Evaluation of virulence and pathogenicity of Alternaria patula on French marigold (Tagetes patula)

Alternaria patula, the cause of French marigold leaf black spot and flower blight, was first isolated from seeds of French marigold cv. Queen Sophia. It is described as a new species of Alternaria and has a considerable morphological variation with a preferential pathogenicity to Asteraceae, Solanaceae and Cucurbitaceae plants. Alternaria patula produces an array of pectin depolymerases that can break 1,4-α-glycosidic bonds either by hydrolysis of polygalacturonases (PG, E.C. 3.2.1.15) or via trans-elimination of pectate lyases (PL, E.C. 4.2.2.2) and pectin lyases (PNL, E.C. 4.2.2.10). This study is the first to emphasize the variability and significance to pathogenesis of the pectinolytic enzymes of A. patula that target various pectic polymers structures during host tissue invasion. Alternaria patula also produced zinniol derivatives as non-host-specific toxins (nHSTs), albeit without phytotoxic symptoms in French marigold. The management of Alternaria blight caused by A. patula via the application of pyrifenox and antagonistic Bacillus amyloliquefaciens effectively reduced disease severity, without adverse effects on French marigold in both in vitro and in vivo bioassays.