Survey of Fungal Pathogens Affecting Watermelon Production throughout Texas Growing Regions

Texas ranks 3rd in US watermelon production, with 40% of Texas counties generating $83.2 million in 2019. Fungal pathogens such as Stagonosporsis spp. and Fusarium spp. can cause significant revenue loss annually. Fungal watermelon pathogens are not well documented in Texas, and the impact of varying environmental conditions on those fungi are not well understood. A survey was conducted to document fungal pathogens in Texas during the 2020 growing season. Thirty symptomatic stem, leaf, and/or root samples were collected from 5 fields in 5 Texas counties and assayed on 25% PDA+ antibiotics. Isolates were identified morphologically and confirmed using PCR. Stagonosporopsis citrulli, Fusarium proliferatum, Fusarium brachygibbosum, Fusarium incarnatum, Bipolaris spp., Alternaria alternata, and Rhizopus oryzae were isolated. Pathogenicity tests were conducted using representative isolates of S. citrulli, F. proliferatum, F, brachygibbosum, F. incarnatum, Bipolaris spp., and A. alternata. All isolates caused disease symptoms on watermelon, though S. citrulli and the 3 Fusarium spp. inoculated plants consistently had the highest disease severity ratings. Environmental data (humidity, temperature, dew point, air pressure, wind speed, wind gust, and precipitation) were collected from weather stations near each location from 6 weeks prior to until time of collection. These data (components) were processed using Principal Components Analysis to determine the most influential factors of the environments and which environments were the most unique. PCA showed humidity and dew point were the most influential components for all counties. Three of the 5 counties had no significant differences among their environmental components, while Maverick and Glasscock counties were significantly different from the other 3. Glasscock county had the most significant differences among the environmental components from all other counties sampled. Glasscock had the lowest total recovery of fungi, and it can be inferred based on the data that environmental components had a role in this outcome. The fungi isolated and documented as pathogenic in this work are more widespread in Texas than previously known and may be associated with reduced plant vigor and reduced yield quality and quantity in the field. More research is needed to determine each pathogen’s production and economic impacts on Texas watermelon