TOMCAST FOR OHIO, INDIANA, & MICHIGAN

Tomato Disease Forecasting System

TOMCAST 1998 - Disease Severity Value (DSV) Hotline: 1-800-228-2905

BACKGROUND

TOMCAST (TOMato disease foreCASTing) is a computer model based on field data that attempts to predict fungal disease development, namely Early Blight, Septoria Leaf Spot and Anthracnose on tomatoes. Field placed data loggers, such as Campbell Scientific CR10's, record hourly leaf wetness and temperature, which when analyzed over a 24 hour period, may result in the formation of a Disease Severity Value (DSV); essentially an increment of disease development. As DSV accumulate, disease pressure continues to build on the crop. When the number of accumulated DSV exceed the spray interval, a fungicide application is recommended to relieve the disease pressure. Some suggested fungicide programs are to be used with TOMCAST are mentioned in a recent VegNet-Newsletter article.

TOMCAST is derived from the original F.A.S.T. (Forecasting Alternaria solani on Tomatoes) model developed by Drs. Madden, Pennypacker, and MacNab at Pennsylvania State University (PSU). The PSU F.A.S.T. model was further modified by Dr. Pitblado at the Ridgetown College in Ontario into what we now recognize as the TOMCAST model used by Ohio State University Extension.

WHAT IS A DSV?

A Disease Severity Value (DSV) is the unit of measure given to a specific increment of disease (early blight) development. In other words, a DSV is a numerical representation of how fast or slow disease (early blight) is accumulating in a tomato field. The DSV is determined by two factors; leaf wetness and temperature during the "leaf wet" hours. As the number of leaf wet hours and temperature increases, DSV accumulate at a faster rate. See the Disease Severity Value Chart below.

Conversely, when there are fewer leaf wet hours and the temperature is lower, DSV accumulate slowly if at all. When the total number of accumulated DSV exceeds a preset limit, called the spray interval or threshold, a fungicide spray is recommended to protect the foliage and fruit from disease development. The spray interval (which determines when you should spray) can range between 15-20 DSV. The exact DSV a grower should use is usually supplied by the processor and depends on the fruit quality and end use of the tomatoes. Following a 15 DSV spray interval is a conservative use of the TOMCAST system, meaning you will spray more often than a grower who uses a 19 DSV spray interval with the TOMCAST system. The tradeoff is in the number of sprays applied during the season and the potential for difference in fruit quality.

USING TOMCAST & THE DSV HOTLINE (800-228-2905)

Tomatoes grown within 10 miles of a reporting station should benefit from the disease management function of TOMCAST to help forecast early blight, Septoria, and anthracnose. Late blight forecasting is also possible from a related program, BLITECAST. If you are unfamiliar with how each forecasting system (TOMCAST & BLITECAST) work and would like more information, a thorough description can be found in the 1998 Ohio Vegetable Production Guide (The Tomato Chapter, Scroll down to the TOMCAST section).

Access to the information generated by TOMCAST is very easy We have set up a toll free DSV Hotline where the most current DSV information can be found, along with any additional bits of information such as insect reports or late blight alerts. The format is simply to call 800-228-2905 and listen or record the DSV for the station(s) of interest.

A phone number is given at the end of the recording to reach Jim Jasinski at: 937-454-5002 if there are additional questions. The DSV Hotline call can be made 24 hours a day, and usually lasts between 1-2 minutes.

If you decide to try TOMCAST this season please keep in mind three very important concepts.

One, if this is your first time using the system, it is recommended that only part of your acreage be put into the program to see how it fits with your quality standards and operational style.

Two, use TOMCAST as a guide to help better time fungicide applications, realizing in some seasons you may actually apply more product than a set schedule program might require.

Three, the further a tomato field is from a reporting site increases the likelihood of distortion in the DSV accumulation, i.e., the reported value may be a few DSV higher or lower than that experienced by the field location. This should be taken into consideration when application of fungicides is likely a few days away. Listen to the DSV reports of nearby stations and triangulate to your own location as the best way to roughly estimate your DSV accumulation.

FIRST SPRAY USING TOMCAST

There has been some discussion over the years regarding the application of the first spray when following TOMCAST. The rule stated in the 1997 Vegetable Production Guide centers around the planting date.

Tomato plants that enter the field before May 20 should have the first spray applied when DSV for that area exceed 25 or when a fail safe date of June 15 arrives. The fail safe is used only if you have not treated since May 20, and is a means to eliminate initial disease inoculum. After the first spray, these tomatoes are subsequently treated when the chosen spray interval (range 15-20 DSV) is exceeded.

Tomatoes planted after May 20 are treated when they exceed the chosen spray interval (range 15-20 DSV) or when they have not been treated by the fail safe date of June 15. Therefore, it is critical to compare the tomato planting date to the date DSV reporting began in that area to guide the spray decision process.

TOMCAST STATIONS

Eleven TOMCAST CR10 units have been deployed and are operational as of [Should be activated by June 1, 1997]. The Table above reports the Disease Severity Value (DSV) accumulations from the initial activation of the unit.

BLITECAST

BLITECAST is a Late Blight disease forecasting model similar to TOMCAST, using weather data to generate daily Severity Values (SV). If the cumulative SV exceed a threshold value of 18, a fungicide application is recommended unless an application has been made in the previous 7 days. The environmental conditions conducive to Late Blight development are generally cool and wet. Late Blight was detected in Ohio and other nearby states last year.

Late Blight Scouting Guidelines
Tomatoes grown under weather conditions conducive to Late Blight development or near potatoes that have been infected with Late Blight require periodic scouting. It is crucial to scout for this disease in areas that are difficult to reach with a sprayer or remain wetter than other parts of the field. If a fungicide has been applied within the past 7-14 days (i.e., sprays called for by TOMCAST), the threat of Late Blight is substantially reduced. If a fungicide has not been applied to a tomato field in an area identified as conducive to late blight development within the previous 7-14 days, a prophylactic spray may be justified. If Late Blight has been identified in a field, immediate use of a fungicide is recommended until harvest.

A Tomato Late Blight Management Guide is available from OARDC Plant Pathology if more information regarding the subject is needed. Ask for PP Dept Series # 99, Potato and Tomato Late Blight, 1995 Update and Recommendations.

Return to Vegetable Crops Homepage