Now is the time to take action against tomato fungal disease. Late Blight (Phytophthora infestans) has been confirmed in Connecticut for the first time this 2010 growing season. Last year Late Blight devastated many home and commercial plantings not protected by fungicides. There are organic fungicides and chemical fungicides labeled for use against Late Blight and other fungal diseases of tomatoes.
Cultural controls also go a long way in keeping tomato plants healthy and strong. The best defense is a strong plant, able to fend off attacks by pathogens. Late Blight resistant varieties of tomatoes are Legend, Santa, Juliet, Mountain Magic, and Plum Regal. Each year rotate planting sites within the garden. Proper soil pH and nutrient levels give the best start to transplants in the garden. Soil tests should have been done prior to planting to determine health of the soil and adjust pH. Tomatoes prefer a pH of around 6.5 but are adaptable. A well balanced fertilizer applied at planting time and again later when blossoms begin to appear is a good rule of thumb. Stake or cage plants to keep foliage and fruit up off of the ground. If tied to a stake and growing tall, remove the lower leaves up to one foot to lessen the chance of leaves coming in contact with the soil where several different disease fungi live. Mulch plants to provide a physical barrier between the soil and the plant. Some diseases live in the soil and are splashed up onto the leaves when it rains or when watered. The mulch stops this action. Mulches can be grass clippings, chopped leaves, bark mulch or plastic strips. Fungal spores that land on a leaf need moisture to germinate. Water plants in morning only so leaves can dry before evening. When watering plants, use soaker hoses or drip irrigation,and avoid overhead watering. Try to not get the leaves wet. This is obviously unavoidable during rain. Space plants far enough apart to promote good airflow and leaf drying. Slow the spread of disease by hand picking off any leaves as soon as you see a spot it and dispose in the garbage.
Other common fungal diseases in Connecticut that attack tomatoes are Early Blight (Alternaria solani) and Septoria Leaf Spot (Septoria lycopersici). All of these cultural controls help to prevent other diseases as well.
Fungicide is used as a preventive measure, stopping the germination of the fungal spores once they land on the plant. Conventional gardeners can use chemical fungicides. These include the active ingredients chlorothalonil, mancozeb or manix. Chlorothalonil can be used on the same day of harvest while mancozeb and manix restrict harvest to five days after application. Several different brands of those named active ingredient are available at better garden centers. Read and follow all label directions for use.
Organic fungicides include copper based fungicides, Bacillus subtilis and few others listed below. Care should be taken when using copper based fungicides as they can cause damage to leaves and fruit. Follow all label directions.
Organic products with late blight on label
• Serenade ASO (Bacillus subtilis) 6 qts/A
• Serenade MAX (Bacillus subtilis) 1-3 lb/A
• Sonata (Bacillus pumilis) 2-6 qts/A
• Oxidate (hydrogen dioxide) 40-128 fl oz/100
• Sporan EC (rosemary, clove, thyme oils) 1-3
• Sporatec (rosemary, clove, thyme oils) 1-3
• Copper fungicides
(list from http://www.umassvegetable.org)
Whichever fungicide is used, thorough coverage is essential and applied regularly to protect new growth.