GardenLine | Vegetables | Blossom-End Rot of Tomatoes

A bad case of blossom-end rot, a condition that can ruin otherwise succulent, mouth watering, home-grown tomatoes, caused one of our callers to comment: "the more you want a vegetable, the more likely something will go wrong with it." A little understanding, though, can go a long way in helping to avoid the problem.

Symptoms: The first symptom is a water-soaked area at the blossom end (opposite the stem end) of the fruit. Blossom-end rot begins when the fruit is about half grown, and continues to develop as the fruit matures. The affected area gradually turns brownish-black and shrivels, leaving a leathery, dry, sunken dark area. With time the rotted area often becomes soft; this is caused by fungal organisms that attack after the rot begins. Blossom-end rot often affects only a small portion of the fruit, but in severe cases, almost the entire fruit is destroyed.

Associated with blossom-end rot is the condition called blackheart, which is the darkening of the flesh inside th fruit, at the blossom end. Blackheart can occur without the other symptoms of blossom-end rot, but is caused by the same environmental conditions. The first fruit formed in the spring are more likely to be affected than later developing fruit. Some varieties, namely the smaller fruited varieties seem not to be as readily affected, as the larger fruited varieties.

Cause: The actual cause of the dry rot is a calcium deficiency i the blossom end of the fruit. A lack of calcium in the soil however, is usually not the problem in Saskatchewan gardens. There are numerous other factors which can lead to this condition. They are as follows:

- irregular fluctuations in moisture levels. Blossom-end rot is worse when excess moisture is followed by dry conditions, such as was experienced this year. Sandy soils tend to show the condition worse than loams or clays, because they dry faster.

- rapidly growing plants are often more affected because rapid growth requires an abundant supply of calcium and water. The calcium is dissolved in the water, and thus if water is deficient, so is the calcium. Calcium is used in the cell walls, and when deficient leads to the breakdown of the cells and tissues. The shrivelling and drying of the blossom end is caused by a lack of water getting to that area.

- high levels of certain nutrients, mainly nitrogen and potassium can also stimulate blossom-end rot. Nitrogen is often in excess in home gardens, thus encouraging rapid growth, and an enhanced chance of the disorder. Potassium is high in most Saskatchewan soils, but is in a form that is not readily available to plants, therefore it is doubtful that this leads to the problems.

- transplanting tomatoes into cold soils may lead to a calcium deficiency, because the organisms that convert the calcium from the unavailable to available form are not active in the cold soils. As the soil warms, the organisms become more active, and thus more calcium is available.

- deep tillage around the plants will damage the roots, and thus prevent uptake of water. This in turn leads to the calcium deficiency in the fruit.

Control: There are a number of procedures to follow that can control this disorder. They are as follows:

- we do not recommend applying calcium to the soil, because it is seldom deficient in the soil. Salinity is caused by an excess amount of salts in the soil, calcium being one of these salts. An excess of calcium, can alter the soil making it less desirable for growing most plants.

- adequate, but not excess amounts of water are desirable. Do not let the soil dry excessively between waterings, as this stimulates blossom-end rot. A steady supply of water is very important. Remember that near by trees take a lot of water from the garden.

- mulching the plants will help ensure the steady supply of water. Dry grass clippings (watch herbicide use on lawn) or finely ground up leaf litter make excellent organic mulches. These should be applied in early July after the plants have had a good chance to grow, but before the fruit is half mature.

- shallow frequent tilling will control weeds without damaging the root system of the tomatoes.

- adequate but not excess nitrogen is important. Excess nitrogen will encourage rapid growth, which encourages the disorder.

- avoid stress to the transplants when they go into the garden. Harden the transplants off adequately, and do not plant into cold soil. Encourage steady growth during the year, but not excess growth.

Wood was an instructor with the Department of Horticulture Science. This column is provided as a service by the Extension Division and the Department of Horticulture Science, University of Saskatchewan.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.