Assessment of Disease Suppression in Organic Transitional Cropping Systems.

In order for agricultural land area to be certified for organic production, current USDA rules require that it goes through a three-year transition period prior to receiving certification in order to minimize the effects of previously applied synthetic pesticides or fertilizers and to improve “soil health”.  Methods for enhancing soil health often include applications of organic matter to the soil.  Adding organic matter to soils has been shown to lower the severity of soilborne/root-infecting diseases, a phenomenon known as organic matter mediated suppression. The aim of this study was to investigate the effect of adding organic matter, through cropping systems (pasture, cash grain, vegetable) and direct organic amendments (cover crops only, cover crops plus manure or compost), on soil suppressiveness during the certification transition period.   Disease suppression of soybean pathogens, Rhizoctonia solani and Fusarium virguliforme was evaluated under the controlled environmental condition during transition, and field evaluations were conducted after transition.  Naturally occurring diseases in the field were evaluated annually in order to compare the organic amendment effects.  In addition, population levels of the Pseudomonadaceae were evaluated for suitability as a suppressiveness indicator.

In greenhouse bioassays, disease levels were consistently higher on plants grown in autoclaved, infested field soil than in the same soil that was not sterilized, verifying the biological nature of the disease suppression studied.  By using root morphology, along with severity ratings, my results provide evidence that adopting pasture as the transitional cropping system may promote organic matter mediated suppression of diseases caused by facultative saprophytes (e.g., F. virguliforme), while encouraging diseases caused by facultative parasites (e.g., R. solani).  Over the three year transition period, there was an increase in soil suppressiveness based on greenhouse bioassay data.  In the R. solani infested field assay, manure amendments resulted in higher emergence, particularly in the cash grain system.  Growers should decide the transition strategies best suited for each field based on their knowledge of the disease history of the field. 

The annual evaluation of naturally occurring diseases showed that diseases caused by biotrophic pathogens were promoted by manure amendments, as shown in higher severity levels of rust on corn, rust on perennial orchard grass, and powdery mildew on winter squash.  A higher incidence of tomato anthracnose was observed in the plots previously transitioned in the vegetable system, resulting in a lower percentage of marketable fruit.  The severity of bacterial pustule on the post-transition soybean crop was highest in the pasture system plots, despite soybeans having the highest yields in those same plots.  In addition, population levels of members of the Pseudomonadaceae in the soil were quantified using a 16S rDNA fragment as a target in a qPCR assay. No significant differences in population levels of this family of bacteria were observed in an individual year as a result of the transition strategies.  However, over the course of the transition period, the population level doubled in all treatment plots, indicating that soil health and suppressiveness were improved.  More study is needed to relate this measurement to the other existing indicators for soil suppressiveness, particularly enzymatic activities.