Forest industry byproducts improve soil quality and increase pepper growth in three soils infested with Phythophthora blight
Phytophthora blight is a serious threat to the Midwest vegetable industry, because the oomycete pathogen responsible for this disease, Phytophthora capsici, has a wide host range, can spread quickly in fields, and produces resilient oospores that can survive in soil for years. Phytophthora capsici has become resistant to commonly used fungicides and resistant crop varieties are rare. Amending soil with complex organic substrates has potential to improve soil quality and suppress soil-borne pathogens including P. capsici. Indiana has a significant forest industry with many residual products that could be used as locally available amendments to meet this goal. However, the mechanisms mediating how amendments induce disease suppressive activity in soil are not well understood, which currently limits their practical application as a disease control strategy. The objective of the experiments described in this thesis were to: (i) determine whether commercially available forest industry byproducts with different compositions and expected rates of decomposition, could suppress Phytophthora blight in pepper, and (ii) determine whether changes in soil physiochemical and biological properties were correlated with the suppressive activity of these amendments. In the first study, four forest industry byproducts were evaluated at a rate of 1% total carbon (w/w) soil. Changes in soil moisture, microbial activity and specific microbial taxa that have previously been associated with disease suppressive activity were monitored during a one month incubation period prior to pepper transplant, and changes in soil chemical properties were quantified at the end of the trial. In the second study, five forest industry byproducts were evaluated, each at one of two rates (1% or 3% total carbon (w/w) soil), and in either a high or low organic matter soil. The soils were amended with P. capsici inoculum to ensure sufficient disease pressure, and a variety of soil physiochemical and biological factors were quantified. The amendments tested in these trials included two biochar products and a locally-available compost derived from woody materials, and kraft pine lignin and sawdust generated as direct byproducts of the forest industry. Both experiments were conducted in the greenhouse using field soil collected from sites with recent outbreaks of Phytophthora blight. Results of these studies indicate that many of the amendments altered soil physiochemical properties including soil moisture, pH, nitrogen, magnesium, potassium, and calcium availability, microbial biomass, and microbial activity, particularly in the low organic matter soil. Several amendments also improved pepper root growth, indicating that they have potential to suppress Phytophthora blight. The suppressive activity of the amendments was not consistently correlated with specific soil physiochemical and biological factors, indicating that different mechanisms may be responsible for the suppressive activity induced by the different types of amendments. Results of these studies indicate that forest industry byproducts have potential to improve soil quality and reduce Phytophthora blight, but field trials and cost-benefit analyses will need to be conducted before these products can be recommended to growers. Additional studies that document differences in the biochemical quality of the amendments and quantify changes in microbial community structure using molecular tools are recommended to better understand how these amendments induce disease suppressive activity.
Hoagland, Purdue University.
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