Using tomato disease forecasts effectively

A system of weather-based disease forecasting called TOMCAST, developed by Dr. Ron Pitblado at the Ridgetown College of Agricultural Technology in Ontario, Canada, can be used to time fungicide applications in tomato. Three fungal diseases; early blight(caused by Alternaria solani), Septoria leaf spot (caused by Septoria lycopersici), and fruit anthracnose (caused by Colletotrichum coccodes) can be controlled using the TOMCAST system.

IMPORTANT CAUTIONARY NOTE:

TOMCAST is only useful on farms that do not have a history of bacterial diseases. If you commonly have problems with bacterial spot, speck, or canker in your tomatoes, you should not use TOMCAST because the recommended spray intervals will not be sufficient for control of bacterial diseases if you’re tank-mixing copper with your fungicide applications.

Leaf wetness and temperature data are converted into a unit called disease severity values (DSV’s) as shown in Table 1. These values are calculated automatically for selected stations on the NEWA network and displayed on the tomato forecast page.

Table 1.

       Leaf Wetness periods (hr) required to produce DSVs of:      

Mean temp. F*	0	2	2	3	4
56-63	0-6	7-15	16-20	21+
64-68	0-3	4-8	9-15	16-22	23+
69-77	0-2	3-5	6-12	13-20	21+
78-85	0-3	4-8	9-15	16-22	23+

Madden, L., S.P. Pennypacker, and A.A. MacNab. Phytopathology 68:1354-1358

* During the leaf wetness period

To use TOMCAST, start accumulating DSVs when the tomatoes are transplanted. The first fungicide application should occur when 25 DSV's have accumulated since transplanting. If 25 DSV’s have not accumulated before July 11, apply the first fungicide at that time*. Subsequent applications should occur when 18-22 DSV's have accumulated since the previous application if Bravo or Quadris are used, or 15 DSV’s if mancozeb fungicides are used. If spray intervals extend more than 14 days using TOM-CAST, and the weather forecast calls for rain, apply a fungicide and allow it time to dry before the onset of rain.

*The July 11 date may not make sense for very late transplanted fields. Use your best judgement to determine the best time for the first fungicide application if DSV’s are accumulating very slowly. Rapidly enlarging fruit or any early blight symptoms would be reasons to initiate the first fungicide application.

Late blight

The potato late blight forecast can be used to help schedule fungicide applications for late blight on tomato. If late blight has been found in your county or a neighboring county, the late blight forecast should determine your spray interval instead of TOMCAST. The late blight forecast calculates units called severity values (SV). SV accumulations over a 7 day period can be used to estimate how favorable weather conditions have been for late blight spore production and infection. Cornell plant pathologists consider weather that accumulates six or more severity values in a week very favorable indicating the need for a 5 day spray interval on potatoes; 3 - 5 severity values indicates moderately favorable weather and the need for a 7 day interval; less than 2 indicates unfavorable weather, when a 10 day interval may be used. The late blight isolate present in an area can also influence the fungicide spray intervals needed. An isolate that is particularly aggressive on tomato will require shorter spray intervals than one that is more of a potato specialist.

Because weather conditions can vary depending on topography and altitude, the forecast information will be most accurate very close to the weather monitor. For locations that are not close to a weather monitor, forecast information should only be used as a *general indication* of how favorable weather has been for disease. Knowing the location of the monitor, and thinking about how temperature and humidity conditions at the monitor differ from particular fields on your farm can help you interpret forecast information. For example, if the weather monitor closest to you is in a valley that tends to dry off slowly compared to your farm, the forecast information would tend to be relatively conservative for your location. If a weather monitor location is in an open area that would tend to dry off quickly relative to your fields, then the forecast information from that location would indicate that conditions had been less favorable for disease than they were at your farm. Comparing the amount of rain that fell at the weather monitor site with the amount that fell on your farm can also help you determine how similar weather conditions have been at the two locations. If rain is forecast, be sure that all foliage has received a fungicide application within the last 7 days, especially if late blight has been found in the area. Late blight spores can be carried on storm fronts, and if large numbers of spores are deposited on unprotected tissue, a lot of infections can get started.

*Overhead Irrigation* can create disease favorable conditions in a field that a weather monitor will not be taking into account. Irrigation that starts when the leaves are still wet from dew in the morning, or continues after dew has fallen at night will extend the wetting period for that day and must be taken into consideration when weekly severity value accumulations are calculated.