Cultural Control Methods that Effect the Development and Spread of Corynespora cassiicola (Berk. & Curt.) Wei on African violet (Saintpaulia ionantha Wendl.)

In recent years, a large commercial grower of African violets (Saintpaulia ionantha Wendl.) in middle Tennessee has experienced epidemics of Corynespora leaf spot caused by Corynespora cassiicola (Berk. & Curt.) Wei (Alan Windham, personal communication). Symptoms of Corynespora leaf spot include rapidly expanding circular lesions on the surface of the leaves and petioles. The disease occurs in propagation material and mature plants of S. ionantha which result in thousands of plants being discarded daily. The objectives of this research were to: 1) determine if irrigation methods affected disease severity, 2) to determine if fungicidal spray intervals could be extended beyond the recommended two week interval, and 3) to determine if leaf age affects the susceptibility of S. ionantha to C. cassiicola.

Three irrigation treatments (drip, mist, and ebb & flow) were evaluated for their effect on disease severity of C. cassiicola. Over a seven week period, plants were observed for the presence of lesions on leaves and petioles. This experiment was repeated four times with four to six replications per repetition. Disease severity was not significantly different in the three irrigation treatments in reducing the spread of C. cassiicola.

Three fungicides (propiconazole, thiophanate-methyl, chlorothalonil) and a water control were applied to symptomatic S. ionantha. Leaves were collected for eight weeks. Overall, leaves sprayed with the thiophanate-methyl treatment produced the lowest amount of sporulation and isolate growth, but no treatment was effective in inhibiting the growth of C. cassiicola in S. ionantha tissue.

The effect of leaf age on disease susceptibility was evaluated using three stages of S. ionantha leaves: juvenile, mature, and senescing. Lesion size was larger on juvenile and senescing leaves than on mature leaves.