Blossom end rot, seen here, might be one of the most confusing and common problems gardeners see on tomatoes and squash.

Blossom end rot might be one of the most confusing and common problems gardeners see on tomatoes and squash. It also occurs occasionally on peppers, eggplant and watermelon. I say it is confusing because the black and brown “rot” is actually a calcium deficiency, but adding calcium is unlikely to solve the problem.

Weather is the actual usual culprit in blossom end rot development. A shift to higher temperatures, soil moisture fluctuations and drought stress are the most common factors affecting calcium uptake that causes blossom end rot. Root damage from deep hoeing or tillage, waterlogged soils, and excessive nitrogen availability also sometimes affect calcium uptake. Actual calcium deficiencies are rare in Kansas soils but can be confirmed with soil testing.

Blossom end rot is recognizable from other plant problems because it only occurs on the blossom end of a developing fruit and only on the fruit mentioned above. Nutrients enter the fruit through the stem. When calcium runs low, the plant tissues farthest away from the stem do not get enough of the nutrient to fully develop. Plant tissues in the deficient region turn black or brown and leathery. Sometimes mold grows on the surface.

The good news is that blossom end rot usually goes away a week or so after it appears, and it is not contagious. You can pick off the affected fruit and compost it. If you continue to see blossom end rot, however, you may need to go to more extreme measures:

• Have the soil tested to determine pH and calcium levels. Soil samples can be submitted for testing through K-State Research and Extension-Douglas County, 2110 Harper St., Lawrence, for a small fee. Maintain the pH around 6.5 as calcium is more available to the plant at this acidity level.

• If using nitrogen fertilizer, use a nitrate type rather than an ammonium type. Avoid excess nitrogen applications.

• Use mulch. Straw, prairie hay and other materials that break down rapidly work best. Mulch reduces soil moisture and temperature fluctuations.

• Water over extended dry periods, and apply water deeply and infrequently to encourage good root growth. An inch to an inch and a half of water per week is adequate for most plants.

• Do not till or hoe in close proximity to plant roots.

• If plants are in an area that remains wet for long periods of time, rotate them to another area of the garden or add compost this winter to improve drainage.

In a fact sheet on blossom end rot, The Ohio State University also notes: “Foliar applications of calcium, which are often advocated, are of little value because of poor absorption and movement to fruit where it is needed.”

Speaking of fertilizing, Kansas State University recommends mixing fertilizer into garden soil before planting rather than during the season. A soil test can determine what your soil needs, but typically in northeast Kansas, fertilizer containing only nitrogen is adequate. Nitrogen is the first number in the three-number ratio listed on all fertilizers sold in Kansas. For maximum production, plants can be sidedressed as follows:

• Tomatoes: one to two weeks before the first tomato ripens, two weeks after picking the first ripe tomato, and one month later.

• Peppers and eggplant: after first fruit sets

• Squash and watermelon should not be sidedressed as excess nitrogen can reduce yield and/or lower fruit quality.

The Master Gardener Demonstration Gardens with raised vegetable garden beds have tomatoes, peppers, squash, and eggplant that are NOT affected by blossom end rot but offer an opportunity to see good mulching practices and drip irrigation that is being used to water the plants deeply and infrequently. The Gardens are located at 2110 Harper St. in Lawrence and are free and open to the public.