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Onion bacterial diseases pose a serious economic risk to growers as they can lead to catastrophic crop losses. Moisture in the plant canopy plays a critical role in onion bacterial disease development by dispersing the pathogens onto plants (e.g., in splashing water) and keeping conditions conducive for bacterial growth. For this reason, irrigation management can be a promising avenue for controlling bacterial diseases in regions where irrigation is used widely, particularly overhead irrigation. This study investigated the effects of in-season irrigation frequency and timing of the final irrigation on onion bacterial bulb rot and marketable bulb yield over the 2020 and 2021 growing seasons in Washington State's semi-arid Columbia Basin, using overhead irrigation. Irrigating twice as often but for shorter periods did not increase the incidence of onion bacterial diseases or affect yield compared to longer, less frequent irrigation events under the conditions of this experiment. However, timing of the final irrigation did influence bacterial disease incidence. Irrigating until the tops (leaves and necks) had fallen over on 90% of onion plants led to culling of more bulbs at harvest as a result of bacterial rot and more bulbs with bacterial rot after five months in storage compared to plots in which irrigation was stopped two weeks earlier, at 5–10% tops down. Ending irrigation at 5–10% tops down did not reduce marketable yield or affect bulb size distribution. In semi-arid regions, careful irrigation management can be an important part of managing onion bacterial diseases, particularly at the end of the season when onion plants are most vulnerable to neck and bulb infection. • In a semi-arid region, ending irrigation one week early did not affect onion yield. • Ending sprinkler irrigation early limited losses to onion rot by 45–81%. • The frequency of irrigations did not affect onion bacterial bulb rot or bulb yield. [ABSTRACT FROM AUTHOR]

Stemphylium leaf spot, caused by Stemphylium vesicarium, and white rust, caused by Albugo occidentalis, can cause significant losses in spinach production. Management of these foliar diseases of spinach has become increasingly challenging with the development of fungicide resistance in some pathogen populations, high planting density and overhead irrigation used for baby leaf spinach production, and the fact that >60% of fresh market spinach production in the United States is certified organic. To identify spinach cultivars with resistance to Stemphylium leaf spot and white rust, a field trial was performed near Crystal City, TX, USA, in 2021 (79 cultivars), 2022 (87 cultivars), and 2023 (63 cultivars). Each year, the plants were inoculated with S. vesicarium and rated for disease severity. Plants were also rated for white rust severity that resulted from natural infection during the 2021 and the 2022 trials. During each trial, 11% to 27% of the cultivars were identified as resistant to Stemphylium leaf spot, and another 29% to 48% had moderately resistant reactions. In contrast, only 5 of 79 cultivars (6%) in the 2021 trial did not develop symptoms of white rust, and all 87 cultivars evaluated in the 2022 trial had symptoms of white rust. Although there was no significant correlation between mean Stemphylium leaf spot ratings and mean white rust ratings during these trials, the cultivars Colusa, Kodiak, PV-1569, and PV-1664 displayed resistant or moderately resistant responses to both diseases in at least two trials. Therefore, processing and fresh market spinach growers have resistant cultivars from which to select to reduce the economic impacts of Stemphylium leaf spot and white rust. [ABSTRACT FROM AUTHOR]

Fusarium wilt of spinach is caused by the soilborne fungus Fusarium oxysporum f. sp. spinaciae and occurs in most regions of spinach production. The disease is favored by acid soils and warm temperatures, and the fungus can survive extended periods as chlamydospores or by asymptomatic colonization of the roots of nonhost plant species. The 10- to 15-year rotation required to minimize losses to Fusarium wilt is the primary constraint on spinach seed production in the maritime Pacific Northwest, the only region of the United States suitable for this cool-season, daylength-sensitive crop. Raising soil pH with agricultural limestone (97% CaCO3) results in a transitory, partially suppressive effect on spinach Fusarium wilt. A field trial was completed from 2009 to 2012 to assess the potential for annual applications of agricultural limestone at 0, 2.24, and 4.48 tons/ha for 3 years prior to a spinach seed crop to improve Fusarium wilt suppression compared with the level of suppression attained from a single limestone amendment at 4.48 tons/ha. Three proprietary female spinach lines were planted that ranged from highly susceptible to partially resistant to Fusarium wilt. Three successive annual applications of limestone at 4.48 tons/ha reduced midseason wilt incidence by an average of 20%, increased spinach biomass by 33%, and increased marketable spinach seed yield by 45% compared with plots amended once with the same rate of limestone in the spring of planting. The suppressive effect increased with increasing rate of limestone amendment, with the greatest difference observed when limestone was applied at between 0 and 2.24 tons/ha annually for 3 years. The effects on seed yield were greatest for the partially resistant female line, followed by the moderately susceptible and highly susceptible female lines. Overall, the results demonstrate that annual applications of agricultural limestone on acid soils of the maritime Pacific Northwest of the United States can enhance suppression of spinach Fusarium wilt, potentially reducing the required rotation interval by as much as 50%, thereby doubling the capacity for spinach seed production in the United States. [ABSTRACT FROM AUTHOR]

The maritime Pacific Northwest is the only region of the United States suitable for production of spinach seed, a cool-season, daylengthsensitive crop. However, the acidic soils of this region are highly conducive to spinach Fusarium wilt, caused by Fusarium oxysporum f. sp. spinaciae. Rotations of at least 10 to 15 years between spinach seed crops are necessary to reduce the high risk of losses to this disease. The objectives of this study were to develop a greenhouse soil bioassay to assess the relative risk of Fusarium wilt in fields intended for spinach seed production, and to identify soil chemical and physical properties associated with conduciveness to this disease. Preliminary bioassays established a protocol for growing spinach plants in a greenhouse environment and inducing Fusarium wilt symptoms so that the bioassay can be completed in <2 months. Test soils with a range of Fusarium wilt inoculum potentials, and three spinach inbred parent lines (highly susceptible, moderately susceptible, and moderately resistant to Fusarium wilt) were used to evaluate sensitivity of the bioassay to different levels of risk of Fusarium wilt. Then, from 2010 to 2013, spinach seed growers and stakeholders submitted soil samples from 147 fields for evaluation with the bioassay. The fields were each under consideration for planting a spinach seed crop, yet the bioassay revealed a wide range in Fusarium wilt inoculum potential among soil samples. Differences in susceptibility to Fusarium wilt of the three inbred lines were key to detecting differences in wilt risk among soils. Visits to spinach seed crops planted in fields evaluated in the bioassay, as well as test plots of the three inbred lines planted in growers' seed crops, confirmed the predictive value of the bioassay for Fusarium wilt risk. Correlation analyses for 23 soil properties revealed significant relationships of 15 soil properties with the Fusarium wilt potential of a soil, but the correlations were influenced significantly by susceptibility of the inbred line to Fusarium wilt (13, 10, and 8 soil properties correlated significantly with Fusarium wilt risk for the susceptible, moderate, and partially resistant inbreds, respectively). Multiple regression analyses identified different statistical models for prediction of Fusarium wilt risk depending on the spinach inbred line, but the best fitting model explained <34% of the variability in Fusarium wilt risk among 121 fields evaluated in the soil bioassay. Thus, no model was robust enough to replace the bioassay for the purpose of predicting Fusarium wilt risk. [ABSTRACT FROM AUTHOR]

Molecular assays to detect and quantify DNA from viable cells of the seedborne pathogen Xanthomonas hortorum pv. carotae in carrot seed were developed and evaluated for use on nontreated and hot-water-treated seed lots. Both a TaqMan real-time polymerase chain reaction (PCR) assay and a loop-mediated isothermal amplification (LAMP) dilution endpoint assay detected and quantified DNA from viable pathogen cells after treatment of carrot seed washes with the livedead discriminating dye propidium monoazide (PMA). The detection limits of the assays were approximately 10¹ CFU for pure cultures of X. hor-torum pv. carotae, and 10² to 10³ CFU/g seed from naturally infested carrot seed lots. X. hortorum pv. carotae in and on carrot seed was killed by soaking the seed in hot water (52°C for 25 min), and a subsequent PMA treatment of these hot-water-treated seed washes suppressed detection of the pathogen with both the real-time PCR and LAMP assays. For 36 commercial seed lots treated with PMA but not hot water, regression of colony counts of X. hortorum pv. carotae measured by dilution plating on a semiselective agar medium versus estimates of pathogen CFU determined by the molecular assays resulted in significant (P < 0.05) linear relationships (R2 = 0.68 for the real-time PCR assay and 0.79 for the LAMP assay). The molecular assays provided quantitative estimates of X. hortorum pv. carotae infestations in carrot seed lots in <24 h, which is a significant improvement over the 7 to 14 days required to obtain results from the traditional dilutionplating assay. [ABSTRACT FROM AUTHOR]

A commercial skullcap (Scutellaria lateriflora, family Lamiaceae) crop with wilted and necrotic plants was examined in Washington State in 2008. Three fungal isolates were obtained and identified as Verticillium dahliae based on morphology and sequencing of the internal transcribed spacer DNA region. All three skullcap isolates caused typical Verticillium wilt symptoms on skullcap and two peppermint cultivars. Inoculations of skullcap with the V dahliae isolates from skullcap and an isolate from peppermint resulted in severe symptoms and a 21 to 78% reduction in aboveground biomass. Isolates from skullcap caused severe symptoms on the susceptible peppermint 'Black Mitcham' and reduced yield by up to 82%. One skullcap isolate caused severe symptoms on the moderately resistant 'Redefined Murray' in three of four trials and reduced biomass up to 71% compared with noninoculated control plants. The pathogen was recovered from 43 to 69% of skullcap stems from plants inoculated with skullcap or peppermint isolates, and was isolated from 2.5% of seed harvested from skullcap plants inoculated with the peppermint isolate of V dahliae. This is the first report of V. dahliae infecting skullcap, and the first demonstration of V. dahliae isolates from skullcap and peppermint causing symptoms on both hosts, as well as the seedborne nature of V. dahliae in skullcap. [ABSTRACT FROM AUTHOR]

A real-time fluorescent polymerase chain reaction (PCR) assay was developed using SYBR Green chemistry to quantify the Botrytis spp. associated with onion (Allium cepa) seed that are also able to induce neck rot of onion bulbs, i.e., B. aclada, B. allii, and B. byssoidea. The nuclear ribosomal intergenic spacer (IGS) regions of target and nontarget Botrytis spp. were sequenced, aligned, and used to design a primer pair specific to B. aclada, B. allii, and B. byssoidea. Primers and amplification parameters were optimized to avoid amplifying the related species B. cinerea, B. porn, and B. squamosa, as well as Sclerotinia sclerotiorum and isolates of 15 other fungal species commonly found associated with onion seed. The primers reliably detected 10 fg of genomic DNA per PCR reaction extracted from pure cultures of B. aclada and B. allii. Conventional assays of surface-disinfested and nondisinfested seed on an agar medium were used to determine the incidence of neck rot Botrytis spp. associated with each of 23 commercial onion seed lots, and the real-time PCR assay was used to determine the quantity of DNA of neck rot Botrytis spp. in each seed lot. A linear relationship could not be found between the incidence of seed infected with the neck rot Botrytis spp. using the conventional agar seed assays and the quantity of DNA of the neck rot Botrytis spp. detected by the real-time PCR assay. However, the real-time PCR assay appeared to be more sensitive than the conventional agar assay, allowing detection of neck rot Botrytis spp. in S of the 23 seed lots that tested negative using the conventional agar seed assay. [ABSTRACT FROM AUTHOR]

The article highlights the Iris yellow spot virus as a threat to onion bulb and seed production. It discusses the history of Iris yellow spot virus in the United States and around the world, the epidemiology of the virus and the onion thrips vector, management of Iris yellow spot and thrips, and outlook and future directions of the virus.

Assays of 77 spinach (Spinacia oleracea) seed lots produced in the United States, Denmark, the Netherlands, or New Zealand in 2000 to 2003 showed that Stemphylium botryosum, causal agent of Stemphylium leaf spot, was present in every lot, at a mean incidence of 29.1% per lot. Either Cladosporium variabile, causal agent of Cladosporium leaf spot, or the morphologically similar species C. macrocarpum, was present in 37 of the 77 lots, at a mean incidence of 1.8% per lot. Some seed isolates of S. botryosum and C. variabile proved pathogenic on spinach. Nonpathogenic isolates resembling C. variabile were identified as C. macrocarpum by the absence of torulose aerial hyphae. Pathogenic isolates of S. botryosum were also detected in each of 12 seed lots stored for up to 11 years at 4.4°C and 60% relative humidity. C. variabile or C. macrocarpum was detected in only 2 of the 11 lots, which had been stored for 3 and 8 years. Component seed assays demonstrated that S. botryosum and C. variabile (or C. macrocarpum) were internal and external in spinach seed. S. botryosum was detected in 5 to 76% of the embryos of five seed lots, but the two Cladosporium species were detected in only 0 to 1% of the embryos of these lots. This suggests greater potential difficulty at eradicating S. botryosum than C. variabile from infected spinach seed using seed treatments. [ABSTRACT FROM AUTHOR]

Cladosporium variabile, Stemphylium botryosum, and Verticillium dahliae are seedborne and seed-transmitted pathogens of spinach. Spinach seed treatments in 1.2% NaOCl for 10 to 60 rain, or hot water (40, 45, 50, 55, and 60°C) for 10 to 40 min, were evaluated for eradication of these fungi from seed. C. variabile and V. dahliae were largely eradicated by chlorine treatment for ≥10 min. Although chlorine treatment reduced the incidence of S. botryosum, this fungus was not eradicated after 60 min in chlorine. Seed germination was not affected adversely by chlorine treatment, even after 60 min. In contrast, germination was reduced significantly by hot water treatment at 50°12 for ≥30 min or 55 or 60°C for ≥10 min. C. variabile was eradicated from seed treated in 40°C water for 10 min. V dahliae was eradicated from seed treated at 55°C for ≥30 rain or 60°C for ≥10 min. S. botryosum was eradicated from a lightly infected seed lot (5% incidence) by hot water treatment at 55 or 60% for ≥10 min, but could not be eradicated from two heavily infected lots (»65% incidence), even at 60°C for 40 min. Using precisely controlled parameters, chlorine or hot water seed treatments can be used to eradicate C. variabile and reduce the incidence of S. botryosum and V. dahliae in spinach seed without damaging germination. [ABSTRACT FROM AUTHOR]

Carrot (Daucus carota subsp, sativus) seed crops in Oregon and Washington were surveyed in 2001–02 and 2002–03 for development of Xanthomonas campestris pv. carotae, causal agent of bacterial blight. For each state and season, 20 plants were sampled from each of 7 to 12 direct-seeded crops twice in the fall or winter and three times from spring to summer; and from each of 2 m 4 steckling (root-to-seed) crops three times from spring to summer. X. campestris pv. carotae was detected in 1 of 15 and 6 of 32 stock seed lots planted in the fall in Oregon and Washington, respectively, and in 2 of 6 steckling shipments planted in each state in the spring. The pathogen was detected at 10² to 108 CFU/g foliage in 1 of 22 and 10 of 14 direct-seeded crops within 2 months of planting in 2001 and 2002, respectively. The prevalence of X. campestris pv. carotae then increased through the season in most seed crops, although bacterial blight symptoms were not observed until April in Oregon and July in Washington in both seasons. In August 2002 and 2003, X. campestris pv. carotae was detected in all 10 and 13 crops surveyed in Oregon, respectively; and in 11 of 12 and 7 of 10 crops in Washington, respectively. The pathogen was typically less prevalent in steckling versus direct-seeded crops. X. campestris pv. carotae was detected in 20 of 22 and 19 of 23 harvested seed lots in Oregon and Washington, respectively, at populations ranging from 1.3 ⊗ 10¹ to 1.4 ⊗ 108 CFU/g seed. Airborne X. campestris pv. carotae, detected ≥1,600 m downwind of crops being threshed in Oregon in September of 2003 and 2004, may provide a source of inoculum for newly planted seed crops between overlapping biennial seasons for carrot seed production. Despite the prevalence of this pathogen in the Pacific Northwest, carrot seed lots free of X. campestris pv. carotae were detected, demonstrating the ability to produce clean seed in this region by adhering to recommended practices for management of bacterial blight. [ABSTRACT FROM AUTHOR]

There are no previous reports of Verticillium wilt in fresh and processing spinach (Spinacia oleracea) crops in the United States. In 2002, a hybrid spinach seed crop in the Pacific Northwest developed late-season wilt symptoms. Assays of the harvested seed and stock seed of the male and female parents revealed 59.5, 44.0, and 1.5%, respectively, were infected with Verticillum dahliae. Assays of 13 stock or commercial seed lots grown in 2002 and 62 commercial lots harvested in 2003 in Denmark, Holland, New Zealand, and the United States revealed the prevalence of Verticillium spp. in commercial spinach seed. Sixty-eight lots (89%) were infected with Verticillium spp. at incidences ranging from 0.3 to 84.8%. Five spinach seed isolates of V dahliae were pathogenic on each of three spinach cultivars by root-dip inoculation. V dahliae was detected on 26.4% of the seed from 7 of 11 inoculated plants but on none of the seed from 6 control plants, demonstrating systemic movement of V dahliae. Seed-to-seed transmission was also demonstrated by planting naturally infected seed lots. This is the first report of Verticillium wilt of spinach in the primary region of spinach seed production in the United States. [ABSTRACT FROM AUTHOR]

Of 12 onion seed lots harvested in the semi-arid Columbia Basin of Washington in 1999 or 2000, 8 were infected and 10 infested with Botrytis aclada at incidences of 1 to 10% and 2 to 26%, respectively. Twenty to forty plants were sampled from each of nine direct-seeded, biennial seed crops in April, June, and July 2001 and assayed for Botrytis spp. Six direct-seeded crops were sampled in October and November 2001 and April, June, and July 2002. One bulb-to-seed crop was sampled in April, June, and July 2002. The incidence of B. aclada increased through each season, reaching 100% in most fields by July. Infections were primarily asymptomatic, with no apparent relationship between plant infection and infection of harvested seed. B, cinerea, B. squamosa, and B. porri were detected in 16, 4, and 4% of the fields, respectively, at lower incidences than B. aclada. Harvested seed from 15 of the fields were infected with B. aclada at <1 to 28%. B, cinerea, B. porri, and B. squamosa were detected in three, three, and none of the harvested lots, respectively. [ABSTRACT FROM AUTHOR]

Two pathogenicity groups of Fusarium oxysporum f. sp. spinaciae, the causal agent of Fusarium wilt of spinach (Spinacia oleracea), were described recently based on virulence of isolates on proprietary spinach inbreds. In this study, a wide range in severity of wilt was observed for 68 spinach cultivars inoculated with an isolate of each pathogenicity group, with 22 (32.4%) cultivars displaying differential responses to the isolates. In a second set of trials, seven spinach cultivars were inoculated with five isolates of each pathogenicity group. The cultivars had similar wilt responses to isolates within each group. In both sets of trials, the most severe wilt developed on cultivars inoculated with pathogenicity group 2 isolates when daylength was shorter and light intensity lower. To test whether light intensity exacerbates severity of Fusarium wilt, three spinach cultivars were inoculated with two isolates of each pathogenicity group and grown with or without shading. Shaded plants developed more severe wilt than nonshaded plants. This difference in wilt severity was greatest for plants inoculated with pathogenicity group 2 isolates. We propose naming isolates of pathogenicity groups 1 and 2 as races 1 and 2 of F. oxysporum f. sp. spinaciae, respectively, and recommend the cultivars Kiowa (susceptible to both races) and Magnetic (susceptible to race 2 and highly resistant to race 1) as differentials. Results of this study should help breeders screen spinach germplasm for resistance to both races of F. oxysporum f. sp. spinaciae. [ABSTRACT FROM AUTHOR]

Although irrigation scheduling has been studied for diverse vegetable crops, much less attention has been given to irrigation scheduling for the seed crops on which these production systems rely. In spinach, for which irrigation scheduling needs are likely to vary greatly between seed and leaf production, this leaves seed producers without adequate resources to make irrigation scheduling decisions. Our research sought to fill this gap by evaluating two alternative irrigation scheduling strategies (a publicly available decision-support tool and soil moisture sensors) and four soil moisture thresholds for irrigation for their impacts on vegetative growth, marketable seed yield, seed quality, and the severity of Stemphylium leaf spot (caused by Stemphylium vesicarium and Stemphylium beticola), a common foliar disease of spinach, under sprinkler irrigation. We found that in all 3 years of the study, earlier and more frequent irrigation increased vegetative growth. However, marketable seed yield only increased relative to the control treatment based on farmers' standard irrigation practices in 1 of the 3 years--a year with an abnormally late planting date. This indicates that vegetative growth is more responsive than seed yield to earlier and more frequent irrigation, and that increases in vegetative growth do not translate directly to increased marketable seed yield. Contrary to the expected increase in Stemphylium leaf spot severity with increasing irrigation, the severity decreased in both years it was measured, likely as a result of the small stature of the spinach seed parent lines used in our study and opportunistic pathogenicity on moisture-stressed plants. These results provide a useful foundation from which spinach seed producers can make irrigation management decisions for their crops that underpin a valuable global industry. [ABSTRACT FROM AUTHOR]

Stunting caused by Rhizoctonia spp. is economically important in irrigated onion bulb crops in the semiarid Columbia Basin of Oregon and Washington, where cereal winter cover crops commonly are planted the previous fall to prevent wind erosion of soil. The cover crop is killed with herbicide application just before or shortly after onion seeding, so that the dead rows of cereal plants provide a physical barrier tall enough to protect onion seedlings against wind and sand blasting but not tall enough to shade onion seedlings. However, the cover crop also serves as a green bridge for Rhizoctonia spp. on cereal roots to colonize the onion roots, potentially resulting in severe stunting of onion seedlings. To determine the effect of timing of application of the herbicide glyphosate to reduce this green bridge effect and, subsequently, onion stunting, three herbicide application intervals preceding onion planting were evaluated in a grower's onion field in each of 2012 and 2014 in the Columbia Basin. The wheat cover crop was killed with a glyphosate application 27, 17, and 3 days before onion seeding in 2012 and 19, 10, and 3 days before seeding in 2014. As the interval between herbicide application and onion planting increased from 3 days to 19 and 27 days, the number of patches of stunted onion plants decreased by >55%, total area of stunted patches decreased by 54 to 63%, and patch severity index decreased by 59 to 65%. Similarly, the Rhizoctonia solani AG 8 DNA concentration in soil sampled from the dead cover crop rows declined as the interval between glyphosate application and onion seeding increased in the 2012 trial but not in the 2014 trial. R. solani AG 3 and AG 8 DNA concentrations in soil sampled from the cover crop rows were significantly positively correlated with the number of patches of stunted onion plants (r = 0.490 and 0.607 at P = 0.039 and 0.008, respectively), total area of stunted patches (r = 0.496 and 0.659 at P = 0.035 and 0.003, respectively), and patch severity index (r = 0.492 and 0.635 at P = 0.038 and 0.005, respectively) in the 2012 trial; however, these variables were only correlated significantly with R. solani AG 3 DNA concentration in the 2014 trial. Increasing the interval between herbicide application to the cover crop and onion planting provides a practical management tool for stunting in onion bulb crops. [ABSTRACT FROM AUTHOR]

Onion stunting caused by Rhizoctonia spp. is an important soilborne disease on very sandy soils in the Columbia Basin of Oregon and Washington. From 2010 to 2013, 251 isolates of Rhizoctonia or Rhizoctonia-like spp. were obtained from soil and onion plant samples collected from inside and outside patches of stunted plants in 29 onion fields in the Columbia Basin. Sequence analysis of the internal transcribed spacer (ITS) region was used to identify the isolates, with 13 anastomosis groups (AGs) or subspecies detected. The most frequent was Waitea circinata var. circinata (25%), followed by Rhizoctonia solani AG 3 (17%), R. solani AG 4 (14%), Ceratobasidium sp. AG A (10%), R. solani AG 8 (7%), Ceratobasidium sp. AG K (6%), R. solani AG 2-1 (6%), W. circinata var. zeae (6%), R. solani AG 5 (4%), Ceratobasidium sp. AG G (2%), R. solani AG 11 (2%), and R. solani AG 1-1B and AG 10 (each <1%). However, the distribution of AGs and subspecies varied depending on whether soil or onion plants samples were collected within or adjacent to patches of stunted onion plants. In an attempt to predict the risk of onion stunting for a field prior to planting, DNA concentrations of AG 2-1, AG 3, AG 4, and AG 8 were quantified from bulk soil samples collected from each of nine growers' fields approximately 1 month before onion sowing in 2012. The preplant DNA concentrations did not show a significant association with the amount of stunting observed in the fields during the growing season. In contrast, the frequency of isolation and DNA concentration of R. solani AG 8 detected in soil samples collected during the growing season were greater from inside patches of stunted onion plants than from adjacent healthy areas of an onion crop sampled in 2012, but not for soil samples collected similarly from an onion crop in 2013. AG 2-1, AG 3, and AG 4 DNA concentrations did not differ significantly in soil sampled inside versus outside stunted patches in the fields sampled in 2012 and 2013. Relationships between the number of bulbs harvested or bulb weight versus severity of stunting were defined using correlation and regression analyses for six onion cultivars grown in seven fields surveyed in 2012 and 2013. Onion stunting reduced the average marketable bulb yield by 25 to 60% within stunted patches of the six cultivars. Stunting did not reduce onion plant stand but consistently reduced the size of bulbs, and yield reduction increased with increasing disease severity. [ABSTRACT FROM AUTHOR]

Thirty-five onion genotypes were evaluated for resistance to stunting caused by Rhizoctonia solani anastomosis group 8 (AG 8) in a growth chamber set at 15 ± 1 °C. The trial was repeated. Resistance tó R. solani AG 8 was defined as a lack of significant difference in plant height, root length, and/or total dry biomass between inoculated and noninoculated plants of the same genotype. Plant height was not reduced significantly by R. solani AG 8 for 14 and 7 of the 35 genotypes in Trials 1 and 2, respectively. In Trial 1, plant height reduction caused by R. solani ranged from 24% for the cv. Lasalle to 62% for the experimental line R14882, and in Trial 2 plant height reduction ranged from 22% for the experimental line PX07713218 to 53% for the cv. Montblanc. However, the extent of reduction in plant height caused by R. solani did not differ significantly among genotypes in either trial. Onion root length was not reduced by R. solani AG 8 for 26 and 18 of the 35 genotypes in Trials 1 and 2, respectively. The degree of reduction in root length ranged from 9% (R14889) to 76% (Sterling and SN232) in Trial 1, and 14% (SN325) to 74% (Sterling) in Trial 2. Onion dry biomass was not reduced by R. solani AG 8 for 19 and 7 of the 35 genotypes in Trials 1 and 2, respectively, and ranged from 18% (Elbrus) to 69% (Sterling) in Trial 1, and 29% (SN232) to 79% (Sterling) in Trial 2. The reduction in onion root length and total biomass did not differ significantly among onion genotypes in Trial 1, but differed among genotypes in Trial 2. Of the 35 genotypes evaluated, 3,16, and 3 demonstrated partial resistance to R. solani AG 8 for plant height, root length, and total biomass, respectively, in both trials. Only four genotypes displayed partial resistance to R. solani AG 8 for at least two of the three growth parameters: plant height, root length, and biomass of PX07713218 were unaffected by the fungus in either trial; and R14885, R14888, and SN307 displayed partial resistance in both trials for two of the three parameters, and in one of the two trials for the third parameter. These four genotypes could be used in onion breeding programs to develop cultivars partially resistant to stunting caused by R. solani AG 8. [ABSTRACT FROM AUTHOR]

Xanthomonas hortorum pv. carotae (Xhc) causes bacterial blight of carrot (Daucus carota L.), is endemic in the primary regions of carrot seed production, and is readily seed-transmitted. Genetic resistance to Xhc is not well documented in commercially available carrot cultivars, and there has been little public research on screening for resistance. Carrot PI lines (n = 66), public inbred lines (n = 2), and commercial cultivars (n = 17) were assessed for response to Xhc in a greenhouse in 2012 based on the incidence and severity of bacterial blight symptoms after inoculation as well as Xhc population [colony-forming units (CFU)/g dry foliage] detected by dilution plating onto XCS agar, a semiselective medium for Xhc. Severity of bacterial blight averaged 8.8% ± 0.4% (mean ± se) with a range of 0% to 50%, and size of the Xhc population detected on the foliage averaged 8.16 x 109 ± 1.07 x 109 CFU/g (range, 1.38 x 104 to 3.28 x 1011 CFU/g) for individual plants. Eight putative resistant PI lines and five highly susceptible PI lines selected from the 2012 screening were evaluated again in 2013 with an additional two PI lines, 12 cultivars, two inbred lines, and 12 carrot wild relatives. In the 2013 trial, severity of foliar blight 6 weeks post-inoculation ranged from 0% to 90% (11.8% ± 0.4%), and Xhc population ranged from 4.90 x 104 to 1.30 x 1011 CFU/g dry foliage (1.00 x 1010±5.29 x 108 CFU/g) for individual plants. Spearman's correlation coefficient between the Xhc population detected and severity of bacterial blight was highly significant in the 2012 and 2013 trials (r = 0.52 and 0.62, respectively, at P < 0.0001). PI lines 418967, 432905, and 432906 were the most resistant based on Xhc population detected and could be used to develop resistant cultivars. Of the 12 carrot wild relatives screened, only Ames 7674 and SS10 OR displayed a relatively low severity of bacterial blight and population of Xhc on the foliage. [ABSTRACT FROM AUTHOR]

The efficacy of 14 seed and drench treatments for control of soilborne damping-off pathogens in organic production of spinach was evaluated in a greenhouse study. The efficacy of each treatment was compared with nontreated seed and seed treated with a conventional fungicide for control of Fusariurn oxysporuin f. sp. spinaciae, Pythiurn ultimurn, and Rhizoctonia solani. Two experimental seed treatments, GTG I and GTG II (each comprised of a proprietary organic disinfectant and the latter also containing Trichoderma harzianum T22), provided equivalent control to the conventional fungicide, mefenoxam, against P. ultimum in one trial and significant reduction of damping-off in the second trial. Natural II and Natural X (Streptomycele products), and Subtilex (Bacillus subtilis) seed treatments each suppressed damping-off significantly in one of the two trials. For R. solani, GTG I and Natural It seed treatments reduced damping-off as effectively as a drench with the fungicide Terraclor (pentachloronitrobenzene). A soil drench with Prestop (Gliocladium catenulatuin) suppressed postemergence wilt caused by F oxysporum in both trials; a compost tea drench and seed treatment with Yield Shield (Bacillus pumilis) each suppressed postemergence wilt in only one of two trials. GTG I and GTG II significantly increased seed germination compared to nontreated seed. No treatment was effective against all three pathogens, and some treatments exacerbated damping-off. [ABSTRACT FROM AUTHOR]

The entire U.S. Department of Agriculture (USDA) spinach (Spinacia oleracea L.) germplasm collection (338 accessions) and 22 commercial cultivars were evaluated for resistance to leaf spot caused by Stemphylium botryosum in a greenhouse trial with two replications in 2004. The resistant and susceptible accessions identified as well as the 22 commercial cultivars were included in a second test in 2005 with four replications to confirm the results. No genotype was completely resistant (immune) to the disease. However, there were significant differences in disease incidence (percent of plants with leaf spot) and severity (percent diseased leaf area) among the genotypes tested. Two accessions from Turkey, PI 169685 and PI 173809, consistently had low disease incidence and severity ratings. Two Spinacia tetrandra and four Spinacia turkestanica accessions screened in these public germplasm tests were all susceptible. None of the commercial cultivars tested consistently had low disease incidence or severity. There was no significant correlation between disease incidence/severity and leaf type (smooth, semisavoy, or savoy). In addition to the public germplasm evaluated, 138 proprietary spinach genotypes (breeding lines and cultivars) were obtained from seed companies and screened along with 10 accessions from the USDA germplasm collection for resistance to Stemphylium leaf spot and Cladosporium leaf spot (caused by Cladosporium variabile) in a greenhouse in both 2004 and 2005. Significant differences in severity of leaf spot were observed among the genotypes for both diseases. For each disease, there was a significant positive correlation in severity ratings of the genotypes between the 2004 and 2005 trials. Information on the relative resistance (or susceptibility) of the spinach germplasm evaluated in this study should be useful for plant breeders to develop leaf spot-resistant cultivars. [ABSTRACT FROM AUTHOR]

Light leaf spot, caused by the ascomycete Pyrenopeziza brassicae, is an established disease of Brassicaceae in the United Kingdom (UK), continental Europe, and Oceania (OC, including New Zealand and Australia). The disease was reported in North America (NA) for the first time in 2014 on Brassica spp. in the Willamette Valley of western Oregon, followed by detection in Brassica juncea cover crops and on Brassica rapa weeds in northwestern Washington in 2016. Preliminary DNA sequence data and field observations suggest that isolates of the pathogen present in NA might be distinct from those in the UK, continental Europe, and OC. Comparisons of isolates from these regions using genetic (multilocus sequence analysis, MAT gene sequences, and rep‐PCR DNA fingerprinting), pathogenic (B. rapa inoculation studies), biological (sexual compatibility), and morphological (colony and conidial morphology) analyses demonstrated two genetically distinct evolutionary lineages. Lineage 1 comprised isolates from the UK, continental Europe, and OC, and included the P. brassicae type specimen. Lineage 2 contained the NA isolates associated with recent disease outbreaks in the Pacific Northwest region of the USA. Symptoms caused by isolates of the two lineages on B. rapa and B. juncea differed, and therefore "chlorotic leaf spot" is proposed for the disease caused by Lineage 2 isolates of P. brassicae. Isolates of the two lineages differed in genetic diversity as well as sensitivity to the fungicides carbendazim and prothioconazole. [ABSTRACT FROM AUTHOR]

Organic vegetable production accounted for 19% of the total organic acreage in Washington State in 2013, with 1,700 ha of certified organic vegetable pea. However, production is challenged constantly with the threat of poor emergence after planting due to damping-off caused by Pythium spp. A survey of Pythium spp. in organic vegetable production areas of the semiarid Columbia Basin of central Washington was carried out in fall 2009 to identify species associated with damping-off during early spring planting. Of 305 isolates baited from soil sampled from 37 certified organic fields, 264 were identified to 16 Pythium spp. by sequencing the internal transcribed spacer region of ribosomal DNA. A soil DNA-CFU regression curve was developed using real-time quantitative polymerase chain reaction assays for each of the three predominant pathogenic species (Pythium abappressorium, the P. irregulare complex, and P. ultimum var. ultimum) found in soil sampled from the 37 fields. The P. irregulare complex, P. abappressorium, and P. ultimum var. ultimum were detected in 57, 78, and 100% of the fields sampled, respectively. A regression analysis was used to determine that P. ultimum var. ultimum ranged from 14 to 332 CFU/g of soil in the 37 fields, the P. irregulare complex ranged from 25 to 228 CFU/g of soil, and P. abappressorium DNA was below the quantifiable limit. In summary, P. ultimum var. ultimum was the most prevalent pathogenic Pythium sp. detected in certified organic fields in the semiarid Columbia Basin of central Washington but multiple Pythium spp. may be associated with damping-off in cool and wet, early spring planting conditions. [ABSTRACT FROM AUTHOR]

Isolates of Rhizoctonia and Rhizoctonia-Wke spp, (n = 179) were baited selectively from soil and plant samples collected from irrigated pea crops in the semiarid Columbia Basin of Oregon and Washington from 2011 to 2013, and characterized to species, subspecies, and anastomosis groups (AG) based on sequences of the internal transcribed spacer region of ribosomal DNA. Rhizoctonia solani comprised 76% of all isolates, and included isolates of AG 4 (31 % of all isolates), AG 2-1(18%), AG 3 (10%), AG 8 (8%), AG 5 (5%), AG 10 (3%), and AG 9 (1%). The isolates of Ceratobasidium spp. (20%) comprised four AGs: AG K (I I %), AG A (6%), AG I (2%), and AG I-like (1%). Waited circinata isolates (4%) comprised two subspecies: W. circinata var. circinata (approximately 4%) and W. circinata var. zeae (<1%). Repeated pathogenicity tests of isolates of the 10 most frequently detected AGs and subspecies on 'Serge' pea at 15°C revealed that R. solani AG 2-1 caused the greatest reduction in pea emergence, followed by R. solani AG 4. R. solani AG 4 caused the most severe root rot, stunting, and reduction in pea seedling biomass, followed by isolates of AG 2-1. R. solani AG 8 did not affect emergence, plant height, and total biomass compared with noninoculated control plants; however, root rot caused by isolates of AG 8 was ranked the third most severe among isolates of the 10 Rhizoctonia subgroups, after that caused by isolates of AG 4 and AG 2-1. Isolates of other AGs and subspecies were either weakly virulent or nonpathogenic on pea. The most common AGs (AG 4 and AG 2-1) detected in pea fields in the Columbia Basin were also the most virulent. In a growers' pea crop grown for seed ('Prevail') planted 5 days after herbicide application and incorporation of a preceding winter wheat crop, severe stunting caused by Rhizoctonia spp. resulted in an average 75% yield loss within patches of stunted plants. In conU'ast, the yield of processing pea from a green pea crop of Serge did not differ significantly for plants sampled within versus outside patches of stunted plants; however, plants within patches were significantly more mature. In the Prevail seed crop, a greater frequency of R. solani AG 8 was detected than AG 2-1 or AG 4 from within patches of stunted plants, indicating that isolates of AG 8 may be associated with the root rot complex in some pea crops in the Columbia Basin. [ABSTRACT FROM AUTHOR]

In 2005, Verticillium dahliae was first reported to be pathogenic to spinach seed crops in the Pacific Northwest, with symptoms only de-veloping after initiation of the reproductive stage of plant growth, and to be prevalent on commercial spinach seed lots produced in Denmark, The Netherlands, and the United States. In this study, the genetic diver-sity, pathogenicity, and virulence were examined for a collection of isolates of Verticillium spp. from spinach as well as other hosts (alfalfa, cotton, lettuce, mint, peppermint, potato, radish, and tomato) from various countries and from different vegetative compatibility groups (VCGs). Of a total of 210 isolates of V. dahliae obtained from spinach seed produced in Denmark, the Netherlands, New Zealand, or the United States, 128 were assigned to VCG 4B (89% of 91 U.S. isolates, 86% of 42 isolates from the Netherlands, 19% of 43 Denmark isolates, and 8% of 13 New Zealand isolates), 65 to VCG 2B (92% of the New Zealand isolates, 79% of the Denmark isolates, 14% of the Netherlands isolates, and 9% of the U.S. isolates), and 3 to VCG 2A (2% of each of the Denmark and U.S. isolates, and 0% of the Netherlands and New Zealand isolates); 14 isolates could not be assigned to a VCG. Although little variation in the sequence of the internal transcribed spacer (ITS) region of ribosomal DNA was observed among isolates within each Verticillium sp., the ITS region readily differentiated iso-lates of the species V. dahliae, V. tricorpus, and Gibellulopsis nigres-cens (formerly V. nigrescens) obtained from spinach seed. Greenhouse pathogenicity assays on spinach, cotton, lettuce, and tomato plants using isolates of V. dahliae (n = 29 to 34 isolates), V. tricorpus (n = 3), G. nigrescens (n = 2), and V. albo-atrum (n = 1) originally obtained from these hosts as well as from alfalfa, mint, peppermint, potato, and radish, revealed a wide range in virulence among the isolates. Isolates of V. tricorpus and G. nigrescens recovered from spinach seed and an isolate of V. albo-atrum from alfalfa were not pathogenic on spinach. In addition, isolates of V. dahliae from mint and peppermint were not pathogenic or only weakly virulent on the hosts evaluated. Although there was a wide range in virulence among the isolates of V. dahliae tested, all of the V. dahliae isolates caused Verticillium wilt symptoms on spinach, lettuce, tomato, and cotton. None of the isolates of V. dahl-iae showed host specificity. These results indicate that Verticillium and related species associated with spinach seed display substantial varia-bility in virulence and pathogenicity to spinach and other plants but the V. dahliae isolates were restricted to three VCGs. [ABSTRACT FROM AUTHOR]

Verticillium dahliae is a pathogen of spinach (Spinacia oleracea) during spinach seed crop production but not in vegetative leafy spinach crops, because plants remain asymptomatic until bolting has been initiated (conversion from vegetative to reproductive growth). The objectives of this research were to evaluate a set of USDA spinach germplasm accessions for resistance to Verticillium wilt and to determine the range in incidence of natural seed infection/infestation in a subset of the USDA spinach accessions screened for resistance. A total of 120 Spinacia spp. accessions from the USDA North Central Regional Plant Introduction Station spinach germplasm collection and 10 commercial S. oleracea hybrids were screened for resistance to V. dahliae in Trials 1, 2, and 3 in 2006, 2007, and 2008, respectively, in greenhouse evaluations. Each accession was grown in either V. dahliae-infested or non-infested potting mix and rated weekly on a seven-step scale from 1 week before bolting to 4 weeks after bolting. A wide range of disease severity ratings was observed among the accessions evaluated. Most of the accessions were highly susceptible. There was no evidence of qualitative resistance but some showed greater levels of quantitative resistance than others. Plants in soil infested with V. dahliae senesced faster and had significantly reduced biomass compared with plants in non-infested soil of the same accession. In addition, in Trial 2 (2007), 34% (20 of 59) of the seed samples assayed of the accessions were infested or infected with V. dahliae; and in Trial 3, (2008) 16% (21 of 130) of the seed samples of the USDA accessions evaluated were infested or infected with V. dahliae, V. tricorpus, or Gibellulopsis nigrescens (formerly known as V. nigrescens). These results are valuable for characterizing potential genetic variability within spinach germplasm for resistance to V. dahliae. [ABSTRACT FROM AUTHOR]

The article presents the observations and trends from more than a quarter century of evaluating sweet corn hybrids in disease nurseries at the University of Illinois in Urbana. It is observed that the host plant resistance often plays an important role in the control diseases of sweet corn. It is pointed out that the commercial success and failure of a disease-resistant sweet corn hybrid is based largely on sales of seed.

During the growing seasons of 2003 and 2004, a disease occurred in several carrot crops in south central Washington with symptoms suggestive of infection by phytopathogenic mollicutes (phytoplasmas and spiroplasmas). In the fall, many affected carrot plants exhibited extensive purple or yellow-purple leaf discoloration, general stunting of shoots and taproots, and formation of bunchy, fibrous secondary roots. For detection of the putative causal agents, polymerase chain reaction (PCR) assays were performed using primers specific to phytoplasmas as well as primers specific to plant-pathogenic spiroplasmas. Restriction fragment length polymorphism (RFLP) analyses of PCR-amplified 16S rDNA sequences revealed that about 81% of affected plants showing dark purple or yellow-purple leaf symptoms tested positive for Spiroplasma cirri. Of affected plants showing mild purple discoloration of leaf margins, 18% tested positive for a phytoplasma strain belonging to the clover proliferation group (16SrVI), subgroup 16SrVI-A, and 11% for another phytoplasma strain belonging to the aster yellows group (16SrI), subgroup 16SrI-A. Nucleotide sequence analysis of cloned 16S rDNA confirmed the phytoplasma group affiliations. Some symptomatic plants were co-infected with S. citri and either aster yellows phytoplasma or clover proliferation group phytoplasma. To our knowledge, this is the first documentation of spiroplasma infection of carrot in the United States. [ABSTRACT FROM AUTHOR]

Blackleg (Phoma stem canker) of crucifers is a globally important disease caused by the ascomycete species complex comprising of Leptosphaeria maculans and Leptosphaeria biglobosa. Six blackleg isolates recovered from Brassica rapa cv. Mizspoona in the Willamette Valley of Oregon were characterized as L. biglobosa based on standard pathogenicity tests and molecular phylogenetic analysis. These isolates were compared to 88 characterized L. biglobosa isolates from western Canada, 22 isolates from Australia, and 6 L. maculans isolates from Idaho, USA using maximum parsimony and distance analysis of phylogenetic trees generated from the ITS rDNA (internal transcribed spacer rDNA) sequence, and the actin and β-tubulin gene sequences. The L. biglobosa isolates derived from B. rapa collected in Oregon formed a separate subclade based on concatenated gene sequences or a single gene sequence, regardless of the analyses. Pathogenicity tests showed that these isolates failed to infect either resistant or susceptible B. napus cultivars, but caused severe symptoms on three B. rapa cultivars (Accession number: UM1113, UM1112, and UM1161), a B. oleracea var. capitata (cabbage) cultivar (Copenhagen Market), and two B. juncea cultivars (CBM, a common brown Mustard, and Forge). These findings demonstrated that the L. biglobosa isolates derived from a B. rapa crop in Oregon were genetically distinct from existing species of L. biglobosa, and constitute a new subclade, herein proposed as L. biglobosa 'americensis'. [ABSTRACT FROM AUTHOR]