the margins of lesions on the detached - land. Infected plants had extensive light gray, diameters reached 15 mm on untreated plates lets, surface sterilized, and transferred to irregularly shaped, slightly sunken lesions up (typically after 5 to 6 d for faster-growing Petri plates containing rye A agar (Caten to 3 cm long on the upper leaves and isolates) and again 2 to 3 d later. The and Jinks, 1968) amended with antibiotics branches. Stems were distorted or dead above percentage reduction in growth on 10 mgÁL–1 (100 mgÁmL–1 ampicillin, 100 mgÁmL–1 nys- points where lesions had coalesced to cause metalaxyl compared with growth on the un- tatin, 50 mgÁmL–1 rifampicin). The plates total or almost complete stem girdling. The amended control plates was calculated. Each were incubated at 20 C for 5 to 7 d to allow disease initially occurred as scattered foci, isolate was tested at least twice. Isolates were mycelia to grow into the medium. Small agar but rapidly spread through the crop causing designated as metalaxyl-resistant if growth blocks containing hyphal tips were cut from significant losses. The disease reappeared in was >60% of the control, -intermediate if the colony margins and transferred to un- the same and adjacent greenhouses in 2002 growth was 10% to 60% of the control, or amended rye A agar for growth and sporula- and 2003, producing severe damage in sev- -sensitive if growth was <10% of the control tion at 20 C. Stock cultures were maintained eral crops. Three isolates were obtained from using the criteria of Shattock (1988). on unamended rye A agar and transferred at sporulating lesions on the leaves and stems. Allozyme assays. Genotypes at two poly- 6- to 12-month intervals. Late blight was observed on potato plants in morphic allozyme loci, Gpi-1 (glucose- Alternate hosts of Phytophthora infestans. fields in close proximity to the petunia glass- 6-phosphate isomerase, GPI, E.C. 5.3.1.9.) Lesions similar to those incited by P. infes- houses and P. infestans isolated. and Pep-1 (peptidase, PEP, E.C. 3.4.3.1.), tans were found on other cultivated and weed Tomato. Samples of tomato foliage and were determined using the protocols of species of the family Solanaceae and isolates fruit with late blight lesions were collected Goodwin et al. (1995a). Chilled supernatants were obtained from these (Table 1) as de- from commercial field crops in New Jersey in containing protein released from mycelial tailed subsequently. 2003 and Pennsylvania in 2004. Three iso- fragments in sterile distilled water were Black nightshade. In 1999, brown necrotic lates were obtained from New Jersey and five loaded onto cellulose acetate plates equili- leaf lesions with pale green margins were isolates from Pennsylvania all from different brated in the appropriate buffer: Tris-glycine found on black nightshade (Solanum nigrum crops. In each case, P. infestans was also (TG) buffer (25 mM Tris-HC1, 192 mM L.) weeds within a potato trial naturally isolated from potato crops in the immediate glycine, pH 8.5) was used for both GPI and infected with P. infestans at Henfaes Research vicinity. PEP. Enzymatic activities were revealed Center, University of Wales, Bangor. Five Mating type determination. Unamended after electrophoresis and staining with the isolates were obtained from leaf fragments. rye agar plates were inoculated with a myce- appropriate agar overlays (Goodwin et al., Isolates were also obtained from infected lial plug of the test isolate, and a plug of 1995a). The genotypes of unknown isolates potato plants in the trial. Similar lesions were a reference P. infestans isolate of either the were determined by comparing their banding observed on black nightshade in the same trial A1 or A2 mating type placed 20 to 30 mm patterns with those of reference isolates area in 2004 and a further isolate obtained. away (four plates for each test isolate, two kindly provided by SB Goodwin, USDA, Hairy nightshade. An important potato with different A1 reference isolates and two Purdue University; isolate (P-83-PI), Gpi production area of northeastern Maine was with different A2 reference isolates). The 90/100 Pep 83/100; R Young, West surveyed during 2004 for the occurrence of dual cultures were incubated at 15 Cin University; US-1 isolate WV-63, Gpi 86/100 late blight on cultivated and noncultivated darkness for 7 to 14 d and then examined Pep 92/100; WE Fry, Cornell University, US-6 host plants. Special attention was directed to microscopically for the presence of oospores isolate CAL 7–5 Gpi 100/100 Pep 92/100; solanaceous weed species growing within where the two colonies interacted. US-7 isolate KKK-W4B, Gpi 100/111 Pep and around fields of blighted potatoes. Plants Metalaxyl sensitivity in vitro. Metalaxyl 100/100; and US-8 isolate NY-01, Gpi 100/ of hairy nightshade (Solanum sarrachoides (as ÔRidomil 2EÕ, Syngenta, Greensboro, 111/122 Pep 100/100 and US-11 isolate Sendtner) with numerous leaf lesions and N.C., 25.1% w/w metalaxyl) was added to 110B, Gpi 100/100/111 Pep 100/100; moderate defoliation were collected from rye seed agar to yield a final concentration of D. Inglis, Washington State University. one location within the border of adjoining 10 mgÁL–1. Each isolate was inoculated onto Identification of mitochondrial DNA potato fields. Typical lesions contained ex- three Petri plates of metalaxyl-amended agar haplotypes. Mitochondrial DNA (mtDNA) tensive, white, superficial mycelia colonizing and three unamended control plates using haplotypes of isolates were determined by abaxial and adaxial leaf surfaces. Eight iso- agar plugs (5-mm diameter) cut from the polymerase chain reaction–restriction frag- lates were obtained from sporulating lesions outer zones of active growth from cultures ment length polymorphism using a modifica- on hairy nightshade and 18 isolates from aged 10 to 20 d. Standard metalaxyl-resistant tion of the method of Griffith and Shaw nearby potatoes. and -sensitive P. infestans isolates were in- (1998). Two-week-old mycelium, grown in Petunia. In April 2001, a new disease was cluded with each set of tests. Plates were pea broth supplemented with 2 gÁL–1 calcium detected on petunia (Petunia · hybrida) incubated at 21 C in darkness and colony carbonate and 0.05 gÁL–1 b-sitosterol, was lyophilized following the methods of Good- win et al. (1995a). DNA extraction was performed using the Qiagen DNAeasy Plant Table 1. Host origin, location, and collection date of isolates of Phytophthora infestans evaluated Mini Kit (Qiagen, Valencia, Calif.). DNA in this study. was amplified using two pairs of oligonucle- Hosts Location Years when sampled Number of isolates otide primers F2/R2 and F4/R4 synthesized Black nightshade Wales 1999, 2004 6 by GibcoBRL Life Technologies (Gaithers- Potatoz 23 burg, Md.) according to the sequences given by Griffith and Shaw (1998). The sequences Petunia Maryland 2002, 2003 3 for the primer pairs were as follows: Potatoz 19 F2 (forward) 5#-TTCCCTTTGTCCTCT ACCGAT-3# Hairy nightshade Maine 2004 8 # z R2 (reverse) 5 -TTACGGCGGTTTAG Potato 18 CACATACA-3# F4 (forward) 5#-TGGTCATCCA Tomato New Jersey 2003 3 # Potatoz 11 GAGGTTTATGTT-3 R4 (reverse) 5#-CCGATACCGATAC Tomato Pennsylvania 2004 5 CAGCACCAA-3# Potatoz 19 Polymerase chain reactions (PCRs) were zLate-blighted potato hosts that were located in bordering, neighboring, or close-by plantings. carried out in a Perkin Elmer Amp 9600

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thermal cycler (Perkin Elmer, Wellesley, insert was PCR-amplified with the oligonu- shade were shown to be metalaxyl-resistant Mass.) in 21 mL final reaction volumes. Each cleotide primers M13 (forward and reverse). (Table 2). Single sporangial isolates derived reaction consisted of 1 mL(55 ng DNA) The product was purified with the Promega from each isolate had the allozyme of template DNA, 2 mL10· PCR buffer Wizard DNA Purification Kit and labeled Gpi 100/111/122 and Pep 100/100, were A2 (Takara, Madison, Wis. 10· Ex Taq Buffer with the NEN Random Primer Fluorescein mating type and mtDNA Ia. DNA fingerprint containing 20 mM MgCl2), 1.6 mL dNTPs (2.5 Labeling Kit (Antifluorescein-AP, NEN Life analysis with probe RG57 further established mM each), 0.2 mL each of the oligonucleotide Science Products, Boston, Mass.). P. infes- that all the hairy nightshade isolates had the primers, 3.2 ml BSA (160 mgÁmL–1), 0.2 ml tans DNA was extracted using Sigma’s Gen- fingerprint characteristic of the US-8 clonal Taq DNA polymerase (Takara Ex Taq poly- eElute Plant Genomic DNA Extraction Kit lineage, the clonal lineage most widely oc- merase; 5U mL–1), and 12.6 mL sterile water. (Sigma, St. Louis, Mo.). The DNA was curring on potato in the United States, and Amplification of DNA was optimized by the transferred to Hybond N+ nylon membrane were identical to local P. infestans isolates following PCR conditions: a denaturation (Amersham Pharmacia, Piscataway, N.J.). from potato. Hairy nightshade infected with cycle at 94 C for 90 s followed by 35 cycles The manufacturersÕ instructions for these kits P. infestans was only found when blight was of 94 C for 40 s, annealing temperatures were followed for membrane transfer, South- already widespread in potato fields. of 60 C (primers F2/R2) or 55 C (primers ern hybridization, and nucleic acid detection. Petunia. The pathogen sporulating on F4/R4) for 60 s, and 72 C for 90 s. The leaves and stems of the petunia plants was reactions were held for 10 min at 72 C Results shown to be P. infestans (Deahl and Fravel, as a final extension step before cooling 2003). Three isolates obtained from symp- down to room temperature. A 10-mL aliquot Black nightshade. Six isolates obtained tomatic petunia stem and leaf tissue were all of each amplified DNA was thoroughly from leaf fragments were subsequently con- of the A2 mating type. The petunia isolates mixed with 20 mL of master mix containing firmed as P. infestans by morphological, bio- were shown to be Gpi 100/111/122, Pep 100/ 1 mL (50 U mL–1) of the restriction enzyme chemical, and molecular characteristics and 100, metalaxyl-resistant, and mtDNA haplo- MspI (for primer pair F2/R2 products) or Koch’s postulates were completed (Deahl type Ia, characteristics associated with US-8; EcoRI (for primer pair F4/R4 products), et al., 2004). The isolates were A1 mating this attribution was confirmed by RG57 anal- 3 mL10· restriction buffer (10· React 1 for type, metalaxyl-sensitive, mtDNA haplotype ysis of three single isolates (Table 2). MspI enzyme or 10· SH buffer for EcoRI IIa (Table 2) with an RG57 fingerprint iden- The petunia isolates were indistinguishable enzyme), and 16 mL sterile water to volume. tical to that reported by Day et al. (2004) as from potato isolates collected in the same Restriction enzyme digestions were incu- RF006. Isolates collected from potatoes in the area in terms of RG57 fingerprints, morphol- bated overnight at 37 C. Five microliters of vicinity of the infected black nightshade plants ogy, and all other assessed characteristics gel-loading buffer (20% Ficoll, 0.02% bro- had identical characteristics to those from (Table 2). mophenol blue, 0.002% xylene cyanol) were black nightshade. RF006 was the third most Tomato. P. infestans isolates obtained added to each digested DNA sample and 15 common fingerprint (9%) among over 1400 from late blighted tomato fields in New Jersey mL of each mixture was loaded into each well isolates collected from potato crops in Eng- in 2003 were all A2 mating type, metalaxyl- of a 2% agarose gel in 1· Tris-acetic acid- land, Scotland, and Wales during 1995 resistant, and mtDNA haplotype Ia. However, EDTA (TAE) buffer stained with ethidium through 1998; all isolates with this fingerprint these isolates were homozygous at the loci bromide (0.1 mgÁmL–1). A 100-bp ladder were A1 mating type and 94% were mtDNA coding for both glucose-6-phosphate isomer- (Promega, Madison, Wis.) was used as a size IIa. ase and peptidase, having Gpi 122/122, Pep marker. Digested DNA patterns were re- Hairy nightshade. When samples of hairy 100/100. RG57 analysis showed that all the solved by electrophoresis run at 5Vcm nightshade plants were placed in a moist tomato isolates from New Jersey had a unique for 2 to 2.5 h and visualized using an chamber to induce sporulation, - and previously unreported fingerprint (Fig. 1). ultraviolet transilluminator at 254 nm. Im- shaped sporangia typical of P. infestans de- Although severe late blight symptoms were ages were captured using the Bio-Rad Fluor- veloped. The identity of the pathogen was present in nearby potato crops, the isolates S System (Bio-Rad, Hercules, Calif.) with confirmed after reisolation onto rye–lima were typical of the US-8 genotype (Table 2). Quantity One software. bean medium. Koch’s postulates were com- Single-lesion isolates from late blighted RG57 Fingerprinting. DNA fingerprint- pleted on S. sarrachoides, potato, and tomato tomato fields in Pennsylvania were also all ing using the moderately repetitive probe with a random selection of the isolates A2 mating type, metalaxyl-sensitive, mtDNA RG57 was carried out as described by Goodwin obtained from S. sarrachoides (Deahl et al., haplotype Ia and were Gpi 100/122, Pep 100/ et al. (1992) except as follows. The RG57 2005). Isolates obtained from the hairy night- 100, characteristics atypical of isolates of

Table 2. Evaluation of the mitochondrial DNA (mtDNA) haplotype, mating type, metalaxyl resistance, allozymes of glucose-6-phosphate isomerase and peptidase, and DNA fingerprint with the RG57 probe of isolates of Phytophthora infestans collected from potatoes, tomatoes, and weed hosts in four U.S. locations and one U.K. location. MtDNA Allozyme genotype Metalaxyl Mating Host haplotype Gpi Pep sensitivityz Type Genotypey RG57 fingerprint Black nightshade IIa 100/100 100/100 S A1 N/A 1,5,9,10,13,14,16,20,21,24,25 Potatox IIa 100/100 100/100 S A1 N/A 1,5,9,10,13,14,16,20,21,24,25

Petunia Ia 100/111/122 100/100 R A2 US-8 1,5,10,13,14,16,20,21,23,24,25 Potatox Ia 100/111/122 100/100 R A2 US-8 1,5,10,13,14,15,20,21,23,24,25

Hairy nightshade Ia 100/111/122 100/100 R A2 US-8 1,5,10,13,14,15,20,21,23,24,25 Potatox Ia 100/111/122 100/100 I/R A2 US-8 1,5,10,13,14,15,20,21,23,24,25

Tomato (New Jersey) Ia 122/122 100/100 R A2 N/D 1,3,5,7,10,13,14,18,20,21,24,25 Potatox Ia 100/111/122 100/100 R A2 US-8 1,5,10,13,14,16,20,21,23,24,25

Tomato (Pennsylvania) Ia 100/122 100/100 S A2 N/D 1,5,13,14,18,20,21,24,25 Potatox Ia 100/111/122 100/100 R A2 US-8 1,5,10,14,16,20,21,23,24,25 zSensitivity to metalaxyl of isolates of P. infestans: R = resistant; S = sensitive; I = intermediate (as defined by Shattock, 1988). yN/A = not applicable, genotype designations not assigned for UK; N/D = not determined, does not conform to any published U.S. genotype. xLate-blighted potato hosts that were located in bordering, neighboring, or close-by plantings.

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apparently no published reports of P. infes- bia, Canada, in 1993 to 1997; several geno- tans naturally infecting S. nigrum in the types were identified, including US-11. There United States, although Peterson (1947) are few reports of infection of S. sarrachoides reported that this species allowed sporula- by P. infestans in the major potato-producing tion when it was artificially inoculated. In- areas of the northeastern United States. deed, some authors have considered Goodwin et al. (1995b) obtained three US-1 S. nigrum as a nonhost of P. infestans (Platt, isolates from hairy nightshade in a field in 1999; Vartanian and Endo, 1985; Vlee- Wisconsin in 1993, whereas Goodwin et al. shouwers et al., 2000). In the present study, (1998) characterized six isolates from New the infected black nightshade plants found in York State collected in 1994, which all Henfaes, North Wales, were low-growing proved to be US-8. Although in the 2004 with large, succulent leaves 4 to 5 cm long survey of potato crops in northeastern Maine, instead of having a more erect habit and particular attention was directed to solana- smaller leaves but were confirmed as ceous weed species, plants of late blight- S. nigrum; their atypical appearance may infected hairy nightshade were collected relate to the known phenotypic plasticity of from only one location and the isolates of this species (Rogers and Ogg, 1981). Al- P. infestans recovered were again typical of though late blight has occurred in potato the genotypes on the surrounding potato plots at Henfaes every year since 1999 and crops. This suggests that there may be the black nightshade weeds have been reg- a host-related factor rather than pathogenic ularly monitored, only in 2004 were a few specialization limiting infection of S. sarra- P. infestans-like lesions again observed; choides in the northeastern United States. Fig. 1. DNA fingerprint patterns (probe RG57) for P. infestans was isolated from one of these. Thus, although solanaceous weeds, in- Phytophthora infestans isolates: standard iso- During a nationwide late blight survey in cluding S. nigrum, S. sarrachoides, and also lates US-1 (lane 1), US-8 (lane 2), US-11 (lane 8), tomato isolates collected in New Jersey The in 1999 and 2000, infection S. dulcamara (Cooke et al., 2002; Flier et al., (lanes 3 and 5) and Pennsylvania (lane 4), of black nightshade by P. infestans was 2003), can act as hosts of P. infestans, they petunia isolate (lane 6), and hairy nightshade observed on several occasions but was are generally only infected when late blight isolate (lane 7). considered to be a relatively rare event and is widespread and by pathogen genotypes not to contribute to the overall disease already occurring in potato crops in the pressure (Flier et al., 2003). The results of same area. The basis of the resistance of the present study would support that view, these species to P. infestans is not well P. infestans from potato in the same area because late blight was only found when the understood, although Flier et al. (2003) sug- (Table 2), which were all of the US-8 geno- disease was already widespread in potato gested that S. nigrum may contain R-genes. It type. The tomato isolates had the allozyme and the P. infestans genotypes isolated were appears unlikely that these solanaceous banding pattern and mating type associated typical of those infecting local potatoes. weeds contribute significant inoculum to late with the US-14 genotype. As with the late Hairy nightshade (S. sarrachoides)is blight epidemics, because the results of this blighted tomatoes in New Jersey, RG57 another common solanaceous weed of potato study found limited diversity, i.e., close analysis (Fig. 1) showed that the tomato crops in the United States, but, as with similarity of the petunia, hairy and black isolates had a unique fingerprint (different S. nigrum, infection by P. infestans appears nightshade isolates with potato isolates. from that found in the New Jersey isolates relatively infrequent, particularly in the east- Nonetheless, even infrequent infection of and not that of US-14), which does not ern United States. Platt (1999) reported spor- solanaceous weeds by P. infestans warrants appear to have been reported previously. ulation on this host when excised leaf and investigation, particularly because some of stem tissue was artificially inoculated with P. these species have been used as sources of Discussion infestans isolates of the US-1 and US-8 supposed nonhost resistance (e.g., Zimnoch- clonal lineages. Similarly, Dandurand et al. Guzowska et al., 2003). Weeds and ornamental plants belonging (2006) also showed infection of detached Petunia (grown to provide flowers and to the family Solanaceae occur very widely. leaves and wounded berries of S. sarra- bedding plants for local markets) and tomato Solanaceous weeds often grow in the close choides inoculated with US-1 and US-8 are two common solanaceous glasshouse vicinity of potato crops, yet reports of natural P. infestans isolates. In the western United crops in the United States, both of which infection of late blight of these plants are States, S. sarrachoides naturally infected by are susceptible to P. infestans and could infrequent. Although P. infestans has been P. infestans was found in cultivated tomato serve as alternate hosts to carry inoculum found to be capable of infecting a wide range fields in southern California in 1981 and 1982 over to subsequent crops, including of solanaceous hosts (Erwin and Ribeiro, (Vartanian and Endo, 1985); all isolates were potatoes. Correspondingly, inoculum from 1996), particularly when artificially inocu- A1 and readily infected tomato. Isolates blighted potato fields can initiate infection lated (Dandurand et al., 2006), natural in- collected from hairy nightshade in Washing- in greenhouse-grown petunias and toma- fection of noncrop Solanaceae has seldom ton State in 1994 (Deahl and Inglis, 1995) toes. Since the first reports of petunia late been observed in the field and integrated were metalaxyl-resistant and A1 mating type. blight in 1856 (Hirst and Moore, 1957), control programs for potato late blight rarely Subsequently, Derie and Inglis (2001) in- occurrence has been sporadic in the United consider a possible role for alternative hosts vestigated virulence of P. infestans isolates States; however, recent disease outbreaks (e.g., Johnson, 2006). However, with the collected from potato, tomato, and other (Becktell et al., 2005a, 2005b; Deahl and introduction of the new P. infestans popula- solanaceous hosts in 1998 and 1999 in the Fravel, 2003) suggest that petunias are very tions in the last 20 to 30 years, the situation same area. Three isolates were obtained from susceptible to current U.S. P. infestans may be changing. S. sarrachoides, two US-8 and one US-11; genotypes and could have an important role Black nightshade (S. nigrum)isacom- virulence testing of one US-8 and one US-11 in the epidemiology of potato and tomato mon annual weed of arable fields in the isolate showed that they were able to over- late blight. During an extensive tomato late and the United States and come eight and six R-genes, respectively, blight outbreak in 1998 (Gavino et al., is resistant to residual commonly which was similar in the number of R-genes 2000), a commercial production greenhouse used on potatoes. Natural infection of black overcome by US-8 from potato and US-11 in California lost both petunias and tomatoes nightshade by P. infestans was observed by from tomato. Punja et al. (1998) obtained to late blight. Subsequently, there were Hirst and Stedman (1960) in but a few (<10) isolates from hairy nightshade in occurrences of late blight on field-grown has not been reported there since. There are their study of P. infestans in British Colum- tomatoes in New York for which late

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blighted petunias were thought to have been from 1996 to 1998. Can. J. Plant Pathol. 22: amplification of DNA from pure cultures or the main source of inoculum (Smart and Fry, 110–116. from host lesions. Appl. Environ. Microbiol. 2001). Most recently, P. infestans has been Dandurand, L.M., G.R. Knudsen, and C.V. Eber- 64:4007–4014. reported to cause leaf blight on petunia in lein. 2006. Susceptibility of five nightshade Hirst, J.M. and W.C. Moore. 1957. Phytophthora the northeastern United States (Deahl and (Solanum) species to Phytophthora infestans. infestans on petunia and datura. Plant Pathol. Amer. J. Potato Res. 83:205–210. 6:76. Fravel, 2003). Day, J.P., R.A.M. Wattier, D.S. Shaw, and R.C. Hirst, J.M. and O.J. Stedman. 1960. The epidemi- In this study, late blight on petunia ini- Shattock. 2004. Phenotypic and genotypic di- ology of Phytophthora infestans II. The source tially occurred as scattered foci but rapidly versity in Phytophthora infestans on potato in of inoculum. Ann. Appl. Biol. 48:489–517. spread through the crop causing significant Great Britain, 1995-98. Plant Pathol. 53:303– Johnson, D.A. 2006. Different Practices Can Help losses. The disease reappeared in the same 315. Manage Late Blight in Potatoes. Spudman. and adjacent greenhouses the next year, pro- Deahl, K.L. and D.R. Fravel. 2003. Occurrence of 5:14–18. ducing severe damage in several crops. In leaf blight on petunia caused by Phytophthora Lebreton, L., J.-M. Lucas, and D. Andrivon. 1999. both years, late blight was observed on potato infestans in Maryland. Plant Dis. 87:1004. Aggressiveness and competitive fitness of Phy- plants in fields in close proximity to the Deahl, K.L. and D.A. Inglis. 1995. Occurrence of tophthora infestans isolates collected from metalaxyl-insensitive Phytophthora infestans potato and tomato in . Phytopathology petunia glasshouses and the pathogen sporu- on Solanum sarrachoides in Northwestern 89:679–686. lated on leaves and stems of the petunia Washington. Plant Dis. 79:540. Lee, T.Y., I. Simko, and W.E. Fry. 2002. Genetic plants. The present study showed the close Deahl, K.L., R. Jones, and L.A. Wanner. 2005. control of aggressiveness in Phytophthora in- similarity of the petunia and potato isolates. Late Blight Caused by Phytophthora infestans festans to tomato. Can. J. Plant Pathol. 24:471– Greenhouse growers who cultivate more than on Solanum sarrachoides in Northeastern 480. one solanaceous species should be aware that Maine. Plant Dis. 89:435. Legard, D.E., T.Y. Lee, and W.E. Fry. 1995. petunia transplants may have incipient Deahl, K.L., D.S. Shaw, and L.R. Cooke. 2004. Pathogenic specialization in Phytophthora in- P. infestans infections, which can serve Natural Occurrence of Phytophthora infestans festans: Aggressiveness on tomato. 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Characterization of Phytophthora 24:188–197. including Legard et al. (1995), Oyarzun infestans populations in western Washington. Platt, H.W. 1999. Response of solanaceous culti- et al. (1998) and Lebreton et al. (1999) and Plant Dis. 83:423–428. vated plants and weed species to inoculation Erwin, D.C. and O.K. Ribeiro. 1996. Phytophthora with A1 or A2 mating strains of Phytophthora investigated by Lee et al. (2002). The diseases worldwide. APS Press, St. Paul, Minn, infestans.Can.J.PlantPathol21:301 P. infestans genotypes isolated from adjacent pp. 346–353. 307. tomato and potato crops were quite distinct Flier, W.G., G.B.M. van den Bosch, and L.J. Punja, Z.K., H. Forster, I. Cunningham, and M.D. and the presence of additional RG57 bands in Turkensteen. 2003. Epidemiological impor- Coffey. 1998. Genotypes of the late the fingerprints of isolates from tomato com- tance of Solanum sisymbriifolium, S. nigrum blight pathogen (Phytophthora infestans)in pared with the US-8 isolates from potato and S. dulcamara as alternative hosts for British Columbia and other regions of Canada suggests that the tomato genotypes were not Phytophthora infestans. Plant Pathol. 52:595– during 1993-1997. Can. J. Plant Pathol. derived from the potato ones. The origins of 603. 20:274–282. these new tomato genotypes are unclear and Gavino, P.D., C.D. Smart, R.W. Sandrock, J.S. Rogers, B.S. and A.G. Ogg Jr. 1981. Biology of Miller, P.B. Hamm, T.Y. Lee, R.M. Davis, and eeeds of the Solanum nigrum Complex (Sola- worthy of further investigation, because they W.E. Fry. 2000. 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