Sunday, July 22, 2012

What is happening to the blossom end?

Ajay Nair
Department of Horticulture, Iowa State University

Lately a number of growers are finding small black spots, often with black concentric rings, on their tomato and pepper fruits. This is a typical symptom of a physiological disorder called blossom end rot. It is caused by lack of calcium uptake from the soil and transfer to the fruits during dry weather. The first symptom of rot is a slight water-soaked area near the blossom end of the fruit. This lesion soon darkens and enlarges in a constantly widening circle until the fruit begins to ripen. The affected area begins to turn black from colonization by saprophytic Alternaria fungal species.  Such fruits become non-marketable and could lead to significant losses. 

Calcium ions mostly move with water in the transpiration stream, up the xylem vessel, toward the upper plant parts. With current drought conditions and moisture stress, plants are not getting enough water and this directly affects calcium uptake. Calcium uptake by fruit can also be affected due to excessive nitrogen fertilization which leads to rapid shoot growth. Rapid shoot growth, occurring simultaneous with fruit growth, causes calcium to preferentially move into growing leaves as opposed to fruits, primarily because of higher transpirational pull from leaves. Transpiration pull from fruit is lower as they are covered with waxy coating. Below are some measures which growers/gardeners could take to mitigate blossom end rot:

1. Supply adequate water especially during high stress period (heat and drought). Uniform watering is critical for a steady flow of water in to the plants
2. Eliminate any other stress (insects, diseases, etc.) by addressing those issues promptly 
3. Avoid excessive use of nitrogenous fertilizers
4. Provide calcium to the plant (calcium fertilizers like calcium nitrate, calcium chloride, or various chelated calcium fertilizer materials). Foliarly-applied Ca fertilizers are not likely to correct or prevent blossom end rot

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