Dennis A. Johnson

High Genetic Diversity, Phenotypic Uniformity, and Evidence of Outcrossing in Sclerotinia Sclerotiorum in the Columbia Basin of Washington State

ABSTRACT Sclerotinia sclerotiorum, the causal agent of potato stem rot, is prevalent and poorly managed on potatoes in the Columbia Basin of Washington. Because of the ubiquitous nature of the fungus and high crop diversity within the Columbia Basin, understanding the population structure and the potential for outcrossing of the pathogen would be helpful in developing disease management strategies. The population structure of S. sclerotiorum in the Columbia Basin from potato was examined using microsatellite markers and mycelial compatibility. Analysis of molecular variance revealed that 92% of the variability among 167 isolates was found within subpopulations, with limited, yet statistically significant impact of the collection date, but not the year or location of collection. Linkage disequilibrium and index of association analyses noted a potential for outcrossing in two locations, which was substantiated by the discovery of recombinant ascospores in three field-generated apothecia from the 12 apothecia examined. Microsatellite haplotypes were not correlated with mycelial compatibility groups. This high haplotypic diversity did not seem to impact pathologically important phenotypes. Greenhouse inoculations of potato plants exhibited no significant differences in aggressiveness on potato stems. Moreover, in vitro studies of response to fungicides and temperature stimuli yielded no significant differences among studied isolates. These findings illustrate the potential for outcrossing in warm temperate regions of North America, where a diversity of crops are planted simultaneously and in neighboring fields. This study also indicates that the unsatisfactory management of potato stem rot is likely not directly attributable to genetic factors, but to gaps in agricultural practices.

Relative resistances of potato clones in response to new and old populations of Phytophthora infestans

Potato cultivars and clones were evaluated in Washington and New York in 1993 and 1994 for field reaction to recent immigrant genotypes of Phytophthora infestans. Plants were visually evaluated at regular intervals for percent blighted foliage. Relative cultivar susceptibilities were compared by ranking the values obtained for areas under disease progress curves (AUDPC) of each line tested. Cultivar rankings in response to infection by new, immigrant isolates of P infestans were nearly identical to rankings obtained previously with isolates prevalent prior to 1990. The cultivars Norchip, Hilite, Russet Norkotah, Goldrush, Superior, and Shepody were more susceptible than Russet Burbank. White Rose and Ranger Russet were similar in susceptibility to Russet Burbank. C008-3008-1, ND-2438-7R, Kennebec, and Elba were less susceptible than Russet Burbank. Comparisons of the number of blighted tubers at harvest showed that foliage and tuber susceptibilities differed among cultivars. Shepody and Russet Norkotah tubers were most susceptible to tuber blight of those tested.

Aggressiveness of isolates of Phytophthora infestans from the Columbia Basin of Washington and Oregon

ABSTRACT The aggressiveness of 22 isolates of Phytophthora infestans collected from naturally infected potato plants in the Columbia Basin of Washington and Oregon was determined on detached potato leaflets at 18 degrees C in an incubator. Selected isolates were evaluated on whole plants in a greenhouse. Aggressiveness was measured by using the area under the lesion expansion curve (AULEC), incubation period, latent period, sporulation capacity, and lesion size on detached leaflets and the area under the disease progress curve and sporulation capacity on whole plants. The detached-leaflet assay was useful in that a large number of isolates were tested, several components of aggressiveness were studied, and significant differences were found among isolates. Significant variation for components of aggressiveness was found within and among isolates classified according to genotype. Significant interactions among isolates and cultivars were found for some components of aggressiveness, so results were pooled according to cultivar. On average, US-8 and US-11 isolates had higher AULEC scores, indicating aggressiveness higher than that of US-7, US-6, and US-1 genotypes. One US-8 genotype isolate had a higher standardized sporulation capacity than isolates of the other genotypes. US-6 genotype isolates were the least aggressive group, as indicated by low AULEC, sporulation capacity, and lesion size values. The replacement of the US-1 genotype by the US-8 genotype in the Columbia Basin may be partially explained by the increased aggressiveness of US-8 isolates. Additionally, potato growers may need to shorten intervals between fungicide applications and begin applications earlier.

Potato Late Blight in the Columbia Basin: An Economic Analysis of the 1995 Epidemic

The cost of managing late blight in potatoes during a severe epidemic caused by new, aggressive strains of Phytophthora infestans in the Columbia Basin of Washington and Oregon in 1995 was documented. The mean number of fungicide applications per field varied from 5.1 to 6.3 for early- and midseason potatoes, and from 8.2 to 12.3 for late-season potatoes in the northern and southern Columbia Basin, respectively. In 1994, a year when late blight was not severe, the mean number of fungicide applications per field made to early- and midseason potatoes was 2.0; whereas late-season potatoes received a mean of 2.5 applications. The mean per acre cost of individual fungicides applied varied from

Characterization of the Phytophthora infestans Population in the Columbia Basin of Oregon and Washington from 1992 to 1995.

4.90 for copper hydroxide to

Control of Potato Tuber Rots Caused by Oomycetes with Foliar Applications of Phosphorous Acid

36.00 for propamocarb + chlorothalonil. Total per acre expenses (application costs plus fungicide material) for protecting the crop from late blight during 1995 ranged from

Effect of foliar infection caused by Colletotrichum coccodes on yield of Russet Burbank potato

106.77 to

Cost of Fungicides Used to Manage Potato Late Blight in the Columbia Basin: 1996 to 1998

110.08 for early and midseason potatoes in different regions of the Columbia Basin and from

Recovery of Verticillium dahliae from North American Certified Seed Potatoes and Characterization of Strains by Vegetative Compatibility and Aggressiveness

149.30 to

Vegetative Compatibility Groups and Aggressiveness of North American Isolates of Colletotrichum coccodes, the Causal Agent of Potato Black Dot

226.75 for lateseason potatoes in the northern and southern Columbia Basin, respectively. Approximately 28% of the crop was chemically desiccated before harvest as a disease management practice for the first time in 1995, resulting in an additional mean cost of