Growing tomatoes in Michigan is fairly simple, but there are a few challenges. Cooler northern temperatures can make tomato plants vulnerable to disease, threatening the quantity and quality of fruit harvested. Knowing the symptoms can help you address treatable diseases early. There are also some precautions you can take to prevent infection in the first place.
Cooler and damp conditions favor fungal problems, like damping off, early blight, late blight and septoria leaf spot. These fungal infections thrive in wet springtime temperatures like those found in Michigan. Damping off occurs when fungal spores attack the stems of young seedlings and destroy them just above the soil level. Early blight appears as brown or black spots up to 1/2 inch in diameter on lower leaves. The spots grow and merge, leaving the rest of the leaf yellow before it falls off. Early blight works its way up the plant. Late blight affects both the tomato plant and leaves. Symptoms are brown spots on the fruit and brown areas on the leaves and stems. Severe cases of late blight kill plants. Septoria leaf spot is distinguished by the light-colored spot that appears in the center of a gray leaf spot. Spores first splash onto lower leaves during rainy weather and progress up the plant if the wet weather continues.
Damping off occurs in young seedlings. Early blight and septoria leaf spot can infect a plant at any time during the growing season. Plants seem to be more vulnerable after the plant sets fruit. Late blight is more prevalent later in the growing season. Fruit affected by late blight is sometimes confused with blossom-end rot. The key difference is that brown spots from late blight appear anywhere on a fruit, whereas blossom-end rot only affects the bottom of tomatoes.
All of these diseases can result in reduced or poor-quality fruit. Left untreated, tomato plant diseases will destroy crops. Disease spreads easily to healthy plants, especially in wet or windy weather. Certain fungal infections, like late blight, can survive in soil and the remains of infected plants through the winter and pass the disease along to next year's crops. Discard infected foliage and fruit, instead of composting them, to avoid future outbreaks.
To avoid damping off problems, use a soil-free seed starting mixture and water seedlings from the bottom to avoid splashing. For the other diseases, consider using a black plastic mulch around the base of the plants to keep mold spores from splashing onto leaves. Space plants in the garden with plenty of ventilation so that leaves are not wet for extended periods. Fungicides are effective for treating blight and leaf spot problems. Your county extension office is an excellent resource for recommending the most effective treatments in Michigan.
Many hybrid plant varieties are resistant to disease. Your county extension office can tell you which varieties grow well in Michigan and are less likely to have disease problems. Make sure that plants have well-draining soil and are fertilized before fruit set and every other week after fruit sets to ensure adequate nutrition. Healthier plants are more capable of resisting disease.