Buckeye Rot

Are damaged areas on tomatoes due to dehydration, deficiency, or disease? Is it blossom end rot, sunscald, or buckeye rot? And what can you do about it?

Buckeye rot of tomato (Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org) CC BY 3.0

Let’s look at all three in more detail so you will know what to do if you see damaged areas on your tomatoes or peppers.

Blossom end rot

Blossom end rot occurs when young fruits do not have enough soluble calcium early in their development. But this does not necessarily mean your soil is deficient in calcium. You can’t know that without a lab-based soil test. [I’ve said it before, and I’ll say it many times again – get your soil tested by a lab every few years. Email me if you need help.]
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Inadequate irrigation is the usual culprit behind blossom end rot. Calcium is an immobile nutrient. Which means it takes a lot of water to move it once absorbed.

Blossom end rot of tomato (Mark) CC BY 2.0

If there isn’t enough water to move the calcium to where it is needed, cell walls surrounding the blossom end of the fruit will collapse, creating a uniformly dark brown or black area ONLY on the lower sidewall or blossom end of the fruit. If you see blossom end rot, irrigate more heavily and more often. If the damaged area is lighter in color and higher up the side of your fruit, it may be sunscald.

Sunscald

Like us, fruits are susceptible to sunburn. Sunburn happens most often when there isn’t enough leaf cover. That can happen when we prune too heavily or when nitrogen levels are too low. Nitrogen is responsible for lush, vegetative growth. 

Sunscald dries to a tan color. The problem is that you don’t want to add nitrogen while plants are fruiting because they will shift from fruit to leaf production. The solution in this case is to provide sun protection with shade cloth or other physical barriers. If the damage is a dark brown bull’s eye, you may be looking at buckeye rot.

Sunburned bell pepper

Buckeye rot symptoms

Buckeye rot is a disease caused by three Phytophthora: P. capsici, P. drechsleri, and P. nicotiana var. parasitica. Phytophthoras are water molds (oomycetes) responsible for several plant diseases. This disease occurs most often after extended periods of warm, wet weather with temperatures ranging from 75°F to 86°F. Buckeye rot can infect tomatoes, peppers, and eggplant. Pineapples, potatoes, and tobacco are also vulnerable to buckeye rot.

​At first, all you may see are smooth, grayish-green water-soaked spots. Damping off and stem cankers may also appear. As the disease progresses, those lesions turn into the classic bull’s eye pattern of large, concentric rings alternating between light and dark brown. The edges of these lesions will be smooth but not clearly marked. Eventually, white fungal growth becomes visible on the lesions.

Other diseases that may exhibit similar symptoms include anthracnose, cucumber mosaic virus (CMV), and late blight. In each case, some differences can help you diagnose the problem:

• Anthracnose – irregular lesions on leaves, as well as fruit
• Cucumber mosaic – downward leaf cupping and deformed midribs
• Late blight – lesions are rough and raised, and leaves are also affected

Buckeye rot management

Starting with healthy, disease-free seeds and plants is the first step toward preventing nearly all plant diseases. Buckeye rot spores can travel on seeds, transplants, clothing, and garden tools. Spores can also be splashed onto healthy plants by rain and overhead watering, so use soaker hoses or water at ground level and allow the soil surface to dry out between waterings. Mulching around plants can reduce the splash effect, as well as reduce weeds and evaporation.

You can reduce the chance of many fungal diseases by providing good drainage and reducing soil compaction. Since buckeye rot spores are in soil, fruits closer to the ground are the first affected. If fruit is held up high with stock panels, tomato cages, or trellising, you can reduce the likelihood of buckeye rot.

Three-year crop rotations also help prevent buckeye rot.

​Now you know.