Genetic diversity of the fungal pathogen Corynespora cassiicola and its fungicide resistance
Type of DegreePhD Dissertation
Entomology and Plant Pathology
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Corynespora cassiicola C.T. Wei is a widespread phytopathogenic fungus that causes disease in tropical and subtropical regions in up to 400 plant species, including fruits, vegetables, ornamentals, forestry, and row crops. Disease symptoms can be observed on plant leaves, stems, roots, flowers, and fruits. Incidence of the disease, target spot, as well as severity has become more frequent across cotton and soybean producing countries, and significant yield losses can occur if it is not properly controlled. The fungus has a parasitic lifestyle coupled with a saprophytic and endophytic lifestyle which complicates disease management. The overall objective of this dissertation was to better understand the pathogen, its interaction with soybean plants, and risks associated with chemical control. The first chapter reviewed the literature on C. cassiicola focused on cotton and soybean, including pathogen biology and genetic diversity, and a detailed discussion on two essential IPM strategies emphasizing their importance to overcome the disease. The second chapter investigated the genetic diversity of C. cassiicola isolates from symptomatic leaves of cotton and soybean by studying the morphology, pathogenicity, and molecular phylogeny based on cassiicolin-encoding genes and four loci. The third chapter revealed the impact of C. cassiicola diversity on screening for resistance to target spot on soybean by comparing two screening methods: leaf wilting bioassay and plant inoculation. The fourth chapter determined the sensitivity profile of C. cassiicola isolates from cotton and soybean to five commercial fungicides, and if there is a fitness loss on C. cassiicola QoI-resistant isolates. Finally, the fifth chapter first reported C. cassiicola isolates from soybean in the United States with the G143A mutation in the cytochrome b gene that confers resistance to QoI fungicides. Altogether, the results of this dissertation provide useful insights for research on the management of target spot disease to reduce the risk of epidemics and yield losses.