Between 2016 and 2018, a study was conducted to evaluate the prevalence and significance of Potato virus Y (PVY) present in seed potatoes distributed through garden stores in western Washington. Seed potatoes, particularly those exhibiting symptoms of cracking possibly caused by PVY, were purchased and grown-out in greenhouse tests. A relatively high incidence (47% in 2016; 26% in 2017; 29% in 2018) of the collections (15, 31, and 49 seed lots in total, respectively) produced plants that were confirmed by ELISA to be infected with PVY. Seed sold for organic production yielded approximately ten times more PVY infections than seed sold for conventional production. Strain typing revealed infections primarily by PVY O , PVY NTN , and PVY N-Wi . Mixed infections of PVY O/NTN and PVY O/N-Wi were detected a few times and no new or unusual PVY strains were found. These PVY strain types often were recovered from plants originating from cracked seed tubers, and the incidence of PVY N-Wi from cracked tubers increased (22% to 69%) during the study. PVY N-Wi remains an important constituent of the region’s PVY strain composition since its first detection in 2011. Garden enthusiasts and organic potato growers need more information about PVY to help mitigate the risk that PVY infected seed tubers could pose to commercial potato production fields.

Five fresh market potato cultivars commonly grown in western Washington were evaluated against Potato virus Y (PVY) O, NTN, and N-Wi using inoculated PVY-free plantlets in the greenhouse in 2015 and successive generations of seed tubers from foliar inoculated plants in 2016, 2017, and 2018. Mosaic displayed strongly on all cultivar by strain combinations, except All Blue by PVY O . Russet Burbank and Yukon Gold infected with PVY O had highest area under symptom progress curve values for leaf drop across all cultivars and years. Yields mostly were lowest for PVY O . More cracked tubers developed on G3 tubers in the field than greenhouse G2 or G1 plants. Chieftain infected with PVY O and PVY NTN , and Yukon Gold with PVY N-Wi had marked increases in tuber cracking by 2018. Cracking began at early plant growth stages; seed tuberborne rather than current season infections had higher cracking incidence; cracking became prevalent with succeeding seed tuber generations; cracking did not contribute to tuber weight loss during storage; and, PVY and tuber cracking substantially impacted yield.

A 3-year study in western Washington from 2010 to 2012 evaluated five tomato cultivars for tomato disease development and yield in open-ended high-tunnel versus open-field settings. Findings in 2010 revealed that severity of late blight, caused by Phytophthora infestans (US-11), was significantly (P = 0.002) lower in high-tunnel compared with open-field experimental plots based on area under disease progress curve (AUDPC) values of 0.02 versus 321, respectively. In spite of rescue foliar fungicide applications to open-field plots in 2011 and 2012, the mean number of late blight infections across cultivars was 1.8 to 30.8 compared with only 0 to 6.5 in high tunnels for these years. Furthermore, accumulated hours of leaf wetness were fewer in high tunnels than the open field each year (857 versus 1,060 in 2010, 598 versus 998 in 2011, and 885 versus 923 in 2012). Cultivar susceptibility to late blight could not be differentiated in high tunnels due to low disease pressure. However, all five cultivars proved susceptible in the open field, with 'Oregon Spring' consistently having the most lesions. In contrast, high-tunnel production contributed to an increased severity of physiological leaf roll compared with open-field production each year, and these values differed significantly (P = 0.0335 and 0.0252) in 2011 and 2012, respectively. AUDPC values for physiological leaf roll showed that Oregon Spring was significantly (P = <0.0001) less susceptible than other cultivars each year. Physiological leaf roll correlated positively (r values of 0.758 to 0. 960) and significantly (P < 0.05) with leaf wetness and air temperature in all years in both high-tunnel and open-field settings but the same was not true for relative humidity. Even with severe physiological leaf roll, high-tunnel production in 2010 resulted in significantly (P < 0.0001) greater total tomato yield than open-field production (35.0 versus 10.6 t ha-1). Although a significant interaction between production system and cultivar occurred in 2011 and 2012, tomato yield always was greater in high-tunnel than open-field plots. Open-ended high tunnels offer tomato growers a potential tool for managing late blight in western Washington while also increasing yield, and could be especially useful in organic production.

Results of studies on survival of sclerotia of and microsclerotia of , demonstrated that soil flooding in western Washington is a possible alternative field rotation practice for (white mold), but not for (Verticillium wilt). Cone-tainer experiments in the greenhouse showed that flooding at 16.5 °C caused sclerotia to lose viability between 12 and 24 weeks while a growth chamber experiment revealed that flooding for 18 weeks at 11 °C or 20 °C was sufficient. microsclerotia appeared resistant to flooding under greenhouse and field settings; recovery ranged within 5 to 10 % of the initial soil population after 6 and 12 months. Potatoes planted into field microplots either flooded or fallowed the previous summer had similar Verticillium wilt ratings and potato yield. Lack of control of by flooding may be due partly to relatively low soil temperatures in a cool, marine climate.Los resultados de estudios de la sobrevivencia de esclerocios de y de microesclerocios de demostraron que la inundación en el oeste de Washington es una alternativa posible como práctica de rotación para (moho blanco), pero no para (marchites por . Experimentos en recipientes en el invernadero mostraron que la inundación a 16.5 °C causaron que se perdiera la viabilidad de los esclerocios de entre 12 y 24 semanas, mientras que un experimento en cámara de siembra reveló que la inundación por 18 semanas a 11 °C o 20 °C era suficiente. Los microesclerocios de resultaron resistentes a la inundación al establecerlos en invernadero y el campo; la recuperación fluctuó dentro del 5 al 10 % de la población inicial del suelo después de 6 y 12 meses. Papas sembradas en microparcelas de campo ya fueran inundadas o sin cultivar el verano anterior, tuvieron calificaciones similares de marchitamiento por y de rendimiento de papa. La falta de control de por inundación pudiera deberse parcialmente a las temperaturas relativamente bajas del suelo en un clima frío marítimo.

Tomato pith necrosis was observed on 2.7% of tomatoes grown in rows covered with black polyethylene, various biodegradable plastics, and an experimental spunbond poly(lactic) acid agricultural mulch in high tunnel and open field experimental plots, in western Washington in 2011. Symptoms developed on 3-month-old plants and progressed acropetally until night temperatures dropped to 10°C. Affected plants had chlorotic leaves, produced adventitious roots, and pith tissue was brown and either corrugated or rotted. Similar symptoms were observed again in 2012 on 2.0% of plants, but only in experimental plots with black polyethylene mulch. Diseased stem tissue was homogenized with a mortar and pestle in sterile water and the extract was streaked onto King's medium B (KMB) agar. Colonies were white and smooth initially, and after 5 days had an irregular surface and margin and produced a tan diffuse pigment. One isolate, Pc.Sl.2011, was gram-negative, grew at 37°C on nutrient broth yeast (NBY) agar, did not fluoresce on KMB (3), and was arginine dihydrolase positive. A partial 16S fragment, 1,387 bp, was obtained via PCR with universal 27f and 1492f primers. The resulting sequence exhibited 99% identity to Pseudomonas corrugata Roberts & Scarlett, and has been assigned GenBank Accession KC812729. Pathogenicity of Pc.Sl.2011 was tested in two greenhouse trials with five replications of one tomato plant per treatment. Seeds of 'Celebrity' were surface sterilized by soaking in 70% EtOH for 30 s and then 10% NaOCl for 30 s, then rinsed with sterile water and sown into 14 cm diameter pots filled with non-sterile Sunshine Mix #1 (SunGro Horticulture Distribution Inc., Bellevue, WA). Seedlings were inoculated at the four leaf stage using 5 ml NBY broth cultures of Pc.Sl.2011 grown at 28°C for 12 h with agitation. A sterile needle was used to inject 10 μl of either sterile water or a bacterial suspension of 1.0 × 1010 CFU/ml into the axil of the second true leaf. Inoculum concentration was confirmed by NBY dilution plate counts. The plants were incubated in clear polyethylene bags for 4 days and placed in a greenhouse at 21.1 ± 1.2°C with a 14-h photoperiod. The first and second trials were sampled at 8 and 9 weeks after inoculation, respectively. Plants inoculated with sterile water had green pith tissue. However, 60 and 40% of inoculated plants had brown pith tissue around the inoculation site in the first and second trial, respectively, but wilting and adventitious roots were not observed. Stem tissue from the inoculation site of symptomatic plants was homogenized as above, and the extract streaked onto NBY agar plates. Three isolates recovered from inoculated plants from both trials had the same characteristics as the original isolate, including similar colony morphology, ability to grow on NBY at 37°C, and lack of fluorescence on KMB. To our knowledge, this is the first documented report of tomato pith necrosis in Washington. Pith necrosis has been reported previously in high tunnel tomato production (4), where excess nitrogen fertilization occurs with cool evening temperatures (3), and when plastic mulch is utilized (2). In the cool climate of western Washington, successful tomato production requires the use of agricultural mulches and covers that trap heat. Since P. corrugata has been isolated from soil and the tomato seeds of inoculated plants (1), local growers attempting to manage pith necrosis need to select tomato seed lots carefully and avoid applying excess nitrogen, especially when using plastic mulch. References: (1) V. Catara. Mol. Plant Pathol. 8:233, 2007. (2) E. J. Sikora and W. S. Gazaway. Online. ACES.edu ANR-0797, 2009. (3) C. M. Scarlett and J. T. Fletcher. Ann. Appl. Biol. 88:105, 1978. (4) X. Xu et al. Plant Dis. 97:988, 2013.

Symptoms of Verticillium wilt were observed on lettuce (Lactuca sativa L.) harvested from high tunnel and open field experimental plots in annual, consecutive spring plantings in western Washington from 2010 to 2012. Leaves had v-shaped, chlorotic lesions, and yellow or brown vascular tissue was noted in the crowns. Total disease incidence increased from 0.2% in 2010 to 1.9% in 2011 and to 14.4% in 2012. Verticillium spp. obtained from infected crown tissues and cultured on half-strength potato dextrose agar medium produced yellow pigment, black microsclerotia, white mycelia, tan chlamydospores, and uniseptate conidia averaging 10.6 × 3.7 μm. Isolates were identified tentatively as Verticillium tricorpus I. (3). Three isolates, Vt.Ls.2010, Vt.Ls.2011-1, and Vt.Ls.2011-2, were evaluated for pathogenicity on 4-week-old ‘Coastal Star’ seedlings in two greenhouse trials. In Trial I, four replicates of two duplicate plants per each isolate, and in Trial II, five replicates of one plant per each isolate were inoculated with conidial suspensions adjusted to 2.0 × 10 6 and 5.0 × 10 6 conidia/ml, respectively. Additionally, in each trial, two sets of control treatments of five plants each were inoculated with either an isolate of V. dahliae at the same conidial concentration or with sterile water. Root tips were cut and exposed to the suspensions for 5 s, then seedlings were transplanted into Sunshine Mix #1 (SunGro Horticulture Distribution Inc., Bellevue, WA), and kept in a greenhouse at 17.7 ± 3.4°C. Plants were harvested 8 to 9 weeks post-inoculation, and symptoms were rated visually. Vt.Ls.2010, Vt.Ls.2011-1, and Vt.Ls.2011-2 caused chlorosis and vascular discoloration on 25, 13, and 13% of the plants in Trial I; and 40, 60, and 20% of plants in Trial II, respectively. V. dahliae caused similar symptoms on 25 and 40% of the plants in the two trials, respectively, but these plants had greater intensity and length of vascular discoloration compared with the three test isolates. None of the water control plants were symptomatic. All V. tricorpus isolates were recovered from inoculated plants, and colony morphologies were similar to the original isolates. The internal transcribed spacer (ITS) rDNA of isolate Vt.Ls.2010 was amplified with ITS4 and ITS6 primer sets. ITS rDNA sequences between Vt.Ls.2010 and two isolates of V. tricorpus in GenBank (Accession Nos. FJ900211 and AB353343) were 100% identical. V. tricorpus is considered a weak pathogen of lettuce crops in California (2), but authors in Japan recently reported pathogenic isolates of V. tricorpus on lettuce (4). To our knowledge, this is the first report of Verticillium wilt caused by V. tricorpus in Washington. Lettuce is the number two crop grown in high tunnels in the United States (1), and cropping lettuce continuously in them can increase the risk of this and other soilborne pathogens. References: (1) E. E. Carey et al. HortTechnology 19:37, 2009. (2) Q.-M. Qin et al. Plant Dis. 92:69, 2008. (3) H. C. Smith. N. Z. J. Agric. Res. 8:450, 1965. (4) T. Usami et al. J. Gen. Plant Pathol. 77:17, 2010.

Results of studies on survival of sclerotia of Sclerotinia and microsclerotia of Verticillium, demonstrated that soil flooding in western Washington is a possible alternative field rotation practice for S. sclerotiorum (white mold), but not for V. dahliae (Verticillium wilt). Cone-tainer experiments in the greenhouse showed that flooding at 16.5 °C caused S. sclerotiorium sclerotia to lose viability between 12 and 24 weeks while a growth chamber experiment revealed that flooding for 18 weeks at 11 °C or 20 °C was sufficient. V. dahliae microsclerotia appeared resistant to flooding under greenhouse and field settings; recovery ranged within 5 to 10 % of the initial soil population after 6 and 12 months. Potatoes planted into field microplots either flooded or fallowed the previous summer had similar Verticillium wilt ratings and potato yield. Lack of control of V. dahliae by flooding may be due partly to relatively low soil temperatures in a cool, marine climate.

Growth, sporulation, and survival of Phytophthora infestans on volunteer potato tubers, was investigated under temperatures representative of winter (4°C, 7°C, and 10°C) and spring (13°C, 16°C, 19°C) soil conditions in western Washington. Inoculated tubers stored at 10°C for 8 days had a significantly (P < 0.05) higher percentage of disease symptoms on tuber surfaces and a higher number of lenticels and eyes with P. infestans sporulation compared to those stored at 4°C or 7°C. Sporulation of P. infestans on cut tuber surfaces was observed following 3-week storage at the three winter soil temperatures. After 12-week storage, tubers inoculated with a US-8 isolate had a significantly higher percentage of late blight on cut surfaces than those inoculated with a US-11 isolate (70% versus 50%, respectively). For spring soil temperature studies, tubers inoculated with the US-8 isolate and held at 19°C had a significantly higher number of lenticels per tuber with P. infestans sporulation than tubers held at 13°C or 16°C. Sporulation of P. infestans on tuber surfaces was detected on infected tubers buried 5-cm deep in potting medium at all tested winter and spring temperatures for 3- or 6-day periods, respectively. The site or depth of tuber inoculation with P. infestans did not influence tuber-to-sprout infection events and whether apical end or stem sprouts become infected. Tubers with late blight that survive the winter in western Washington and support sporulation of P. infestans via lenticels and eyes may enable the transmission of P. infestans from infected tissues to sprouts of volunteer plants. However, the impact of these events on primary inoculum production by P. infestans in the region is probably limited by the mild, winter conditions favoring tuber break-down in soil.

A segregating introgression population, established by crossing an accession of Solanum hougasii from the central highlands of Mexico with two successive recurrent corky ringspot resistant parental lines, was screened against Phytophthora infestans . Foliage and tuber reactions were compared under natural epidemics of US-8 and US-11 P. infestans at Mount Vernon, Washington, natural epidemics at Toluca, Mexico, and by laboratory assays with US-8 and US-11 P. infestans at Pullman, Washington. Relative area under disease progress curve (RAUDPC) values in the field ranged from 3 to 80 and 2 to 42 for the 1998 and 1999 populations at Mount Vernon, respectively, and from 5 to 63, 2 to 79, and 4 to 76 in 1998, 1999 and 2000 for the populations at Toluca, respectively. Of the progeny lines tested during 1998 and 1999 at Mount Vernon, 7% were resistant, 60% were intermediate, and 33% were susceptible. Of those tested during 1998, 1999, and 2000 at Toluca, 33, 31, and 36% were resistant, intermediate and susceptible, respectively. RAUDPC values in the field at Mount Vernon and Toluca were significantly ( P = 0.0001) correlated. Late blight severity on detached leaflets inoculated with US-8 and US-11 P. infestans in the laboratory ranged from 0 to 64% or 65%, respectively. Severity of infection on inoculated tubers ranged from 0 to 68% for US-8 and 0 to 80% for US-11. Disease severity on leaflets in laboratory tests was significantly correlated with field RAUDPC values, but tuber severity in laboratory tests was not, although some lines exhibited resistance in both the foliage and tubers. Foliar resistance in the field was characterized by leaf chlorosis, as well as limited lesion expansion and sporulation. Estimate of broad-sense heritability was relatively high. Only 15% of this BC 1 population showed significant instability, giving little indication of specific interactions between genotypes and populations of the P. infestans pathogen that would be indicative of R-gene interactions. The introgression population (BC 1 ) appears to be expressing highly heritable durable resistance. The high heritability estimate suggests that utilization of highly resistant and stable BC 1 genotypes, such as 53.78, as a parental source of foliar late blight resistance will transmit substantial nonrace specific genetic resistance to future progenies, and that ultimately, after several cycles of backcrossing and selection, this genetic potential could be deployed in new potato cultivars.