The Diseases Behind Spotted Tomatoes

Have you noticed a few spots on your tomatoes? While the recent weather has been good for building up the moisture levels of our soils, the extra wetness and warmth is also great for diseases. With several central Wisconsin counties already reporting late blight, it’s time to pay attention to the spots showing up on your tomatoes.

Here are some of the big players when it comes to spotted tomatoes: early blight, septoria leaf spot, and late blight.

Early blight lesions

 

  • Early blight, as the name suggests, is one of the first spotted diseases to appear. It is often confused with septoria leaf spot which occurs around the same time. Early blight symptoms show up on the older leaves as irregularly shaped brown spots, often developing yellow rings around the lesions. Eventually the spots can get bigger and connect, causing leaf death and drop. Fruit can be affected, developing dark, leathery, sunken spots near the stem end.

 

 

  • early blight and septoria

    Septoria leaf spot (R), early blight (L)

    Septoria leaf spot begins on the older leaves as small, circular spots on the upper surface of the leaf. These spots can sometimes have a water-soaked appearance. Over time, the spots spread and connect to create dead sections of leaves. The spots may develop a tan or light-colored center, and have tiny black fruiting bodies in the middle of the lesions. Fruit are not affected.

 

 

Late blight

Late blight

  • Late blight is a devastating foliar disease feared by commercial potato and tomato growers. It first shows up on older leaves as pale-green areas that become brownish-black, water-soaked, and oily looking. The areas can spread, causing entire leaves to die back. Stems can develop dark-brown and black areas, and fruit will develop sunken, dark looking spots with rings. Plants can die within weeks due to the quick spread of this disease.

 

 

What should you do if you have them?

First, prevention is key. When planting your tomatoes, make sure to provide enough space between each plant. The increased air flow will allow the leaves to properly dry, making the environment less disease friendly. Try mulching around your tomato plants, and avoid letting the leaves or plant parts touch the soil. Also, don’t plant your tomatoes, peppers, or potatoes in the same spot every year!

Furthermore, if any plants are diseased, make sure to remove the diseased plant material from your garden. This includes end-of-season garden care! Those diseased plant parts provide a fresh source of disease inoculum the following spring if not removed. And, make sure you use disease-free compost to avoid this problem in future years.

If you have a disease beginning to show up on your tomatoes, remove the bottom diseased leaves as able. This will avoid the disease spreading to new, healthy leaves. If you detect late blight, remove the plant entirely to avoid spreading to healthier tomatoes.

Any potential late blight infections can be sent to the UW-Madison Plant Disease and Diagnostic Clinic for diagnosis at no-charge. While you learn what exactly is bothering your tomatoes, researchers can use the data to track disease spread. Bring a sample into the UW-Extension office, and we’ll ship the sample for you.