The ends of my watermelons on the vine are turning black and mushy. What is causing this?

Blossom-end rot is a nonparasitic disease of tomato, pepper and watermelon. Losses can vary from a trace to more than 50 percent.



Symptoms



The first symptom is a slight water-soaked area on or near the blossom end of the fruit. The affected area soon darkens and enlarges in a constantly widening circle until the fruit begins to ripen. The tissues are dark and shrunken and have a dry, leathery appearance. With pepper the rot is tan in color and should not be confused with sunscald, which is white. The affected area may be merely a speck or it may involve half or more of the fruit (see illustration on tomato). Secondary microorganisms may grow on the decayed area.



Cause



Blossom end rot is caused by a lack of calcium in the developing fruit. This may be due to a lack of calcium uptake from the soil or to extreme fluctuations in water supply. Incidence of blossom-end rot is also increased where there is a low ratio of calcium to certain other nutrients such as potassium and nitrogen.



Control



Although the most desirable calcium levels for preventing blossom-end rot have not been determined, the application of lime to fields known to be low in calcium has helped to prevent the disease. Soil should be limed according to recommendations of soil analysis report, usually to pH 6.5-6.8. The use of gypsum (land plaster), at rates of 500 to 1000 pounds per acre (1 to 2 pounds per 100 square feet) as a supplement to liming on calcium deficient soils has proven beneficial. Lime and/or gypsum should be applied several months in advance of planting.



After tomatoes are planted, good results have been obtained by spraying foliage and stems with anhydrous calcium chloride (4 pounds per 100 gallons of water per acre) four times on a weekly schedule beginning when the second forms, or when symptoms first appear. Calcium chloride solutions are also available under several trade names for small scale garden use. Application in the same tank with fungicides and/or insecticides is suggested. This helps to rapidly place calcium directly where it is needed in the fruits. Do not exceed the recommended rate. Calcium chloride is suggested only for tomatoes. Application should be done while temperatures are cool in the morning



Since blossom-end rot is so closely related to extremes in the water supply, an important aid in control is to regulate moisture supply in the soil. Cultivation and hoeing can be avoided if proper weed-control chemicals are used. If cultivation is necessary, it should be shallow to avoid root pruning. Mulching, which serves to maintain an even supply of soil moisture, should be practiced where feasible. If irrigation of any kind can be used, it may prove profitable during hot, dry weather. To reemphasize, either an inadequate or excess moisture stress favors blossom-end rot development. In general, plants need at least one inch of water per week in the form of rain or supplemental irrigation.



Young plants should not be grown too quickly nor should the plants be subjected to severe "hardening-off" before transplanting. A steady growth rate as a seedling and in the field will discourage this trouble.



Removing affected fruits when symptoms are first observed may be worthwhile for subsequent development of other fruit on the plant. This is particularly recommended for tomatoes.

The ends of my watermelons on the vine are turning black and mushy. What is causing this?
Blossom End Rot.