Anthracnose is a serious fungal disease causing circular, sunken lesions on ripe tomato fruit. These lesions reduce the marketability of the fruit and allow other fruit rot organisms to invade. Although the symptoms only show up when fruit ripen, control must begin much earlier in the season.

The anthracnose organism is spread primarily by splashing water. It can infect tomato foliage, providing a source of inoculum close to the fruit. Foliar symptoms, however are inconspicuous. Anthracnose can infect both ripe and green fruit, but symptoms will not show up until the fruit begin to mature.

The anthracnose organism can be present in crop debris, weeds, and tomato foliage. It prefers wet conditions and temperatures of 10-30°C. At 20-25°C, it only takes 12 hours of leaf wetness (on unprotected leaves or fruit) for infection to occur. Within 5-6 days of a spore landing on a ripe fruit, a lesion can be seen.

Anthracnose control must start well before fruit ripen. Once green fruit are present, the crop needs to be protected with fungicide. If the TOMCAST disease model is available in your growing area, then tomatoes transplanted before May 25 should have their first fungicide application once 35 DSVs have accumulated since transplanting. Tomatoes transplanted after May 25 need a fungicide application by 45 DSVs after transplanting. Apply a fungicide by July 11 (July 18 for very late plantings) even if 35 or 45 DSVs have not accumulated yet, as anthracnose can start to infect by then if the plant is not protected. Fungicide should be reapplied every 20 to 25 DSVs (18-20 DSVs if mancozeb is being used).

If TOMCAST is not available in your area, begin a preventative fungicide spray program once fruit are present. Repeat at 5-7 day intervals during continuous moist weather, but the schedule can be extended to 12-14 days in warm, dry weather if diseases are under control. Applications should continue until mid-September.

Remember, infections can start on green fruit, but they will not be visible until the fruit starts to ripen. At that point it is too late to control them. These infections can occur if the fungicide program is started too late, or spray intervals are extended too long.

For more information on controlling anthracnose, including pesticide recommendations, consult OMAFRA Publication 363, Vegetable Production Recommendations. TOMCAST information for parts of Ontario can be found at www.weatherinnovations.com/