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Tomato/ Potato Early Blight Update

Posted: August 7, 2012

Early blight is showing up in commercial tomato fields as well as in home gardens.

Early blight, caused by the fungus Alternaria solani, develops under warm temperatures (75 to 85°F) and high relative humidity, heavy dews or significant rainfall. The lesions are initially small, brown to black in color and may be surrounded by a yellow halo. As the disease progresses, larger lesions will develop the characteristic concentric ring pattern.

Managing early blight includes the use of practices such as crop rotation, good nitrogen management, as well as minimizing leaf wetness. However, it is not uncommon for growers to also use fungicides for improved in-season management. The TOM-CAST disease forecasting model is a tool for helping tomato/potato growers decide when to apply fungicides for managing early blight as well as Septoria leaf spot (tomato only). The model uses number of hours of leaf wetness per day and the average temperature during that wetness period to assign disease severity values (DSV) on a scale of 0 to 4. A DSV of 0 indicates that the environmental conditions are not favorable for the early blight development while a DSV of 4 indicates very favorable conditions. The first fungicide application is recommended when a total of 35 DSV have accumulated since planting and then subsequent sprays are recommended after the accumulation of an additional18 DSV.

Below are the daily, weekly and total summed disease severity values for 16 locations across Pennsylvania. The TOM-CAST model is run using interpolated atmospheric weather data by ZedX, Inc. (not specific field-based weather stations). Since environment varies, sometimes within relatively short distances, the spray recommendation information should be considered in combination with your local environmental conditions.

The majority of locations have already reached the 35 cumulative DSV threshold to trigger the application of a fungicide. Please consult the 2012 Pennsylvania Vegetable Production Guide for specific product and rate recommendations (http://extension.psu.edu/vegetable-fruit/).

Keep in mind that for potato early blight inoculum is typically present in the field around July 4th. For earlier planted cultivars this can coincide with the transition of the crop from flowering to tuber formation, a more susceptible stage in the phenology of the crop. For tomatoes, once any fruit start to ripen, regular fungicide applications may be warranted.

Table 1. Actual daily disease severity values (DSV) from July 31 through August 6, forecasted DSV for Aug 7 through Aug 10 and total accumulated DSVs since June 1, 2011 for 16 locations across Pennsylvania.
Town County August daily DSVs





Total weekly accumulated DSV Forecasted Aug DSVs


Total accumulated DSV since June 1


31 1 2 3 4 5 6
7 8 9 10
Fairview Erie 0 0 0 0 0 0 1 1 0 0 0 2 28
Corry Erie 3 1 2 1 0 1 2 10 0 0 1 3 66
Sweden Valley Potter 0 1 2 2 0 1 3 9 0 0 0 0 49
Butler Butler 3 0 1 0 0 1 3 8 0 0 2 2 53
Finleyville Washington 0 0 1 0 0 0 2 3 0 0 0 1 41
Loretto Cambria 0 2 2 1 1 1 3 10 0 0 0 0 64
Rock Springs Centre 0 1 2 1 1 1 2 8 0 0 0 0 55
Jersey Shore Lycoming 0 1 2 1 0 1 3 8 0 0 0 2 42
Clarks Summit Lackawanna 2 2 1 2 2 2 3 14 0 0 2 3 80
Wyoming Valley Luzerne 3 3 1 1 1 1 3 13 0 0 2 3 77
Germansville Lehigh 3 3 1 2 1 1 3 14 0 1 2 3 61
Kutztown Berks 3 3 3 3 0 0 3 15 0 1 1 3 54
Ringtown Schuylkill 2 3 3 3 3 3 2 19 0 0 2 3 80
Gratz Dauphin 1 1 1 1 1 1 3 9 0 1 1 3 57
Leola Lancaster 1 2 2 2 1 0 2 10 0 1 1 3 48
Landisville Lancaster 0 0 0 0 0 0 1 1 0 0 0 2 44