Be Aware of Late Blight Threat in Tomatoes

Conditions are almost perfect for late blight to develop in tomatoes this season. Growers are encouraged to scout their fields frequently for this disease and be prepared to apply a registered effective fungicide to reduce the risk of late blight. Many growers watched their tomato crops melt away at the end of 2000 due to a severe infestation of late blight. Some fresh market tomato growers in particular, lost significant yields that year. This disease is the same one that caused "The Irish potato famine" during the 1840's that resulted in the death and migration of millions of people. It has already been found on potato plants in Prince Edward Island, Long Island, New York and near Simcoe Ontario this year. With the cool wet weather experienced this past spring, the wet weather experienced in July and the discovery of the disease in New York and Ontario, late blight in both tomatoes and potatoes could become a significant problem this season.

The pathogen can infect leaves, stems and fruit of tomato. Once infection begins, the disease can spread rapidly and destroy a crop within a few days. Scouting for late blight regularly and applying effective fungicides is extremely important.

On leaves, the disease appears as a brown, sometimes black, advancing irregular shaped lesion. Often a yellow or chlorotic area surrounds the dark brown lesion (Figure 1 (a)). Most strains in Ontario also cause brown irregular lesions on stems and petioles that quickly enlarge and girdle the tissue (Figure 1 (b)). Lesions on infected fruit first appear as water soaked olive coloured greasy spot that eventually enlarges as the fungus advances throughout the entire fruit. Infected fruit eventually appear brown, hard and corky (Figure 1 (c)). Rotting tomato fruit and vines infected with the late blight pathogen often emit a distinctive foul odor.

Figure 1. Symptoms of late blight on infected tomato (left) leaf, (middle) stem and fruit (right)

| Top of Page |

Late blight is very weather dependent and can destroy a potato or tomato crop within a few days under optimum conditions (91-100% relative humidity and temperatures of 3-26°C, optimum 18-22°C). Cool nights with dew and warm days are ideal for disease development much like what is experienced in Ontario the past few days. The pathogen is not active during very hot temperatures (>30°C).

The disease spreads long distance on infected tomato transplants but is not considered a seed borne disease. It can sometimes be found on infected seed potato tubers, which is a major source of the disease that can result in epidemics under cool wet conditions.

The pathogen Phytophthora infestans that causes late blight produces asexual sporangia that can either dislodge and infect near by susceptible tissue or during very wet conditions burst open and release special zoospores that actually swim short distances to susceptible tissue. The sporangia appear as a white fungal growth on the undersides of leaf lesions. They may also be produced on infected stem lesions and on infected fruit under very humid conditions. Sporangia are spread by long distances during prolonged rainy periods. In one study conducted in Washington State, sporangia survived for 24 hours during cool wet conditions. On several occasions, they survived for up to four hours. Researchers estimated that with a wind speed 20 to 40 km/hour, sporangia could travel 80 to 160 km in four hours. That is why late blight is considered a community disease. Once it is found in a region, all susceptible crops nearby are threatened.

Managing late blight requires a proactive and integrated approach including scouting, cultural methods and fungicide application. The following are some tips to consider for managing this disease.

Crop Scouting:

Tomato growers should try to monitor their crops twice a week for the disease. If a few plants are observed to be infected, they should be placed in a sealed plastic garbage bag, removed and destroyed away from the field as soon as the disease is observed. Volunteer tomato or potato plants in nearby fields should also be destroyed since they may be a source of the disease. If a large area of plants are found to be infected it will be impractical to remove them from the field. Disking under large areas of infected plants and applying a fungicide with some kick back has proved to be successful in managing the disease while minimizing losses for some growers, provided the epidemic has been caught early.

| Top of Page |

Weed Control:

Good weed control particularly of host plants such as hairy nightshade is also important. Dense weeds also reduce drying within the crop which prolongs the leaf wetness period for infection and makes fungicide penetration into the canopy more difficult.

Fungicide Application:

Growers should be vigilant this year about keeping up with scheduled fungicide applications to reduce losses and inoculum from building up. There are several good fungicides registered on Tomatoes for late blight control in Ontario (See OMAFRA Vegetable Production Recommendations Publication 363). Remember to rotate fungicides with different modes of action to reduce the possibility of selecting for resistant strains of the pathogen. The first application of a registered fungicide should been done no later than just after the tomato fruit first appeared on vines and before wet weather occurs. If an application of a registered fungicide has not yet been made to the crop, it is a good idea to consider making an application soon before the disease appears. Remember fungicides work best against this disease when they are on plant tissue before the pathogen infects. Good spray coverage is also very important. Adjust water volumes, pressure and ground speed to ensure the fungicides are adequately covering lower leaves and stem. During wet cool periods, a fungicide should be applied every 5 - 7 days. If the weather conditions become dry, the spray intervals may be extended to every 12 -14 days.

Cull Pile and Volunteer Plant Destruction:

It is very important that growers burn or bury cull piles if they haven't already. Tomato and potato debris left on fields should also be cultivated deep into the ground to encourage decomposition and reduce the potential of unsuspecting infested debris from becoming a source of the disease.

Field Selection:

Fields should be selected away from potential sources of the disease such as backyard gardens where tomatoes and potatoes grow without the use of fungicides.

Crop Rotation:

The fungus does not survive very well outside of infected plant tissue unless it mates with an opposite mating type and produces a resting spore called an oospore. Crop rotation for 3 to 4 years will significantly reduce field inoculum provided persistent oospores were not produced.

| Top of Page |