Author’s note: This article is adapted from a previous article published in Vegetable Crops Hotline, Issue 598, by Dr. Dan Egel
Many growers have already finished transplanting watermelons and cantaloupes, while others completed planting a few weeks ago and may be preparing to begin fungicide applications. In many areas of Indiana, rainfall has been relatively high, which can create favorable conditions for disease development. When possible, fungicide applications should be initiated preventively. If you plan to use the Melcast system to schedule applications, be sure to record your first spray date so that EFI (Environmental Favorability Index) values can be used effectively. Instructions for using and recording EFI values can be found in Extension bulletin BP-67-W, which is available on the MELCAST website. You may also contact me if you would like a copy or need assistance.
A common question among growers is when and how fungicides should be applied. Applying fungicides according to a weather-based system is straightforward for cantaloupe and watermelon growers. MELCAST was developed at Purdue University by Dr. Rick Latin to help growers apply foliar fungicides to control Alternaria leaf blight, anthracnose, and gummy stem blight. When MELCAST is followed, fungicides are applied when needed most, based on leaf moisture and temperature. The system was managed by Dr. Dan Egel for several decades, and I am now pleased to continue managing it. Stay alert for the weekly updates sent by the Southwest Purdue Agricultural Program. If you would like to be added to the MELCAST email list, please contact me or Barbara Joyner at joynerb@purdue.edu.
The MELCAST program uses weather information from one of 15 sites located across Indiana. It also serves growers in Georgia, Michigan, and Ohio. Cantaloupe and watermelon growers should farm within approximately 50 miles of a MELCAST site. If rain events, dew formation, and temperatures at one of the MELCAST sites are similar to those at your farm, MELCAST should be effective for you. Growers using MELCAST typically apply foliar fungicides every 14 days unless weather thresholds described below indicate that an application should be made sooner. A step-by-step guide to using MELCAST is provided below:
- Start fungicide application at or before vines touch within a row. Applications can begin earlier if disease pressure is high.
- Check the EFI value for the day of fungicide application.
- Calculate the threshold for the next application by adding 20 (cantaloupe) or 35 (watermelon) to the EFI value in step 2. It is important for cantaloupe and watermelon growers to use the EFI values designed for their crop. To get a MELCAST calendar to keep track of EFI values, call Cesar Escalante or download the spreadsheet from the MELCAST website.
- Apply the next fungicide application 14 days after the first, or sooner if the EFI threshold has been reached.
- Check the EFI values on the day you make your next fungicide application and recalculate the threshold for the next application.
It is best to apply fungicides before the threshold is reached rather than after. For example, if you are a watermelon grower and the EFI threshold has reached 33 with rain expected soon, consider applying a fungicide ahead of time. Use the thresholds of 20 and 35 EFI units as general guidelines, and consider using a lower threshold if disease pressure is high. Finally, note that fungicide applications for downy mildew and powdery mildew cannot be scheduled using MELCAST. Likewise, applications for bacterial diseases cannot be scheduled based on MELCAST.