Karl Foord - Extension Educator, Horticulture
In anticipation of spring tomatoes, please consider your tomato systems and avoid one of the scourges of gardening - tomato blossom end rot.
Water soaked areas at the blossom end of the fruit usually appear when the fruits are one third to one half full size (Photo 1). This enlarges and darkens as the fruit matures (Photos 2 & 3).
Causes and the role of calcium
Blossom end rot is a "physiological disorder" induced by a localized calcium deficiency in the fruit. The incidence of the disorder is usually not due to a lack of calcium in the soil, but rather due to factors affecting the uptake and translocation of calcium.
On a cellular level calcium is a critical component of cell walls (a structural nutrient). So when calcium is limiting cell walls cannot form properly and rapidly growing parts of the plant suffer breakdown. As a structural component calcium once incorporated into a cell wall is not mobile within the plant.
At the plant level, the end of the fruit is an area of rapid growth and has a high need for calcium as do other rapid growth areas like meristems. If 90% of the calcium that a mature fruit contains is already in the fruit by the time it is ½ - ¾" in diameter then the critical time for calcium uptake is early in the development of the fruit.
Soil calcium and plant uptake
Calcium uptake is associated with water uptake. Thus anything that interferes with water uptake can create calcium deficiencies. Dry or wet soils interfere with water uptake in different ways but both can lead to calcium problems. The strongest sink for calcium is actively transpiring leaves because they are actively pulling water. Other plant structures are not transpiring near the degree that leaves are and thus function as poorer calcium sinks. A waxy cuticle develops on the fruit when it is ½ - ¾ "in diameter which reduces transpiration and thus weakens the fruit as a calcium sink.
The fruit is competing with leaf tissue for calcium so a higher fruit to leaf ratio reduces the relative strength of the leaf as a calcium sink allowing more calcium to be allocated to growing fruits. This adds to the logic recommending pruning of tomato sucker shoots.
Control of Blossom End Rot
Maintain even and adequate soil moisture; mulch aids in this process. Avoid poorly drained and cool soils. Avoid over- fertilizing with nitrogen which creates excessive vegetation. Avoid ammonium based fertilizers as ammonia inhibits uptake of calcium. Use nitrate as the main source of N in fertilizers. Choose cultivars that have fewer tendencies to demonstrate blossom end rot. Use soil test data to maintain proper nutrition and optimum pH in the 6 - 6.5 range.
Foliar applications of calcium
There seems to be disagreement about the effectiveness of foliar applications of calcium. The logic on the ineffective side is that calcium is immobile in the plant and will not translocate to the fruit from material sprayed on the leaves. The response is to spray on the fruit. However, a waxy cuticle develops on the fruit when it is ½ - ¾ "in diameter which reduces transpiration and perhaps direct absorption of sprayed calcium.
Often blossom end rot decreases as the season progresses. This could be due to weather effects, warmer soils, or a slowing of vegetative growth all of which would make it appear that early applications of calcium have been effective.
Perhaps foliar sprays applied on plants prior to the first cluster of fruit or directly on small fruit can be used to supplement calcium. Calcium nitrate and chelated calcium are the safest sources of calcium to be applied as a spray. However spray applications of calcium are no substitute for proper nutrition and water management.
If calcium is best allocated by the plants xylem water conduction system, then keeping this system functioning optimally is the best course of action.
Editors note: This article was developed from a presentation created by Dr. Carl Rosen and Michelle Grabowski and delivered by Dr. Carl Rosen at the Upper Midwest Regional Fruit and Vegetable Growers Conference.