Early Blight on Tomatoes

Early Blight, cause by the pathogenic fungus Alternaria solani is a very common and important disease on tomatoes. Early Blight first affects foliage on tomato plants, which decreases the plant’s photosynthetic activity and can lower yield. The Early Blight fungus can also directly infect tomatoes before harvest, causing a direct impact on yield.

Early Blight is easy to identify on leaves, small brown lesions starting on the bottom sides of the leaves. As it spreads, they become target like rings. These symptoms indicate that infection of the plant has already occurred and the disease may be difficult to control at this stage.

Most fungicides labelled for Early 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 in the United States and Canada have created the ‘TomCast’ model for predicting conditions on Tomato which lead to Early Blight infection. This model is available on the Pest Prophet app.

Tomato Early Blight Risk Model as a Management Tool

The TomCast 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 DSV (Daily Severity Value) based on leaf wetness duration and temperature. These DSV’s should be counted daily and once 15-20 DSVs have accumulated, a preventative fungicide spray should be made. This resets the model until 15-20 DSVs have accumulated again. The exact amount of DSV intervals should be determined by the grower. Using 15 DSV as an interval will typically provide better control, but using 20 DSV may also work well enough, while reducing the overall number of sprays. Susceptibility of the variety, history of past infection, and grower preference should determine the DSV interval.

Initiating the Risk Index Model

For producing areas where Early 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. First spray should be after 15-20 DSVs have accumulated, ( or 25 DSV if planted before May 20), or June 15 if DSV threshold has not been met (for the northern hemisphere).

Calculating Daily DSV

The following table should be used to calculate DSV:

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:

Pitblado, R. E. 1992. The development and implementation of TOM-CAST: A weather-timed fungicide spray program for field tomatoes. Ministry of Agriculture and Food, Ontario, Canada.

Madden L., Pennypacker, S. P., and McNab, A. A. 1978. FAST, a forecast system for Alternaria solani on tomato. Phytopathology 68:1354-1358.