MaryAnne Spinner, Chicago Master Gardener

Avid home tomato growers generally fall into two categories. The first take tomato growing very seriously. Winter is spent poring over dozens of seed catalogs in search of the perfect varieties. Late spring is indoor seed-starting time, with expanding peat pellets, heat mats, miniature greenhouses, and canopies of special lights. In May, the plants are set out, surrounded by protective devices (or at least an old Marshall Field's shopping bag), and are checked three times daily by their nervous godparents. First fruit? Time for celebration, bragging rights, and journal entries.

Then there is the other type of tomato grower. Pick up a few plants from the local garden center in June, and plunk 'em into a big pot.

What all tomato growers have in common, however, is that long, frustrating wait for ripe fruit. You peek through the foliage–that big tomato down there is getting just a touch yellow, don't you think? And then the yellow turns orangey, then a tentative red, and then...what is that little tan spot on the bottom of that tomato? Please, please, tell me it's not going to turn brown, then black, then spread up the sides. It's the bane of ripening tomato, the dreaded Blossom End Rot (BER).

Identifying the Culprit

Based on the questions sent to the Extension's Electronic Plant Clinic, this is the number one tomato problem around the country. Blossom End Rot is not a disease per se, but a physiological disorder. When a tomato is affected by BER, the blossom end of the fruit (the portion at the opposite end of the stem, for those of us who still need to count on our fingers, and say "Thirty days hath September") breaks down and rots. The first signs are small tan water-soaked spots that resemble small bruises or mimic another condition called sunscald. The spots enlarge ominously, becoming dark brown or black, and look sunken and leathery. Not a pretty sight.

Half the fruit may eventually become affected, and often bacteria or fungi opportunistically invade the discolored areas, leading to further decay. Although the rotted portions can be cut away and the rest of the fruit can still be used, many tears will have been shed over the injustices inflicted on our lovely tomatoes. And note that tomatoes are not alone in the BER pantheon; other vegetables, such as peppers, eggplant, and even zucchini (can anything really stop a zucchini?) are at risk for the disorder.

Calcium is Not Just for Bones

BER lesions are symptoms of a lack of calcium in the fruit. This deficiency may be due to an innately low level of calcium in the soil, but more likely results from a plant's impaired ability to take up calcium and transport it to the fruit. Several conditions can inhibit calcium uptake: drought stress, alternating soil moisture extremes–too much rain, followed by drought–damage to a plant's roots, waterlogged or cold soil, and high concentrations of ammonium, potassium, or magnesium in the soil.

Movement of calcium in plants depends on the process of transpiration (the loss of water through above-ground plant parts). Because leaves transpire more than fruits, calcium moves more easily into the leaves, and remains there, bypassing the fruit. And calcium is not subsequently redistributed from leaves to fruit. This calcium deficiency in an area of rapid growth–the end of the fruit–causes cells to collapse, leaving the iconic sunken-lesion symptom of BER. This situation, where calcium ends up in the leaves at the expense of the fruit, can be aggravated by over-fertilizing with nitrogen, which promotes excessive foliage production.

It is advisable to avoid overfertilizing in the form of side dressing during early fruiting, especially with ammoniacal forms of nitrogen. In fact, the first fruit clusters on a plant are typically more at risk for BER. This is attributed to the combination of several ingredients: rapid plant growth, a large leaf area for water transpiration, water stress, and fruit enlargement. Even a temporary water stress during early fruit enlargement can cause the disorder, because the fruits are the last to receive adequate calcium. In addition, high levels of relative humidity (think Chicago in August), or low levels of relative humidity in combination with hot, windy weather (Chicago again!), can limit transpiration, thus preventing calcium from traveling to the fruits.

Consistency is not a hobgoblin

The key to controlling BER is to make sure your plants have a consistent supply of moisture. In this current drought period, the last thing we are worried about is too much rain, but it should be remembered that too much moisture can cause as much damage as too little; I have often noted problems with BER after a thunderstorm. Thus, if you are hand-watering your tomato plants, try not to overdo it. Plants generally need one inch of moisture per week for proper growth and development, whether from rain or from irrigation, increasing to 1.5 inches per week when fruiting.

Irrigate evenly, and mulch the soil to retain moisture during dry periods. In recent years, research has shown that red plastic mulch not only conserves moisture, but actually can increase tomato yields. But any sort of mulch–such as straw, pinestraw, decomposed sawdust, or newspaper–applied properly, will add a protective factor against BER. It has also been suggested that in times of drought, the use of plastic may actually increase the incidence of this disorder if the plants are not watered enough.

Research has shown that the ideal soil pH for growing tomatoes and preventing BER is 6.5 to 6.7. Home gardens not limed in the past two to three years might benefit from an application of two cups of lime for each plant, and the lime should be worked into the soil 12 inches deep. To verify the need for this amendment, however, you should have your soil tested first.

Another strategy is to grow your tomatoes in large plastic tubs specially designed to retain water. These tubs have a water reservoir in the bottom, and a layer of capillary matting is placed under the soil to help further retain moisture. Watering is done through a tube that goes directly into the reservoir, and the plants then take up only as much moisture as they need. This is the same general strategy as employed by earthboxes; to see these in action, visit the Demonstration Garden at the Garfield Park Conservatory.

Note that there are many so-called remedies, both commercial and homemade for BER. Garden centers and catalogs sell foliar sprays and crushable tablets usually containing chelated calcium, "guaranteed" to stop BER in its tracks. However, foliar sprays are of little value, because of poor absorption and movement to the fruit where it is needed. Tomato fruits do not have openings in the epidermis (skin) where moisture can be lost or where calcium can enter the fruit from surface application. Thus, direct application of calcium to fruit is ineffective. And, remembering that this disorder may arise from a plant's inability to take up calcium from the soil, adding more calcium to the soil through chemical supplements will be to no avail. That being said, I must admit that my garden closet contains an assortment of sprays and tablets, like tomato talismans, just in case.

Finally, as with all other disease- or disorder-prone plants we want to buy, try to find tomato varietals that are resistant to BER. Seed catalogs often state this resistance where it is known, though it is less likely that you will get adequate information when you buy transplants at a garden center, unless the staff is particularly knowledgeable. It has been observed that tomato varietals with large amounts of foliage tend to be more susceptible to BER (because the foliage takes in the calcium at expense of the fruit), so you could try filtering with this characteristic.

Blossoms End Happily Ever After

Have faith that BER can be controlled, leading to perfect ripe, juicy tomatoes. There are a number of factors that affect a tomato plant's susceptibility, but remember the two most important points for control: maintain a consistent moisture level, and mulch. Then with a plethora of ripe tomatoes, all we'll have to worry about is the second-most feared tomato problem: squirrels that take little bites out of every tomato and then toss them aside. That's another story.

Do you have a gardening question? Email the Extension's Electronic Plant Clinic at rwolford@uiuc.edu, and our Master Gardeners will be glad to assist you.