Verticillium isaacii is a Pathogen and Endophyte of Potato and Sunflower in the Columbia Basin of Washington.

The objective of this research was to test the hypothesis that Verticillium isaacii causes diseases of sunflower and potato plants. Two sunflower genotypes and one potato cultivar were inoculated with five V. isaacii isolates and three pathogenic V. dahliae isolates. Biomass, disease expression, and stem colonization were quantified. Overt wilt symptoms were observed on both sunflower genotypes and potato plants inoculated with a subset of the V. isaacii isolates. Biomass of both sunflower genotypes was not affected by V. isaacii infection. Tuber yields either decreased in response to infection by one V. isaacii isolate or were not affected by infection. Stems of sunflower and potato plants were infected by at least four of the five V. isaacii isolates. A new disease of sunflower and potato is documented. Evidence that V. isaacii exhibits different lifestyles including pathogenicity and endophytism is presented. Finally, this research documents variation in fungal lifestyles that can exist in samples from a single field.

Pathogenicity and Aggressiveness of Three Alternaria spp. on Potato Foliage in the U.S. Northwest.

Alternaria spp. were collected from potato foliage showing symptoms of early blight and brown spot in the Columbia Basin, WA and Bonners Ferry and Rupert, ID between 2009 and 2011. The aggressiveness of three Alternaria spp. on potato was quantified on nonwounded and wounded detached leaves of 'Russet Norkotah' potato; wounded detached leaves of 'Alturas', 'Ranger Russet', 'Russet Burbank', and 'Umatilla Russet'; and whole plants of Russet Norkotah. Mean infection frequencies (MIF) and area under the lesion expansion curve (AULEC) were significantly greater for Alternaria solani (P = 0.0072 and 0.0002, respectively) than for A. arborescens or A. arbusti on nonwounded leaves. Wounding of tissue significantly increased MIF and AULEC for A. arbusti (P = 0.008 and 0.0047, respectively) and AULEC for A. arborescens (P = 0.01) relative to nonwounded tissue. AULEC did not differ significantly among the three Alternaria spp. when inoculated onto wounded foliage of whole plants (P = 0.34); the AULEC of whole plants was positively and significantly correlated with AULEC on detached leaves (P = 0.03). Umatilla Russet was the most susceptible and Russet Burbank was the least susceptible based on MIF and AULEC for all three pathogen species. Results indicate that A. solani was the more aggressive pathogen of potato in the Columbia Basin, because both A. arborescens and A. arbusti require wounds and A. arbusti lesions do not expand significantly in comparison with A. solani or A. arborescens following inoculation.

Identification and Enumeration of Small-Spored Alternaria Species Associated with Potato in the U.S. Northwest.

A number of Alternaria spp. have been isolated from potato worldwide but only Alternaria solani and A. alternata have been described as pathogenic to this host in the United States. These taxa are easily differentiated based on conidial morphology but species delimitation among the small-spored Alternaria spp. associated with potato are much more challenging. Accurate identification methods for small-spored Alternaria spp. are necessary so that a more thorough understanding of Alternaria epidemiology can be obtained. Isolations of Alternaria fungi from lesions on potato leaves collected in the U.S. Northwest were made between 2008 and 2011. Large-spored taxa (putatively A. solani), were isolated less frequently than small-spored taxa (putatively A. alternata sensu lato), except in 2010. Colletotrichum coccodes was isolated from necrotic lesions in 2008 to 2010 but not in 2011. Frequency of isolation ranged from 0.05 (5%) to 0.11 (11%) during the 3 years the fungus was detected. Anonymous genomic region OPA1-3, previously used for Alternaria systematics, allowed for the discrimination of phylogenetic lineages among 210 small-spored isolates. When OPA1-3 was restricted using enzyme ApaI, 65 isolates (31%) displayed a restriction banding pattern consistent with previously characterized morphospecies A. alternata and A. tenuissima and 145 (69%) displayed a restriction banding pattern consistent with the previously characterized morphospecies A. arborescens. Morphological characterization of a subsample of 59 small-spored Alternaria isolates randomly selected with each restriction pattern was compared with phylogenetic lineage. In all, 54 (92%) isolates were consistently assigned to the same group by both methods. Three isolates exhibited conidial morphologies that were inconsistent with any described morphospecies. A small number of isolates were identified as A. arbusti (infectoria group) via sequencing of the glyceraldehyde-3-phosphate-dehydrogenase locus and BLAST searches.

Effect of Powdery Scab Root Galls on Yield of Potato.

Powdery scab of potato, caused by Spongospora subterranea f. sp. subterranea, has increased in incidence since 1981 to become a major concern for potato production in the Columbia Basin of south-central Washington and north-central Oregon. Disease symptoms consist of pustules on the tuber surface and wart-like galls on roots, stolons, and tubers. In the current study, the effect of powdery scab root galls on potato yield was quantified by comparing yields of cultivars susceptible and resistant to root galling grown on soil naturally infested and noninfested with the powdery scab pathogen. Potato tuber yields for the three yield components of yield per plant, number of tubers, and weight of tubers were not negatively affected by powdery scab galls on roots of Umatilla Russet and Shepody, as indicated by yield ratios ≥1.0 in 3 of 4 years, and also by regression analyses in all 3 years. Regression lines with a nonsignificant slope or slopes that significantly increased with gall index also indicated that root galls did not negatively affect yield. Mean tuber weight increased as gall index increased for Umatilla Russet in 2 of 3 years. Consequently, control tactics directed solely at reducing powdery scab galls on roots are not justified economically under conditions encountered in the Columbia Basin of Washington and Oregon states. Effort has been made to develop resistant cultivars and search for soil applied chemicals to reduce root galls.

Effect of Extended Crop Rotations on Incidence of Black Dot, Silver Scurf, and Verticillium Wilt of Potato.

Potato tubers were collected and evaluated for symptoms and signs of black dot, silver scurf, and Verticillium wilt to determine the effect of extended crop rotations on disease incidences in the Columbia Basin. Incidence of tubers with black dot collected from storage significantly decreased as the number of years between potato crops increased from 3 to 5 years and beyond and significantly increased as the number of previous potato crops increased to 16. The highest incidence of black dot (range of 73 to 98%) was from fields rotated out of potatoes for 1 to 3 years. The mean incidence of black dot was 56% for fields out of potatoes for 0 to 4 years and 12% for fields out of potatoes 5 and more years. A low incidence (0 to 9%) of black dot was detected at 15 years out of potatoes. Years out of potato and number of prior potato crops accounted for 71% of the variability associated with the incidence of black dot. Severity of black dot on tuber periderm peels significantly increased as incidence of tuber periderm peels with Colletotrichum coccodes increased. Coefficient of determination was 0.87 for log severity on regressed on black dot incidence. Incidence of silver scurf was highest from fields out of potatoes for 1 year. Incidence of silver scurf infected tubers significantly increased as the number of previous potato crops increased due to short rotations between potato crops. Incidence of tubers with Verticillium dahliae was not related to years between potato crops or number of previous potato crops. The present study confirmed that black dot can be reduced with rotations out of potatoes greater than 5 years.

Accuracy of Rain Forecasts for Use in Scheduling Late Blight Management Tactics in the Columbia Basin of Washington and Oregon.

Accuracy of prediction was analyzed for 17- and 30-day rain forecasts at two locations in the Columbia Basin to determine whether forecasts were sufficiently accurate to be included as a model component to schedule fungicide applications for potato late blight. Accuracy was partitioned into specificity (percentage of forecasted nonrainfall events classified correctly) and sensitivity (percentage of forecasted daily rainfall events classified correctly). An adjusted sensitivity, which included the forecasted rain day plus the next 2 days, was also used to give a wider target than only 1 day for evaluating accuracy of forecasted rain events. For 17-day forecasts, specificity during the seasonal test period was ≥70% from mid-June through September and specificity over the days of the forecast was >70% for the first 8 days at both locations both years. Adjusted sensitivity over days of the forecast was initially >80% and then decreased as forecasts increased from 7 to 17 days for 17-day forecasts at both locations and years. Sensitivity and adjusted sensitivity during the seasonal test period were both positively correlated with the number of rainy days while specificity was negatively correlated. Adjusted sensitivity was considerably higher for May (month with highest incidence of rain) than July (month with lowest incidence of rain) at both locations. For 30-day forecasts, specificity during the test period was >75% in July and August and adjusted sensitivity ranged from 60 to 100% for time periods occurring in May and June during both sample seasons. Specificity was generally above 80% as days of the forecast increased and adjusted sensitivity varied greatly over days of the forecasts, with extremes between 0 and 100% at both locations and years for the 30-day forecasts. Specificity of 17- and 30-day rain forecasts and adjusted sensitivity of 17-day rain forecasts have utility in scheduling late blight fungicides in the Columbia Basin.

Slow-Rusting Resistance in Native Spearmint to Puccinia menthae.

Rust, caused by Puccinia menthae, is often a serious yield-reducing problem on native and Scotch spearmint in south-central Washington State. Rust resistant mint cultivars would reduce a dependence on fungicides for disease management; however, conventional breeding practices are not possible because commercial mint plants are sterile. Mutant Scotch and native spearmint lines induced by irradiation were evaluated for partial resistance to rust. Latent period and number of uredinia per leaf from urediniospore inoculations in the greenhouse and aecial and uredinial development in the field were quantified. Length of latent period, number of uredinia per leaf, incidence of shoots with aecia, and area under the disease progress curve (AUDPC) for uredinia development all varied significantly among lines and standard cultivars. Length of latent period was significantly longer and number of uredinia per leaf was significantly lower for native spearmint line N87-1 than for most of the other mutant lines, native spearmint, and Scotch spearmint. Line N87-1 had the lowest AUDPC values of all lines and standard cultivars for 3 years in the field. However, line N87-1 had a relatively high incidence of shoots with aecia both of two years in the field. Length of latent period was significantly and negatively correlated with AUDPC 2 of 3 years. Mean incidence of shoots with aecia was not correlated with AUDPC or with length of latent period after urediniospore infection. Rust in native spearmint in south-central Washington State could be satisfactorily managed with slow-rusting resistance when coupled with good irrigation water management and sanitation tactics that limit aecia development on early spring mint foliage.

Fungicide Resistance of Two Species of Alternaria from Potato in the Columbia Basin of Washington.

Fungicides are extensively used in the Columbia Basin of Washington in an effort to manage early blight and brown spot on potato caused by Alternaria solani and A. alternata, respectively. Azoxystrobin and boscalid interfere with cellular respiration, and fungicide resistance is expected to occur in fungal populations due to the narrow modes of action dictated by each fungicide. The objective of this research was to assess A. solani and A. alternata populations in the Columbia Basin of Washington for azoxystrobin and boscalid resistance by initially screening isolates at a threshold concentration using mycelial growth and spore germination. Fifty isolates of A. solani and 58 of A. alternata were randomly selected for screening from 671 isolates collected from potato in the Columbia Basin from 2009 to 2011. Screening at a threshold concentration was done to estimate the range of fungicide resistance within the populations of A. solani and A. alternata in the region. Resistance of five isolates that differed in sensitivity to azoxystrobin and boscalid were then assessed for mycelial growth and spore germination at three fungicide concentrations in order to determine where rates were no longer effective. A. solani mycelial growth ratios and percent spore germination on azoxystrobin-amended media did not change from 2010 to 2011. A slight decrease in resistance of mycelial growth to azoxystrobin was exhibited among A. alternata isolates collected between 2010 and 2011. No change in sensitivity to azoxystrobin was observed in mycelial growth ratios among A. alternata isolates collected between 2010 and 2011. Resistance to boscalid by A. solani increased as indicated by a significant increase in mean mycelial growth ratio from 2010 to 2011, whereas A. alternata showed no change. Spore germination of A. solani was less at 100 µg/ml than at 1 or 10 µg/ml azoxystrobin. Spore germination of A. alternata did not differ among 1, 10, or 100 µg/ml azoxystrobin. Spore germination of A. solani and A. alternata did not differ among 0.5, 5, or 50 µg/ml boscalid. Resistance to azoxystrobin and boscalid in A. solani and A. alternata populations was shown to be present in the Columbia Basin and will likely become widespread with continued used of site-specific fungicides.

Verticillium dahliae populations from mint and potato are genetically divergent with predominant haplotypes.

In total, 286 Verticillium dahliae isolates from mint, potato, and other hosts and substrates were characterized for mating type, vegetative compatibility group (VCG), and multilocus microsatellite haplotype to determine population genetic structure among populations infecting mint and potato. Populations from mint and potato fit a clonal reproductive model, with all isolates a single mating type (MAT1-2) and multiple occurrences of the same haplotypes. Haplotype H02 represented 88% of mint isolates and was primarily VCG2B, while haplotype H04 represented 70% of potato isolates and was primarily VCG4A. Haplotypes H02 and H04 typically caused severe disease on mint and potato, respectively, in greenhouse assays regardless of host origin. Principal coordinate analysis and analysis of molecular variance indicated that mint and potato populations were significantly genetically diverged (P = 0.02), and identification of private alleles and estimation of migration rates suggested restricted gene flow. Migration was detected between infected potato plants and seed tubers, infested tare soil, and field soils. Genetic differentiation of V. dahliae from mint and potato may be due to the occurrence of a single mating type and differences in VCG. Populations of V. dahliae in potato and mint were characterized by the presence of aggressive, clonally reproducing haplotypes which are widely distributed in commercial mint and potato production.

Incidence and impact of Verticillium dahliae in soil associated with certified potato seed lots.

Verticillium dahliae causes Verticillium wilt of potato and can be found in soil associated with potato seed tubers. The purpose of this research was to quantify V. dahliae in soil associated with certified seed tubers and determine if this potential inoculum source is related to disease development in the field. Approximately 68% of seed lots assayed contained V. dahliae-infested soil on seed tuber surfaces (seed tuber soil). Over 82% of seed lots contained V. dahliae in loose seed lot soil obtained from bags and trucks used to transport seed tubers. Most samples contained ≤50 CFU/g but some contained >500 CFU/g. Most isolates (93%) were vegetative compatibility group 4A. Populations of V. dahliae in stem sap increased with increasing inoculum densities in field soils only when V. dahliae concentrations in seed tuber soil were low. High concentrations of V. dahliae in seed tuber soil resulted in greater stem sap colonization when V. dahliae inoculum densities in field soil were low (P < 0.01) and resulted in greater pathogen inoculum densities in postharvest field soils (P = 0.04). Seed tubers contaminated with V. dahliae-infested soils may introduce the pathogen into fields not previously cropped to potato or recontaminate those which have received preplant management practices. Long-term management of V. dahliae requires reducing propagules in soil associated with seed lots.

A Plant Stem Inoculation Assay for Assessing Transmission of Phytophthora infestans from Potato Seed Tubers to Emerged Shoots.

A stem inoculation assay was developed to assess transmission of Phytophthora infestans from the base of belowground potato stems to emerged aboveground shoots. Sporangia of P. infestans subsequently developed on aboveground stems after belowground stems were inoculated with isolates BF05 (US 8 genotype), WA10.1 (US 24), and 110B (US 11) by (i) placing a filter paper square saturated with a spore suspension onto nonwounded belowground stem tissue, (ii) placing a filter paper square saturated with a spore suspension onto wounded belowground stem tissue, and (iii) dipping the base of the cut belowground stem into a spore suspension. Plant stems were then incubated in a humidity chamber at 10 to 21.5°C for 7 to 28 days. Incidence of aboveground stems with sporulation was significantly less when stems were inoculated with isolate 110B than with isolates BF05 and WA10.1 but length of latent period did not differ among the three isolates. Incidence of aboveground stems with sporangia was significantly (P < 0.05) greater for 'Russet Norkotah' than for 'Umatilla Russet' when inoculated near the point of seed piece attachment. Incidence of aboveground stems with sporulation did not differ among incubation temperatures of 10, 15, and 21.5°C. Latent period significantly increased as temperature decreased. Age of stem did not appear to affect incidence of transmission. Incidence of transmission of P. infestans to aboveground stems from the base of belowground stems was significantly lowered when aboveground stems were treated with selected fungicides 1 h before inoculation.

Stability of Slow-Rusting Resistance to Puccinia asparagi and Managing Rust in Asparagus.

The stability of slow-rusting resistance to Puccinia asparagi in several asparagus cultivars was evaluated in two replicated field trials. Rust epidemics were monitored in each trial for 8 years spanning a period of 13 years (1983-1990 and 1987-1995). Inoculum of P. asparagi, an autoecious macrocyclic rust, originated each year as teliospores. In the first trial, the cultivars Jersey Titan, Jersey Centennial, Jersey Giant, Delmonte-361, and UC-157 had consistently lower area under the disease progress curve (AUDPC) values than Wash T2 and WSU-1. Cultivar Mary Washington was intermediate between the two groups of resistant and susceptible cultivars in 6 of 8 years. Jersey Titan consistently ranked number 1 for resistance with the lowest AUDPC values all 8 years. In the second trial, Jersey Giant, Delmonte-361, and UC-157 had consistently lower AUDPC values than Larac, Gynlim, Cito, Largo 17-3, and Franklim in each of 8 years. Jersey Giant, Delmonte-361, and UC-157 always ranked low (1, 2, or 3) for AUDPC. A shift from rust-susceptible to rust-resistant asparagus cultivars began in central Washington around 1996. In 2011, resistant cultivars made up nearly 96% of the asparagus plantings. From 1996 to 2011, rust was not considered a problem in commercial fields with slow-rusting resistant cultivars. Use of durable, slow-rusting cultivars, along with sanitation practices that reduced levels of aecia in nonharvested nurseries and on volunteer asparagus plants and judicious irrigation management, has effectively managed asparagus rust in commercial fields for at least 29 years in south-central Washington.

Roles of Infected Seed Tubers and Soilborne Inoculum on Verticillium Wilt of 'Russet Burbank' Potato.

Verticillium dahliae, causal agent of Verticillium wilt of potato, persists in soil as microsclerotia and can be found in infected tubers used for seed. The effects of naturally infected tubers and soilborne inoculum on Verticillium wilt symptoms were compared in the greenhouse. Infected and noninfected tubers were grown in infested and noninfested potting soil. Chlorosis and necrosis were measured and converted to area under senescence progress curves (AUSPC). Aboveground stems and progeny tubers were assayed for V. dahliae. Plants from infested soils exhibited significantly greater AUSPC than plants from noninfested soil. Plants grown from infected and noninfected tubers had similar AUSPC and interactions between infected tubers and infested soil were not observed. The pathogen was isolated from the vascular system of 94% of plants grown in infested soils and 8% of plants grown from infected tubers in noninfested soil. Plants grown in infested soil contained microsclerotia on 46% of stems while plants grown from infected tubers in noninfested soils exhibited microsclerotia on <1% of stems. Infected progeny tubers were only recovered from plants grown in infested soil. Seed tuber infection did not contribute to premature senescence or potential inoculum production, indicating that management efforts should focus on reducing soilborne inoculum.

Impact of Seed Lot Infection on the Development of Black Dot and Verticillium Wilt of Potato in Washington.

The fungi Colletotrichum coccodes and Verticillium dahliae are two pathogens of potato which are prevalent in potato field soils and seed tubers. The objective of this study was to investigate the influence of seed lot infection by C. coccodes and V. dahliae on disease severity and yield using infested seed lots and fields with and without potato in their recent cropping histories. Greater incidence of C. coccodes in seed led to more root colonization in 2008 (both field types) but not in 2007 and did not influence tuber quantity or yield. Incidence of infection in seed lots by V. dahliae was not related to canopy necrosis, infection of progeny tubers, or yield. However, seed lots exhibited more V. dahliae aboveground vascular colonization, canopy necrosis, infested plant debris, and infected progeny tubers in 5-year potato rotation fields compared with long potato rotation fields. In general, foliar disease symptoms and yield reductions were not related to levels of either pathogen in seed lots. Symptoms of both diseases were more severe and yields were lower in fields with potato in their recent cropping histories, emphasizing the importance of soilborne inoculum in the development of black dot and Verticillium wilt of potato.

Verticillium Wilt of Skullcap and Potential for Pathogen Dissemination via Seeds and Stems.

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.

Development and Suppression of Aerial Stem Rot in Commercial Potato Fields.

The development of aerial stem rot of potato was quantified in relation to soil type, previous crop, and atmospheric temperature in the Columbia Basin. Incidence of bacterial stem rot was assessed for several weeks beginning before row closure in 18 commercial potato fields and once at the estimated peak of disease development in 38 fields over 4 years. Aerial stem rot was first observed between 25 to 36 days following row closure for 78% of the fields assessed multiple times per season. Slopes of disease progress curves initially increased very rapidly. The partial slope estimate for mean daily maximum temperature indicated an increase of disease with an increase of mean daily maximum temperature (P < 0.0001). Estimates of partial slopes for soil type (sandy loam versus silt loam) and previous crop (sweet or field corn versus other) were significant at P < 0.001 and P = 0.046, respectively, suggesting that cultivation of potato in sandy loam-type soils or following sweet or field corn as a previous crop increases the probability of aerial stem rot. Incidence of bacterial stem rot was significantly reduced in the sections of fields treated with famoxadone plus mancozeb and famoxadone plus mancozeb plus copper hydroxide mixtures.

Transmission of Phytophthora infestans from Infected Potato Seed Tubers to Emerged Shoots.

Transmission of Phytophthora infestans from infected seed tubers to emerged potato sprouts, infectivity of sporangia deposited on whole tubers before burial in soil, and infectivity of sporangia in a loamy fine sand to leaflets were investigated in the greenhouse under simulated spring planting conditions of the Columbia Basin. Incidence of late-blight-infected shoots from infected seed tubers was significantly greater when foliage was exposed to wet periods in mist chambers (mist for 45 s every 15 min) for either 24 or 48 h than when not exposed to a wet period. Proportion of infected shoots from infected tubers was 0.210 to 0.261 in a moist environment versus 0.013 to 0.052 in a nonmist environment. Development of chlorosis, necrosis, and sporangia occurred on shoots that emerged from infected, symptomatic tubers buried in soil. However, approximately 20% of the infected shoots produced sporangia before stems had visible discoloration of late-blight symptoms. Sporulation was sparse and formed near the soil line on some of the shoots after 24 h in the moist environment. The latent period or time from inoculation to sporulation on young stems of Russet Burbank was 5 to 6 days, which is too long to account for an infection from either sporangia or zoospores at the soil level of shoots during the wet period in this study. Sporangia were infective when placed directly on eyes of whole tubers before planting. Leaflets touching a loamy fine sand infested with sporangia developed typical late-blight lesions beginning at the leaflet tip within 7 days after a 24-h wet period and the infested loamy fine sand was infective when splashed on leaflets.

Evaluation of Verticillium Wilt Resistance in Mentha arvensis and M. longifolia Genotypes.

Verticillium wilt, caused by Verticillium dahliae, is a major constraint to mint (Mentha spp.) production in the United States, and the use of resistant cultivars is an important component of Verticillium wilt management. Two Mentha arvensis and four M. longifolia genotypes were evaluated for resistance to Verticillium wilt in the greenhouse using V. dahliae isolates obtained from different hosts and belonging to different vegetative compatibility groups. Isolates of V. dahliae obtained from peppermint (M. × piperita) caused significantly higher disease severity, plant mortality, and yield reduction than isolates obtained from other hosts. Disease severity, plant mortality, and pathogen incidence in aboveground stems were higher and yields lower in peppermint, the susceptible standard, compared with the resistant standard, native spearmint (M. spicata). Root-dip inoculations of M. arvensis and M. longifolia with isolates of V. dahliae obtained from peppermint produced severe symptoms; however, both species displayed the ability to recover from infection and produce asymptomatic growth from rhizomes. Both M. arvensis cultivars exhibited lower mean disease severity ratings following cutback and regrowth and were not significantly different than native spearmint. The restriction of pathogen movement in aboveground tissue and ability to recover from infection may be important components of V. dahliae resistance in perennial mint cropping systems.

Latent Infection of Potato Seed Tubers by Phytophthora infestans During Long-Term Cold Storage.

Latent infection of potato seed tubers by Phytophthora infestans was determined following inoculation of tubers and typical seed tuber storage conditions in the Pacific Northwest. Severity of late blight increased over 182 to 209 days during two storage seasons at mean temperatures of 4.1 and 4.2°C. From 0 to 44% of inoculated tubers sampled at given intervals were asymptomatic. However, P. infestans sporangia were observed on slices from these tubers when incubated in a humidity chamber at 15°C or late blight symptoms developed in asymptomatic tubers obtained following storage when incubated at 22 to 23°C for 3 weeks. Development of P. infestans sporangia and symptoms of late blight in asymptomatic seed tubers indicated latent infection of tubers by P. infestans during long-term cold storage. Sporulation was observed after 21 to 24 h on symptomatic tubers and 6 to 20 days on asymptomatic tubers that were removed from storage and incubated in a humidity chamber at 15°C. Latent infection of seed tubers and production of viable sporangia of P. infestans were demonstrated after long-term cold storage of infected potato tubers.

Association of Solar Irradiance and Days of Precipitation with Incidence of Potato Late Blight in the Semiarid Environment of the Columbia Basin.

The effects of cumulative solar irradiance and rainfall on incidence of potato late blight (caused by Phytophthora infestans) in the Columbia Basin of south-central Washington and north-central Oregon were investigated using meteorological data collected near Othello, WA from 1990 through 2007 and Prosser, WA from 1990 through 2006. An association between solar irradiance and seasonal differences in late blight epidemics has not been quantitatively determined. Incidence of late blight in the Columbia Basin significantly increased as cumulative solar irradiance decreased during 1 April to 31 July and 1 June to 31 July at both Othello and Prosser. Incidence of late blight also increased as number of rainy days increased during 1 April to 31 July at Othello and Prosser and from 1 June to 31 July at Prosser. Coefficients of determination for significant models on late bight incidence ranged from 0.28 to 0.43 for cumulative solar irradiance and 0.27 to 0.38 for number of rainy days. Late blight incidence significantly increased as day of the year for first occurrence of late blight decreased in the Columbia Basin. A multiple regression model that included cumulative solar irradiance from 1 June to 31 July at Prosser and day of year for first occurrence of late blight as independent variables accounted for 58% of the variation for late blight incidence (adjusted R2 = 0.58), and a model that included number of cloudy days without rain combined with number of rainy days accounted for 62% of the variation for late blight incidence. This is the first report demonstrating a quantitative association of solar irradiance and incidence of late blight in commercial potato fields.