Management of Fungal Plant Pathogens through Advanced Diagnostics and with Biorational Fungicides

Due to the emergence of new pathogens or more virulent strains of existing pathogens and limited effective chemical fungicide options, it is critical to find new approaches to prevent significant crop losses. This dissertation adds to current knowledge on plant disease management by focusing on the identification and/or control of both emerging and established fungal pathogens through advanced molecular diagnostics and exploration of alternative options to manage plant diseases. A diagnostic tool was developed to identify an aggressive Neopestalotiopsis sp. affecting strawberries and calcium propionate (CaP), a Generally Recognized As Safe (GRAS) product, was investigated for its mode of action and ability to control fungal pathogens and the diseases they cause. Chapter 1 describes the development of a molecular method that accurately and rapidly distinguished an aggressive Neopestalotiopsis sp. impacting strawberries from other less aggressive Pestalotia spp., often found on symptomatic strawberry leaves. The study identified a mutation in the β-tubulin gene unique to the aggressive Neopestalotiopsis sp. and enabled the development of a RFLP test based on restriction enzyme BsaWI that selectively cut the β-tubulin gene into uniquely sized fragments. This method improved the identification of the aggressive Neopestalotiopsis sp. strain, which until then relied on visual disease symptoms and fungal signs for diagnosis. Alongside adoption of enhanced diagnostic tools, the exploration of safer fungicides is needed to reduce pesticide risk. Chapter 2 discusses the efficacy and mode of action of CaP, an FDA-approved GRAS product to preserve bread and meats, in managing botrytis blight of ornamental flowers. Results showed suppression of mycelia of fungal plant pathogen, including Botrytis cinerea, Colletotrichum acutatum, Monilinia fructicola, and Alternaria alternata, that were sensitive and resistant to various chemical classes of fungicides in vitro. Results suggested CaP had a mode of action different from Fungicide Resistance Action Committee (FRAC) codes 1, 2, 3, 7, 9, 11, 12, and 17. The fungicide suppressed botrytis blight of petunia and cut roses caused by B. cinerea at a concentration of 0.1%, pH 6.0, but higher concentrations did not improve performance. At that concentration, CaP inhibited infection cushion formation by about 50%. The phytotoxic effects at higher concentrations, combined with its fungistatic rather than fungicidal properties, highlighted the need to find ways to enhance the efficacy of CaP in agricultural use. Chapter 3 showed that combining CaP 0.1% (pH 6.0) with commercially available biological fungicides, Howler EVO (a.i. Pseudomonas chlororaphis metabolites) and Theia (a.i. Bacillus subtilis), suppressed fungicide-induced atrB gene expression resulting in synergistic effects against botrytis fruit rot of grape caused by B. cinerea non-MDR isolates. This gene encodes a cell membrane-bound ATP-Binding Cassette transporter and is a known genetic determinant of fludioxonil resistance in B. cinerea. The identification of synergy partners may open a new avenue for CaP to be used in agriculture and to boost the efficacy of biological control products. Chapter 4 presents first field data evaluating the efficacy of CaP against fungal diseases. In this study, CaP was used for peach scab control. At 0.8% (pH 5.1-5.7), it significantly reduced disease incidence and severity over two years, with no observed phytotoxicity. Unlike its performance at concentrations higher than 0.1 % on ornamental flowers, CaP showed no phytotoxicity and was effective against peach scab, likely due to the tougher peach fruit and leaf tissues compared to more fragile flower tissue. These findings suggest that CaP may be useful to reduce pesticide risk if integrated in conventional spray programs.