Evaluation of postinfection control of citrus scab and melanose with benomyl, fenbuconazole, and azoxystrobin

Citrus scab, caused by Elsinoe fawcettii, and melanose, caused by Diaporthe citri, are serious diseases of citrus in Florida that reduce the exterior quality of fruits produced for the fresh market. The availability of fungicides with postinfection activity could increase the flexibility of growers in managing these diseases. The effectiveness of benomyl at 0.96 g a.i./liter, fenbuconazole at 120 mg a.i./liter, and azoxystrobin at 235 mg a.i./liter was evaluated on rough lemon seedlings for scab and on grapefruit for melanose. Potted seedlings in the greenhouse were pruned to stimulate formation of uniform susceptible shoots, which were inoculated and kept moist for 16 h. Fungicides were applied prior to inoculation and at various intervals from 16- to 96-h postinoculation and subsequently evaluated for disease intensity. All three fungicides were effective as preinoculation sprays for scab control. Benomyl was effective if applied up to 72 h after inoculation, and fenbuconazole and azoxystrobin were effective if applied within 16 to 48 h after inoculation. For melanose, azoxystrobin was highly effective as a preventive spray but benomyl and fenbuconazole were not. None of the fungicides was very effective for melanose control as postinfection treatments. Use of postinfection sprays under field conditions appears to be promising for scab control but not for melanose control.