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.

Genetic Variation within Clonal Lineages of Phytophthora infestans Revealed through Genotyping-By-Sequencing, and Implications for Late Blight Epidemiology.

Genotyping-by-sequencing (GBS) was performed on 257 Phytophthora infestans isolates belonging to four clonal lineages to study within-lineage diversity. The four lineages used in the study were US-8 (n = 28), US-11 (n = 27), US-23 (n = 166), and US-24 (n = 36), with isolates originating from 23 of the United States and Ontario, Canada. The majority of isolates were collected between 2010 and 2014 (94%), with the remaining isolates collected from 1994 to 2009, and 2015. Between 3,774 and 5,070 single-nucleotide polymorphisms (SNPs) were identified within each lineage and were used to investigate relationships among individuals. K-means hierarchical clustering revealed three clusters within lineage US-23, with US-23 isolates clustering more by collection year than by geographic origin. K-means hierarchical clustering did not reveal significant clustering within the smaller US-8, US-11, and US-24 data sets. Neighbor-joining (NJ) trees were also constructed for each lineage. All four NJ trees revealed evidence for pathogen dispersal and overwintering within regions, as well as long-distance pathogen transport across regions. In the US-23 NJ tree, grouping by year was more prominent than grouping by region, which indicates the importance of long-distance pathogen transport as a source of initial late blight inoculum. Our results support previous studies that found significant genetic diversity within clonal lineages of P. infestans and show that GBS offers sufficiently high resolution to detect sub-structuring within clonal populations.

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.

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.