Home gardeners know it has not been the best year in terms of weather conditions for vegetable production. While we've not had to water, the cool rainy conditions also favor the spread of plant disease.

One disease that has made the news recently is tomato late blight, the disease that was responsible for the Irish potato famine in the mid-19th century. Caused by a funguslike pathogen, it can infect and destroy the leaves, stems, fruits, and tubers of potato and tomato plants.

The disease spreads rapidly during conditions of high moisture and moderate temperatures between 60 and 80 degrees. The spores that infect healthy plant tissue disperse via rain splash or on wind currents. Airborne spores can travel for 5 to 10 miles or more, beginning another cycle of the disease.

Late blight symptoms are easily visible to the grower. The edge of the water-soaked lesion, on either the top or bottom of the leaf surface, is covered with a white fungal growth that contains the disease-spreading spores. Lesions enlarge rapidly and turn dark brown to purplish-black. In dry weather, the infected leaf tissues will quickly dry and the white mold growth disappears.

On the stems, late blight lesions will appear brown to almost black. Similar lesions also will be seen on the fruit and might not develop until they have sat on the kitchen counter for a few days. The lesions are not harmful to humans, so the tomatoes can be used if the affected area is removed.

If symptoms of the disease are present in your garden, there are several management practices that will help to control the spread of the disease.When infected plants have been identified, allow the plant surfaces to dry and then remove all of the leaves and fruit that show signs of the disease. Diseased plant material should be placed in a sealed plastic bag and allowed to sit in the sun for several days to destroy the spores before disposing of in the garbage. Infected plants should never be placed in a compost pile because the spores will continue to spread the disease.If disposing of whole plants, do not try to pull the plant. The force of pulling the plant will dislodge spores that will become airborne, furthering the spread of the disease. The best option for disposal of the entire plant is to put a garbage bag over the plant, cut the plant at the soil surface, and then tie the bag closed to contain the spores. Place the bags containing whole plants in the sun for several days to destroy the spores.By appropriately disposing of diseased plant material as soon as the disease is found in a home garden, the potential of spreading late blight to neighbors and nearby commercial growers is reduced.

Home gardeners also can purchase fungicides that will help control late blight. Fungicides are effective if used before the disease appears and are then reapplied every few days if wet weather continues. When purchasing a product, look for the active ingredient cholorothalonil on the label. Be sure to read and follow all directions and safety precautions on the label, handle the fungicide carefully and store it in the original, labeled container out of reach of children and pets.After harvest, remove plants immediately to help control the spread of the disease in future years. If your plants are affected by late blight, avoid planting tomatoes in the same area of the garden next year. Choose a sunny spot that allows adequate room between plants to facilitate drying off plants during rainy periods.This material is submitted by Donna Zang, extension director, and the staff of the Penn State Extension office at the Sunnyview Nursing and Rehabilitation Center complex.