When tomatoes start to turn gray or dark at the stem end and then drop off the plant, the problem could be botrytis blight, also known as gray mold. Unlike blossom end rot, which causes a blackening of the tomato from the blossom end of the fruits, botrytis usually starts on the stems and eventually makes its way to where the fruit joins the stem.
Every backyard vegetable gardener knows that tomatoes are no strangers to disease. There is always some wayward spore or pathogen that causes gray or black spots on tomato leaves or, worse, on the tomatoes themselves. Most tomato problems are easier to avoid than to cure, and that is true of gray mold. Gardeners need to be on the alert for the first signs of problems and take action fast. You may not be able to save the tomato plant that is showing symptoms, but at least you can prevent the problem from spreading.
What Is It?
Botrytis blight, or gray mold. is a wide-ranging fungal disease that attacks over 1,000 kinds of plants, including tomatoes, strawberries, and peonies. As if that’s not frustrating enough for gardeners, gray mold can also cause a variety of other diseases to take hold, from damping off disease (seedling death) to various blights affecting stems, buds, fruits, and flowers.
As its name implies, gray mold appears on tomatoes as a gray-colored, fuzzy mold. Symptoms often start as pale, or "ghost," spots on the fruit, which eventually starts to rot.
Often the first symptoms of gray mold occur on the tomato stems. They may show up as dark spots or as dark rings that completely girdle the stem. In humid conditions, spotting will be evident on the stems and leaves and may start as low as the soil level. Many gardeners don't notice these symptoms unless they are looking for them. Once gray mold takes hold, it can spread to all parts of the plant. The immature fruits may turn a light brown or whitish color, with the inside flesh getting soft and moldy.
When a plant is susceptible to as many problems as tomatoes, it is wise to keep a close eye on them for any signs of trouble. Since so many tomato diseases have similar early symptoms, you should consider taking a sample to your local cooperative extension office for a diagnosis.
Unfortunately, there are no tomato plant varieties with natural resistance to gray mold. Fungicides can help prevent it, but they will not cure a plant that is already infected. The best you can do is remove and destroy the affected plants and fruits and apply a preventative spray on any healthy-looking plants. Use an organic fungicide labeled for use on edible plants; look for something with copper, sulfur, or neem.
Next season, be sure to rotate your crops. Botrytis spores are good at overwintering in the area. If you have similar weather conditions again—or high humidity or damp, cool nights—you may want to consider a preventative spray of your organic fungicide early in the season. Then, be on the lookout and remove any plants that show signs of gray mold as soon as possible.
Rupp, Sabrina, et al. Botrytisfragariae, a New Species Causing Gray Mold on Strawberries, Shows High Frequencies of Specific and Efflux-Based Fungicide Resistance. Applied and Environmental Microbiology, vol. 83, no. 9, 2017, doi:10.1128/aem.00269-17
Botrytis Blight. Missouri Botanical Garden