Bioassay of Pythium and Rhizoctonia in a Peat-based Substrate Amended with Pine Wood Chip Aggregates

Substrates used for the production of horticultural crops vary in their occurrence and severity of root rot diseases.  A bioassay of two common soil-borne pathogens was conducted on peat-based substrates amended with either 20% perlite or 20% pine wood chips (PWC) to determine disease severity.  The PWC were produced by chipping freshly harvested loblolly pine trees (Pinus taeda) and hammer milling through a 6.35 mm screen. Pythium ultimum and Rhizoctonia solani were cultured and inoculated separately via pulverized rice grains into both substrates. Pythium was inoculated at the rate of 0.1 g/L substrate and Rhizoctonia was inoculated at the rate of 0.05 g/L substrate. Uninoculated controls of each substrate were also evaluated. Containers (1.3 L) were filled with each substrate and 10 cucumber (Cucumuis sativus ‘Straight Eight’) seeds were evenly spaced and direct sown in the containers at a depth of 1 cm. Disease severity was assessed on each seedling 17 days after planting using the following scale: 1) healthy vigorous seeding; 2) seedling emerged but stunted; 3) seedling emerged and diseased; and 4) seedling dead or did not germinate.  Ratings of all seedlings were then averaged within each treatment. Disease severity ratings were similar in uninoculated controls of both perlite (1.13) and PWC (1.08) amended substrates, which indicate no increased disease occurrence/introduction from the fresh PWC component.  Disease severity rating of Pythium was highest in the perlite amended substrate (3.05) compared to the PWC amended substrate (1.20). The PWC rating (1.20) was similar to the uninoculated controls. Disease severity rating of Rhizoctonia was highest in perlite amended substrates (2.94) compared to the PWC amended substrate (2.58) demonstrating slight disease suppressiveness. Results indicate potential suppressiveness of both Pythium and Rhizoctonia in greenhouse substrates amended with PWC but further screenings with floriculture crops is needed.  Future investigations are also needed to assess how disease occurrence may change with varying rates of PWC aggregates.