Gummy Stem Insensitivity to Quadris and Abound

Syngenta warning on reduced sensitivity to azoxystrobin (Quadris®) in some Didymella bryoniae isolates causing gummy stem blight disease in the United States.

Isolates of Didymella bryoniae (causal agent of gummy stem blight disease on cucurbits) with reduced sensitivity to the fungicide azoxystrobin (Quadris®) have been identified. Isolates were obtained from areas where the fungicide did not control the disease at satisfactory levels (Sussex county, Delaware; Wicomico county, Maryland and Union/Cordele counties, Georgia). The reduced sensitivity has been detected in laboratory tests (in-vitro). Additional tests are being conducted in plants (in-vivo) to validate the results obtained today. Extensive resistance monitoring is scheduled in 2001 in the US and Mexico to determine the frequency of these isolates and their impact on the control of the disease.

Syngenta recommends the following agronomic practices for cucurbits disease control:

· In the locations where Didymella bryoniae isolates with reduced sensitivity have been detected, Quadris® is not recommended for gummy stem blight control. Fungicides with a different mode of action than Quadris®, such as chlorothalonil are recommended.

· In other areas where isolates of D. bryoniae with reduced sensitivity have not been detected, Quadris® can be used at the recommended commercial rate to control the disease. Preventative applications are recommended, alternating after one spray with fungicides that have a different mode of action, such as chlorothalonil.

· Quadris® at the recommended rate should be used to control powdery mildew, downy mildew and belly rot. It is recommended to alternate Quadris® after 1 spray with fungicides with different mode of action, such as chlorothalonil.