Managing field tomato fungal disease

Updated July 5, 2016: Added link to fungicide efficacy tables

With the pattern of wet weather that seems to have set in after what was, in some areas, a dry start to spring, the moisture-loving tomato diseases will thrive.

Although research has shown no significant benefit to spraying for bacterial spot on tomatoes (except for the marginal benefits of an intensive copper + Actigard program), it is very important to have a fungal disease management program in place now.

Septoria leaf spot, early blight, anthracnose
A preventative fungicide program is key to managing these three common field tomato diseases in Ontario. For areas in which the TOMcast program operates, you can register to access TOMcast data at http://www.vegtools.ca/register.cfm (free). This model was designed to assist in timing fungicide applications for when they are needed to prevent economically damaging disease development.

Using the TOMcast model, properly timed fungicide applications can occur. This is in contrast to spraying by the calendar, in which some sprays will be too early and some too late. Another approach — waiting for symptoms — will result in economic losses as infections have already occurred. In the case of anthracnose, in particular, infections can be initiated on green fruit, but symptoms will not show up until ripening. In order to avoid fruit lesions, the fungicide program has to be in place once fruit are present to prevent the infection. Weeks after infection, when the symptoms show up on the ripening fruit, there is no rescue treatment.

In areas not covered by the TOMcast system, start fungicide applications when first fruits appear and re-spray at 7 day intervals during continuous warm, moist (dews, fogs, or rain) weather or at up to 12-14 day intervals in warm, dry weather if diseases are under control.

Remember, the TOMcast model is based on the biology of septoria, early blight, and anthracnose. It does not guide fungicide timing for late blight or other fungal diseases of tomato and it does not address bacterial diseases.

Late blight
Although certain fungicides used to protect against septoria leaf spot, early blight, and anthracnose are active against late blight, a TOMcast-based application timing will not be sufficient to protect the crop if spores are present in the area and the weather is favourable for late blight.

If late blight is found in the area, tomato growers should:
• Tighten up spray intervals – During wet weather, a fungicide should be applied every 5 – 7 days to protect against late blight. If the weather conditions become dry, the spray intervals may be extended. Wet conditions include not only rain, fog and dew, as well.
• Scout fields often. Know the symptoms. Refer to the Tomato Late Blight Photo Gallery and Late Blight Look-Alikes for photos of late blight and possible look-alikes on tomato.
• Monitor ONvegetables.com for updates on late blight in Ontario and surrounding areas.

Contact OMAFRA at 519-674-1690 or janice.leboeuf@ontario.ca if you suspect you have found late blight in Ontario. This helps us alert others of late blight in the region and if possible, we will try to collect samples for late blight researchers studying the disease.

Learn more

Fungicide efficacy summary tables for management of diseases in field tomatoes