Early Blight on Potatoes

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. It appears first on older foliage, or on stressed or senescing plants. Severely infected leaves can turn yellow and drop. Infected tubers show a brown, corky dry rot. 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 have adapted the ‘TomCast’ model for predicting conditions on Potato which lead to Early Blight infection. This model is available on the Pest Prophet app.

Potato Early Blight Risk Model as a Management Tool

The adapted TomCast model is a useful tool to make smart decisions regarding preventative fungicide sprays based on the weather and optimum temperatures for reproduction. For Potatoes, the temperature threshold is lowered to 10C compared to the version of the model used for tomatoes. The model calculates DSV (Daily Severity Value) based on leaf wetness duration and temperature. These DSV’s should be counted daily and, for potatoes, once 10 DSVs have accumulated, a preventative fungicide spray should be made. This resets the model until  10 DSVs have accumulated again.

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 10-15 DSVs have accumulated, ( or 25 DSV if planted before May 20), or June 15 if DSV threshold has not been met (Dates for northern hemisphere).

Calculating Daily 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.

Meto, et al. 2020. Modification of the TOMCAST Model with Aerobiological Data for Management of Potato Early Blight. Agronomy 2020, 10(12), 1872