Alternaria Leaf Blight on Watermelon

Alternia Leaf Blight, or Alternaria Leaf Spot, is caused by the pathogenic fungus Alternaria cucumerina is a very common and important disease on melons. Alternaria Leaf Blight symptom starts as small circular leaf spots on older leaves on cantaloupe, cucumber and watermelon. This can eventually spread to form spots on all leaves, which decreases the plant’s photosynthetic activity and can lower yield.

Most fungicides labelled for Alternaria Leaf Blight work best as a preventative application before infection occurs. To help growers know when is the optimal time to use preventative sprays, and at what intervals, researchers at Purdue University have created the ‘MelCast’ model for predicting conditions on Melons which lead to Alternaria Leaf Blight infection. This model is available on the Pest Prophet app.

Watermelon Alternaria Leaf Blight Risk Model as a Management Tool

The MelCast model is a useful tool to make smart decisions regarding preventative fungicide sprays based on the weather and optimum temperatures for reproduction. The model calculates EFI units (environmental favorability index) based on leaf wetness duration and temperature. These EFI units should be counted daily and once 35 EFI units have accumulated, a preventative fungicide spray should be made. This resets the model until 35 EFI units have accumulated again.

Initiating the Risk Index Model

For producing areas where Alternaria Leaf Blight is common, we should assume overwintering and presence of the disease. For this reason, the best time to start calculating risk, and adjusting spray intervals is based on the growth of the plants. Model should be first used immediately following planting. Initial fungicide sprays on melons should occur between the time that plants are beginning to vine and the time that vines of adjacent plants within rows begin to make contact with each other.

Calculating Daily EFI

Temperature and leaf wetness data can be obtained from in field thermometers, or temperature loggers or through weather stations placed near the field. Sensors placed within the plant canopy, at the leaf level will typically give the most accurate data, but ambient temperatures are still very useful when in-field sensors are not practical or reliable. Some weather station models will calculate daily risk. Growers seeking an option without weather stations should consider using the Pest Prophet mobile app.

To learn more about this app, and how you can get started using it for free, please visit www.pestprophet.com or start using the app.

Source:

Everts, K. L., Korir, R. C., and Newark, M. J. 2016. Re-evaluation of MelCast for fungicide scheduling in mid-Atlantic watermelon. Plant Health Prog. 17:51-52.