Submitted to: International Plant Protection Congress
Publication Type: Proceedings
Publication Acceptance Date: 4/29/1998
Publication Date: N/A
Citation: Interpretive Summary: In the Lower Rio Grande Valley of Texas (LRGV), Didymella bryoniae commonly causes lesions in the crown and foliage but rarely on the melon fruit. The spring of 1997 was unusually wet and despite routine benomyl applications, a severe gummy stem blight epidemic occurred resulting in numerous corky- brown lesions (1-3 mm diameter) below the epidermis of cantaloupe fruit. Overall, fruit losses for the spring-1997 season were approximately 68.4% of the cantaloupe crop, amounting to an estimated $15 million loss for south Texas producers. It was determined that greater than 90% of the fungal isolates were benomyl resistant. As a result, several fungicides were evaluated for the control of the gummy stem blight pathogen. The fungicides CGA-245704, Quadris, or Bravo provided significant control of the disease as compared to benomyl and control treatments. In addition, higher marketable yields were obtained in treatments receiving CGA-245704, Quadris, or Bravo. Alternating these fungicide compounds is presently recommended for the control of gummy stem blight of melons in the LRGV. This also reduces the likelihood of developing resistance to these compounds.
Technical Abstract: In vitro tests indicated that the EC50 for benomyl for mycelial growth was greater than 5.0 ug/ml for 92% of the D. bryoniae isolates 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 rdisease 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 yeild, 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.