Sclerotinia homoeocarpa overwinters in turfgrass and is present in commercial seed

Abstract

Dollar spot is the most economically important disease of amenity turfgrasses in the United States, yet little is known about the source of primary inoculum for this disease. With the exception of a few isolates from the United Kingdom, Sclerotinia homoeocarpa, the causal agent of dollar spot, does not produce spores. Consequently, it was assumed that overwintering of this organism in soil, thatch, and plant debris provides primary inoculum for dollar spot epidemics. Overwintering of S. homoeocarpa in roots and shoots of symptomatic and asymptomatic creeping bentgrass turfgrass was quantified over the course of a three-year field experiment. Roots did not consistently harbor S. homoeocarpa, whereas S. homoeocarpa was isolated from 30% of symptomatic shoots and 10% of asymptomatic shoots in the spring of two out of three years. The presence of stroma-like pathogen material on leaf blades was associated with an increase in S. homoeocarpa isolation and colony diameter at 48 hpi. Commercial seed has also been hypothesized to be a potential source of initial inoculum for S. homoeocarpa. Two or more commercial seed lots of six creeping bentgrass cultivars were tested for contamination with S. homoeocarpa using culture-based and molecular detection methods. A viable, pathogenic isolate of S. homoeocarpa was isolated from one commercial seed lot and contamination of this lot was confirmed with nested PCR using S. homoeocarpa specific primers. A sensitive nested PCR assay detected S. homoeocarpa contamination in eight of twelve (75%) commercial seed lots. Seed source, but not cultivar or resistance to dollar spot, influenced contamination by S. homoeocarpa. Overall, this research suggests that seeds are a potential source of initial inoculum for dollar spot epidemics and presents the need for further research in this area.

Figure 1. Sclerotinia homoeocarpa isolation and weather data for pathogen overwintering studies.

Roots and shoots were collected from symptomatic and asymptomatic turf in the fall of each year to determine starting S. homoeocarpa populations. The same areas were resampled in the spring to assess pathogen survival. ANOVA values for season, treatment, and season by treatment effects are reported for each year. Weather data includes daily air temperature (dashed line) and precipitation (solid line) averages. The average temperature in 2011–2012 was c. 4°C higher than in the other two years and a major snow events occurred in early November 2011–2012 and early February of 2012–2013 (arrows).

Figure 2. Stroma on creeping bentgrass leaf blades and its effect on isolation on S. homoeocarpa.

A, typical stroma-like material observed on a turfgrass leaf blade was brown in color and appeared as flaky, plate-like material. B, The percentage of CRB leaf blades with and without visible stroma from which S. homoeocarpa was isolated. P-value from ANOVA using the binary distribution for presence/absence of S. homoeocarpa and α = 0.05. C, Average diameter of S. homoeocarpa colonies 48 h after plating leaf blades with or without visible stroma. P-value is from ANOVA with the normal distribution and α = 0.05. Error bars represent ± one standard error of the mean (n = 5).

Figure 3. Results of nested PCR detection of Sclerotinia homoeocarpa DNA in creeping bentgrass commercial seed lots.

Columns reflect the average proportion of samples positive for S. homoeocarpa contamination across the two lots of each cultivar. Shading indicates cultivar source: ‘96-2’, ‘Focus,’ and ‘SR1150’ were from a single source while ‘Shark,’ ‘Penncross,’ and ‘Declaration’ were from different seed distributors. Error bars represent ± one standard error of the mean. Source and cv(source) P-values are from ANOVA of the data with the normal distribution and α = 0.05. The P-value for R versus S cultivars was obtained from a pre-planned orthogonal contrast.