Role of Organic Acids in Suppression of Sclerotium rolfsii During Anaerobic Soil Disinfestation (ASD)

Anaerobic soil disinfestation (ASD) is a non-chemical method used for controlling soilborne plant pathogens. Individual elements of the ASD (also referred to as biological soil disinfestation or BSD) process, including application of organic amendments or soil saturation, have been studied for over 50 years for suppression of various soilborne plant pathogens. More recent research, primarily in the Netherlands, Japan, and the United States has been targeted at developing a soil disinfestation process based on anaerobic decomposition of labile soil amendments that can be integrated into modern horticultural production systems as a soil fumigant. The process leads to the creation of several fermentation by-products including the generation of short chain organic acids or volatile fatty acids (VFAs), other volatile compounds, and subsequent lowering of soil pH. These byproducts act as pesticides for soilborne pathogens. The saturated soil, changes in soil microbial communities, and byproducts of fermentation give rise to an environment inhospitable to many plant pathogens. Volatile fatty acids (VFAs), including acetic and n-butyric acid, are reported to play a role in the suppression of plant pathogen inoculum during anaerobic soil disinfestation (ASD), but it is unclear how VFAs affect sclerotia of Sclerotium rolfsii. To evaluate the effect of VFA, VFA concentration, soil pH and soil texture on germination of S. rolfsii sclerotia, a series of anaerobic growth chamber trials was conducted. Alongside anaerobic growth chamber trials, greenhouse trials were conducted to evaluate endemic soil populations of Trichoderma spp. following the same ASD treatments. These studies show Acetic and n-butyric acids in soil solution are likely a primary factor in suppression of germination and colonization of S. rolfsii during ASD treatment, and activity against these fungal propagules is dependent on concentration, solution pH and soil texture. Sclerotial germination was generally reduced by exposure to either acetic or n-butyric acids, and most notably reduced by VFAs in autoclaved soil (29% germination). Overall, mean germination rates in n-butyric acid were significantly lower than those of acetic acid (32% and 44%, respectively). Germination rates were reduced as acid concentration increased to 4, 8, and 16 mmol/kg concentration (56%, 38% and 19%, respectively)