As a vegetable gardener, you're probably familiar with the good feeling most gardeners get from harvesting their own crops. It's disappointing, though, to see garden produce such as green peppers (Capsicum spp.) develop black areas and stop growing when they're only partially formed. Several problems can cause blackening of pepper fruits. So identifying the specific cause is an important first step in dealing with the problem and saving the remaining crop.
Peppers usually are grown as annual plants in all parts of the United States, but they are perennials in U.S. Department of Agriculture plant hardiness zones 7 through 11. Whether grown as annuals or perennials, they thrive in well-drained soil and full-sun exposure.
Most pepper types, including bell pepper (Capsicum annuum), a common type, start setting fruits about six weeks after they are planted. The fruits gradually enlarge and, depending on the variety, may stay green or turn yellow, orange or red when fully mature. Although usually trouble-free plants, peppers can be susceptible to plant diseases that may damage and discolor their fruits.
Several fungal disorders can cause ripening pepper fruits to develop black spots. One of the disorders is anthracnose, which leads to round lesions on pepper fruits; the lesions sometimes have black rings in their center and can enlarge enough to cover and collapse entire fruits. Another fungal disease, called Phytophthora blight, also can cause black areas on pepper fruits, with those areas eventually covering entire fruits. Phytophthora blight also produces black areas on pepper plant stems and overall wilting of the plants.
The best way to prevent fungal problems is to inspect transplants carefully and choose only those without any wilted or damaged leaves. Also, choose varieties certified as disease-resistant. Water the plants at their bases to keep their foliage dry, which helps prevent fungal growth, and space the plants at least 18 to 24 inches apart to give them good air circulation.
Once you see signs of a fungal problem in a pepper plant, you might be able to save the plant by pruning away and discarding its affected parts, cutting behind diseased areas and into healthy tissue with sharp shears that you wipe with rubbing alcohol between cuts to prevent spreading the disease. If the entire plant is affected, discard it to prevent the fungus' spread to other plants. It's also helpful to rotate planting areas because fungi live in the soil and can reinfect the next year's crop.
An infection caused by the tomato spotted wilt virus may cause light-colored rings on pepper fruits, and the rings may slowly turn black. The affected plant's leaves become bronze-colored and wilt, and a severely infected plant could die. This virus is spread by a thrip, a tan or yellowish brown insect about 1/25th inch long. If thrips or tomato spotted wilt virus symptoms appear on a pepper plant, then mix 5 tablespoons of insecticidal soap with 1 gallon of water, and spray the mixture on all parts of the plant until they are dripping wet. Use the spray on a windless day when the temperature is at or below 90 degrees Fahrenheit. If an entire plant shows symptoms of the virus, it's best to discard it.
A problem called blossom end-rot also can produce black spots on pepper fruits. Each spot looks sunken and rotted, and it appears on the fruit's blossom end -- not the fruit's stem end. If you see a small, black spot on an otherwise healthy pepper fruit, then it's fine to pick the fruit and use its unaffected areas, but discard its blackened end. If, though, the damage involves most of a fruit, then pick and discard that fruit, allowing the plant to put its energy into other, healthy fruits.
Blossom end-rot develops when the soil is poor in calcium or when plants have difficulty using the calcium that's present. The best way to prevent the problem is to avoid moisture stress to the plants by ensuring they get at least 1 inch of water weekly. Add about a 2-inch-thick layer of mulch on the soil surface under each pepper plant to conserve soil moisture, keeping the mulch back a bit from every plant's center to discourage fungus.
If you encounter blossom end-rot in your pepper plants, check their soil's pH level with a soil test kit before the next growing season, ensuring that it's pH 6.5 to 6.8, a level that helps make soil calcium available to the plants. If necessary, pre-treat the soil the next spring as recommended in the soil test result. If, for example, the soil's pH is 7.5, then add 1 1/5 pounds of aluminum sulfate for each 10 square feet of area to lower the soil's pH to 6.5.