by Michael Celetti, OMAFRA
Managing late blight requires a proactive and integrated approach including scouting, cultural methods and fungicide application. Here are some tips to consider for managing this disease.
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.
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.
Growers should be vigilant 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. 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.