Alternaria Leaf Blight on Carrots

Alternaria Leaf Blight, cause by the pathogenic fungus Alternaria dauci is a very common and important disease on carrots. Alternaria Leaf Blight first affects leaf blades and petioles, causing brown lesions that can eventually kill the entire leaves. 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 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 have adapted the ‘TomCast’ model for predicting conditions on Carrots which lead to Alternaria Leaf Blight infection. This model is available on the Pest Prophet app.

Tomato Alternaria Leaf 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. The model calculates DSV (Daily Severity Value) based on leaf wetness duration and temperature. These DSV’s should be counted daily and once 20 DSVs have accumulated, a preventative fungicide spray should be made. This resets the model until 20 DSVs 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 after July 5 (in the Northern Hemisphere), or after 35 DSVs have accumulated. Treatment should continue until 7-10 days before harvest.

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.

Bolkan, H. A., and Reinert, W. R. 1994. Developing and implementing IPM strategies to assist farmers: an industry approach. Plant Dis. 78:545-550.