PENNSYLVANIA VEGETABLE DISEASE UPDATE
JUNE 25, 2013
Beth K. Gugino
Assistant Professor Vegetable Pathology
Department of Plant Pathology
and Environmental Microbiology
219 Buckhout Lab
University Park, PA 16802
Phone: 814-865-7328
Email: bkgugino@psu.edu
Web: http://extension.psu.edu/plants/vegetable-fruit
TOMATO/POTATO EARLY BLIGHT UPDATE
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 locations below have not yet reached the 35 cumulative DSV threshold to trigger the application of a
fungicide. Please consult the 2013 Pennsylvania Vegetable Production Guide for specific product and rate
recommendations (http://extension.psu.edu/plants/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 June 18 through June 24, forecasted DSV for June
25 through June 28 and total accumulated DSVs since June 1, 2013 for 16 locations across Pennsylvania.
June daily DSVs
Town
County
18
19
20
21
22
23
24
Total
weekly
accum.
DSV
Forecasted Aug
DSVs
25
26
27
28
Total
accum.
DSV June
1 to 24
Fairview
Erie
1
1
0
0
0
0
0
2
1
3
3
4
20
Corry
Erie
1
1
0
0
0
1
0
3
1
3
3
4
35
Sweden Valley
Potter
1
0
0
0
0
0
1
2
1
2
3
4
30
Butler
Butler
1
0
0
0
0
0
1
2
1
3
3
4
21
Finleyville
Washington
2
1
1
0
0
0
0
4
1
2
2
3
17
Loretto
Cambria
2
1
1
0
1
1
1
7
1
2
3
4
30
Rock Springs
Centre
1
1
0
0
0
0
1
3
0
2
3
4
22
Jersey Shore
Lycoming
0
0
0
0
0
0
0
0
0
0
3
4
11
Clarks Summit
Lackawanna
0
0
0
0
0
0
0
0
2
3
4
5
16
Wyoming Valley
Luzerne
1
0
0
0
0
0
0
1
2
3
4
5
18
Germansville
Lehigh
1
0
0
0
0
0
0
1
1
1
3
3
23
Kutztown
Berks
1
1
1
0
0
0
1
4
1
1
2
3
20
Ringtown
Schuylkill
2
1
0
0
0
0
0
3
1
1
3
3
27
Gratz
Dauphin
1
1
0
0
0
0
1
3
0
1
3
3
20
Leola
Lancaster
2
2
3
1
0
0
1
9
1
1
2
2
26
Landisville
Lancaster
1
3
3
1
0
0
1
10
1
1
2
2
27
----Information provided is intended for consideration by the user, but is not intended to be a recommendation. Production decisions
should be based on consideration of many types of information (scientific, experimental, economic, legal, etc.) available to the user.
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