Nobody wants to invest time, effort and money into planting a garden only to have it all go to waste. Tomato plants are vulnerable to cool spring weather, especially in Michigan. Coupled with damp conditions, you may start to notice leaves on your tomato plant turning yellow with brown spots. More than likely, these symptoms indicate a fungal infection known as early blight.
Early blight appears on tomato plants that have been exposed to cool weather and plenty of moisture. Spring weather patterns in Michigan tend to favor this pattern. Early blight first appears on the lower leaves of the plant, usually as brown spots that may be up to 1/2 inch in diameter. The infected leaves turn yellow and eventually fall off. Left untreated, early blight will work its way up the entire plant. Since early blight is a fungal infection, it can spread to healthy plants.
Tomato plants are more susceptible to early blight during a period of cool and wet weather. For tomato plants, this means daytime temperatures that don't reach 75 degrees Fahrenheit during the day and fall below 50 degrees Fahrenheit at night. Excessive moisture that doesn't allow the leaves to dry also promotes development of early blight and other fungal infections.
Tomato plants infected with early blight will have a smaller harvest, if any at all. Early blight can occur after a large fruit set, which puts a huge demand on the plant as the fruit grows. Early blight can spread to the stems of the plant, affecting water and nutrient absorption. As temperatures increase, the likelihood that early blight spores will spread to healthy plants also increases. As the plant defoliates, developing fruit exposed to excessive direct sunlight can develop sun scald and discolor the fruit.
Use a plastic mulch around the base of your tomato plants to prevent early blight. In addition to warming the soil, it will prevent early blight spores from splashing onto leaves from the soil during rainy weather. When watering tomato plants, don't sprinkle from the top; instead, water at the base of the plant or consider a drip irrigation system. Provide the tomato plants with adequate fertilizer, as healthy plants are less likely to become infected. Leave plenty of space between tomato plants in your garden so that there is adequate ventilation and foliage can dry. Last, rotate the location of tomato plants in your garden so that you are not replanting in the same area for three years.
Treating plants with early blight can help save the plant. Side dress plants with fertilizer to support growth. Manually removing infected leaves can slow the progress of the disease; infected leaves should be removed from the garden and discarded (not composted). Early blight spores can survive in the soil for several years. Consider plowing under infected plants if possible. Several fungicides are effective for treating early blight. Also, contact your county extension office for effective treatments in your area.