Extension Educator, Clearwater and Polk Counties
Plants need three factors for disease to develop. The host plant must be susceptible, the pathogen must be present (usually in the soil), and the environmental conditions must be right. This typically involves wet leaves over some period of time.
There are several simple cultural techniques that you can implement before considering chemical treatments. Fungicides (that innocent looking white dust) are dangerous and are often over applied, so avoid using them until you have exhausted the following cultural options. Why add pesticides to your food when it can be avoided?
Step 1. Begin with a proper crop rotation. This simple practice alone will significantly reduce disease. Do not plant tomatoes where you had potatoes, peppers -- and of course tomato -- last year. Rotation is important and is your first defense in disease prevention.
Step 2. Plant a variety of tomato that is resistant or tolerant to leaf blight, especially if blight has rained on your parade in the past. Sometimes, disease can be prevented simply by variety selection. If you start your own transplants, this information is supplied on the package. If you purchase transplants, your local garden center horticulturalist will have this information.
Step 3. Do not crowd the tomatoes, lack of air circulation favors disease development. Leaf diseases need longish periods of uninterrupted wetness for disease development, and overcrowding prevents leaf drying.
Use a trellis or cage on plants spaced a minimum of 24 inches apart to keep leaves and fruit off the ground. This aids in the plant drying and keeps the disease inoculum further away from the leaves; remember, the disease inoculum is in the soil. Also, it keeps the fruit cleaner and reduces the incidence of spoiled fruit.
Step 4. This may be the single most important step of all. If you need to irrigate, water the ground, not the leaves. Sprinkler irrigation keeps the leaves wet and splashes the disease inoculum from the soil onto the leaves. Soaker hoses, drip irrigation or careful hand watering are better alternatives.
Step 5. Consider using plastic mulch. Plastic mulch laid on the soil surface prior to transplanting is a common practice in commercial production and works well in gardens too. Commercially available plastic is available in 3' and 4' wide rolls in a rainbow of colors. Clear and black are the most readily available to the gardener, but red and green are often available at garden centers. Clear and red mulch will allow weed growth beneath the plastic, unlike the black and green colors. If you need to water the plants, water through the transplant hole. Often, however, the plastic reduces water evaporation so watering is not necessary. Drip irrigation is another option when mulch is used, but increases the complexity of "simple tips".
Plastic mulch's other benefit is warming the soil, which significantly increases the yield of warm season crops like tomatoes, peppers and melons. Research in northern Minnesota shows a two-fold increased fruit yield using plastic mulch as compared to bare soil. The increased yield is due to warmer soil temperatures, reduced disease pressure, reduced water evaporation, and greater consistency in water uptake by the plant.
Step 6. Pinch off the "suckers" growing at the leaf axis. Suckers produce unnecessary foliage and decreases air circulation around the plant. Commercial growers will often leave one leaf beneath the first flower and remove all other leaf from that point and down. This does not make for a pretty tomato plant, but it's effective.
Step 7. Don't apply too much fertilizer, especially nitrogen. Excess fertilization promotes succulent leaf growth which is often more prone to disease. My fertility choice is compost, from either manure or other vegetative materials. Compost is advantageous as it provides all the essential elements necessary for plant growth in a slow release form in sync with plant growth.
If you follow these recommendations, you will hopefully enjoy juicy red tomatoes with an added benefit - fruit without pesticide residue.
If all else fails and you're tempted to use a fungicide, remember that fungicides only prevent new infections, they will not cure existing leaf disease. Fungicides should be applied according to the product label.