The Residual Effect of Mustard (Brassica juncea L. Czern.) Seed Meal for Suppression of Sclerotinia Homoeocarpa.

Dollar spot (Sclerotinia homoeocarpa F.T. Bennett) is a common turfgrass disease in the United States. Repeated applications of fungicides to turf over the past 40 years have led to dollar spot resistance to almost all registered materials. Consequently, it has become necessary to develop environmentally friendly bio-pesticides to reduce dependence on ineffective conventional fungicides.  Early reports found that volatile compound isothiocyanates (ITCs), derived from glucosinolates (GSLs) contained in mustard (Brassica juncea L. Czern.) seed meal (MSM), effectively suppressed dollar spot under field conditions. However, the residual effect of isothiocyanates was short-lived, and more than half of the ITCs were lost within days. Our own preliminary studies provided evidence that mixtures of the original MSM with deheated or autoclaved meals, which contain inactivated myrosinase (the enzyme that catalyzes  hydrolysis of GSLs to ITCs), resulted in prolonged suppression of pathogen growth. Therefore, the objective of this study was to explore the potential of mixing original and deheated or autoclaved MSM to extend residual efficacy of MSM. Two in vitro studies were initially performed to identify effective MSM rates for S. homoeocarpa suppression.  Original MSM at rates equivalent to 0, 5, 10, 15, 20, 40, 60, 80 and 100 kg/ha were applied to the surface of PDA (Potato Dextrose Agar) medium in petri dishes inoculated with S. homoeocarpa. Chlorothalonil applied at the label rate was included as a treatment as well.  Plates were incubated at 24°C in dark and pathogen growth was measured daily for 14 days.  The experiment was a completely random design with 8 replications and repeated once.  Results showed that MSM applied at or below 40 kg/ha did not reduce pathogen growth significantly compared to growth on non-amended PDA.  MSM applied at 60 kg/ha or above suppressed pathogen growth for more than 1 week. MSM applied at highest rate of 100 kg/ha appeared to be lethal to S. homoeocarpa, and no growth was observed over 2 weeks of incubation.  Growth was suppressed for an additional 14 days after transferring the pathogen to fresh PDA without MSM. Pathogen growth was suppressed by chlorothalonil during the 2-week incubation period.  However, growth resumed when transferred to non-amended PDA. Based on the results, a second experiment has been established to evaluate pathogen suppression in relation to mixing ratios of original and deheated or autoclaved meals. Results of this experiment will be presented.