FUNGICIDAL SENSITIVITY OF DIDYMELLA BRYONIAE ISOLATES FROM SOUTH TEXAS
ME MILLER1, T ISAKEIT1, BD BRUTON2 and JX ZHANG2
1Texas A&M University, Weslaco, TX 78596, USA; 2USDA-ARS, Lane, OK 74555, USA
Background and objectives In south Texas, Didymella bryoniae (Auersw.) Rehm commonly causes lesions in the crown and foliage of melons (Cucumis melo L.) but rarely on fruit. Despite routine benomyl applications, a severe gummy stem blight (GSB) epidemic occurred in Spring-1997 resulting in numerous corky-brown lesions (1-3 mm dia.) below the epidermis of cantaloupe fruit. Overall, cantaloupe fruit losses were approximately 68%, amounting to an estimated US$15,000,000 loss. The purposes of these studies were 1) to describe and document cantaloupe fruit symptoms and losses caused by D. bryoniae; 2) to determine the sensitivity of south Texas isolates of the fungus to benomyl; and 3) to screen fungicides for their efficacy to control D. bryoniae.
Materials and methods D. bryoniae isolates were obtained from fruit, stems and roots of cantaloupe, honeydew and watermelon from south Texas. Benomyl sensitivity was determined by growing 137 isolates on PDA amended with 1.0, 5.0 or 50.0 µg/ml and measuring growth. Eight fungicide were tested in vitro at 1.0, 1.0, 5.0 and 50.0 µg/ml for their effect on mycelial growth of 15 D. bryoniae isolates. Ten fungicides were also tested for their efficacy to control GSB under field conditions. Plants were inoculated twice with 376,00 and 300,000 viable conidia/ml, respectively, of an aqueous suspension of a D. bryoniae isolate resistant to benomyl. Fungicides were applied four times at weekly intervals. Data were recorded on incidence of stem and fruit lesions, foliage disease severity, and marketable fruit yield.
Results and conclusions In vitro tests indicated that the EC50 for benomyl for mycelial growth was >5.0 µg/ml for 92% of the D. bryoniaeisolates from south Texas, indicating a high level of resistance in the fungal population to benomyl. Most isolates that were resistant to benomyl were also resistant to thiabendazole. Reports of resistance of D. bryoniae isolates to benzimidazole fungicides appeared in 1983 in Europe  and Keinath and Zitter  suggested that the occurrence of pathogenic, benomyl-insensitive D. bryoniae isolates might limit the usefulness of benzimidazole type fungicides for GSB management. This data indicates that this fungicide group is no longer capable of effectively controlling GSB in south Texas.
Cantaloupe plants treated with azoxystrobin at 224.0 g/ha had significantly less (p=0.05) fruit with GSB lesions, higher marketable yield, lower foliar disease ratings, and fewer stem lesions than plants treated with other fungicides and control in field studies. Chlorothalonil at 2.5 kg/ha and cyprodinil at 280.0 g/ha also effectively controlled GSB on fruit, stems, and foliage significantly better than most of the other fungicides and control, but to a lesser degree than azoxystrobin. Marketable yield, percentage of fruit with lesions, percentage of plants with stem lesions, and foliar disease ratings were not significantly different on plants treated with benomyl as compared with the control.