Credit: Pixabay

Tomato holds the distinction of being America's favorite home garden vegetable. Unfortunately, it also probably causes the greatest number of problems for home gardeners because of its vulnerability to insects, diseases, and disorders. August is a good time to assess this year's tomato crop and learn from what might have gone wrong. Listed below are some of the more frequently asked questions we receive about growing tomatoes.

More gardeners grow tomato than any other vegetable. (Credit: Helios4Eos/Getty Images)

What is the best way to fertilize tomatoes?

Fertilizer recommendations should be based on soil test results. In the absence of a test, incorporate a complete garden fertilizer at the rate of about one pound per 100 square feet at the time the soil is prepared for planting. Use a fertilizer low in nitrogen (N), high in phosphorous (P) and medium to high in potassium (K). Among the best analyses for tomatoes are 8-32-16 and 6-24-24. Avoid using ammonia-based fertilizers such as urea or ammonium nitrate for tomato fertilization.

At planting time, use a fertilizer relatively high in phosphorus (e.g., 9-45-15) as a soil drench at the base of the plant. One cup of transplant solution formulated by dissolving one heaping tablespoon of fertilizer per gallon of water is sufficient.

Finally, side-dress with a fertilizer high in nitrate nitrogen when the first cluster of fruit begin to enlarge. Side-dressing is the application of fertilizer 2-4 inches beside a row of vegetables. The fertilizer is left on the soil surface rather being dug in. This fertilizer will then need to be watered in. Calcium nitrate is an excellent choice of fertilizer for tomatoes. Apply at the rate of one pound per 100 feet of row and repeat every 10 days to two weeks.

My tomatoes bloom but do not set fruit. What might be wrong?

The problem could be temperature related. High temperatures, especially if accompanied by low humidity, hinder fruit set through failure of viable pollen to form and/or fertilization to occur. Temperatures above 90°F during the day or above 70°F at night usually result in poor flowering and reduced fruit set. Research indicates that night temperature likely is more critical than day temperature, with the optimal range for the former being 59 to 68°F.

It must be noted, however, that temperature cannot always be blamed for poor fruit set. A heavy fruit load combined with inadequate nutrition can reduce fruit set on flower clusters located on the middle-to-upper part of a tomato plant.

The failure of tomatoes to set fruit most often is temperature related. (Credit: University of Maryland)

What advantage is there to staking and pruning tomatoes?

Staking tomatoes improves light interception, reduces diseases, and lessens incidence of sun scalding. Pruning limits foliage thereby making disease management easier. Pruning also limits the number of tomatoes allowed to set on a plant resulting in larger individual fruit with earlier maturity.

Why are my tomato fruit misshapen?

Misshapen fruit is likely the result of poor pollination. Pollination is inhibited by low temperatures (less than 55°F), high temperatures (day more than 92°F; night more than 70°F) or cloudy weather at the time of flowering or fruit set.

Why do tomato fruits crack?

There are two types of fruit cracking in tomato: radial and concentric. Radial cracks develop at the pedicle (stem end of the fruit) and radiate outward. Concentric cracks form in a circular pattern on the fruit around the pedicle. Both types are the result of a fluctuation in the growth of the tomato fruit. Anything that affects steady, even growth of the tomato can induce cracking. Irregular soil moisture probably is the greatest cause for this disorder. Planting "crack resistant" varieties and keeping the soil uniformly moist will help to reduce the incidence of fruit cracking.

Tomato fruit cracking is caused by an irregular supply of water. (Credit: Missouri Botanical Garden)

The leaves on my tomatoes are turning yellow and dying from the bottom of the plant and progressing upward. What is wrong and what can I do?

One likely cause of the symptoms described is early blight. Early blight is a fungus (Alternaria) that attacks tomato leaves beginning in June in Missouri. A distinguishing characteristic of early blight is the lesions on the leaves and stems. Early blight lesions have a distinct "bullseye" appearance. To control this disease, stake and mulch the tomatoes, increase plant spacing to reduce humidity around the plant and remove some of the lower leaves. Also, avoid getting tomato foliage wet when irrigating. Finally, the use of fungicides might be necessary when infestations are heavy. Products containing chlorothalonil as their active ingredient have been shown to provide good control. It is important to remember that fungicides such as chlorothalonil are preventative and not curative in action.

What causes a sunken, dark spot on the bottom of a tomato fruit?

The dark spot on the bottom (blossom end) of the tomato most likely is a physiological disorder called blossom end rot. Blossom end rot is caused by a lack of calcium and inadequate amounts of moisture. Because of the influence of moisture, it is a problem that often develops in dry, hot weather. Make sure your garden soil contains adequate amounts of lime (pH about 6.5) and that the tomatoes are adequately watered. Most garden plants use between one and two inches of water per week.

Blossom-end rot is a physiological disorder caused by a lack of calcium in the tomato fruit. (Credit: Missouri Botanical Garden)

Why are my tomato plants wilting?

Tomatoes wilt any time they lose more water through their leaves than they take up with their roots. There are several disease organisms which, after invading the plant, block the tissue responsible for water movement and cause plants to wilt. If the wilting is caused by a disease organism, the plant usually dies. Causes for wilt in tomato include:

• Fusarium wilt This soil-borne disease is caused by the fungus Fusarium oxysporum. The fungus enters the plant vis the root system and blocks the xylem tissue which results in wilting. Often, one side of the plant exhibits wilting symptoms first. Fusarium wilt is more prevalent in warmer weather. Most hybrids carry genetic resistance; heirloom varieties do not.
• Verticillium wilt Much like fusarium wilt, except the causal pathogen is one of six different species in the genus Verticillium. Much less common than fusarium wilt, this disease favors cooler weather. Most hybrid tomato cultivars have genetic resistance.
• Bacterial wilt Caused by a bacterium in the genus Pseudomonas this disease results in a sudden wilt of the plant without leaf discoloration. The center of the stems of diseased plants will be water-soaked at first and then turn brown. In time, they may become hollow. Strict sanitation is the best way to avoid this wilt disease.
• Nematodes Nematodes that can cause wilt are soil-borne roundworms that attack the root system of tomatoes preventing nutrient and water uptake. Roots of infected plants look thick and gnarled. Once soil is contaminated with nematodes, planting resistant varieties is the best way to deal with this disease.
• Tomato spotted wilt virus TSWV is a relative new disease of tomato that produces a number of symptoms, including leaf and fruit lesions as well as wilting. The disease is transmitted by thrips. Prevention is the only cure.
• Walnut wilt Walnut and related species secrete a chemical from their roots called juglone. The latter is very toxic to tomato plants. Prevention consists of making certain there are no walnut trees in the vicinity of a planting of tomatoes.
• Waterlogged soil The roots of plants need oxygen to take up water. The soil atmosphere of waterlogged soil is low in oxygen. As a result, plants wilt even though they literally might be standing in water. The wilting is more severe when there is bright sunshine and high temperatures.

The leaves of my tomato plants are curling. Should I be concerned?

Although tomato leaves tend to cult when exposed to an herbicide, the most common type of curl is known as physiological leaf curl. This type of leaf curl is thought to be a reaction to environmental stress. Causes for the stress vary from excessive moisture and/or nitrogen, high temperatures, insufficient water, severe pruning, or root damage of some type. In the case of physiological leaf curl, symptoms occur first on the lower, older leaves. In contrast to leaf curl caused by herbicides, physiological leaf curl leaves tend to roll upward followed by an inward curling of leaflets. Leaves often appear to be thick and leathery but remain a normal green color. If stress conditions persist, all leaves on a plant might ultimately show symptoms.

The good news is that, as alarming as physiological leaf curl might appear to be, the disorder appears to have little detrimental effect on yield. Therefore, preventative measures are seldom worth the effort.

Physiological leaf curl is a response to environmental stress and rarely reduces yield. (Credit: Texas A&M University)

What causes small, irregular yellow patches on the skin of otherwise red tomato fruit?

The fact that the yellow patches are irregular in their location on the fruit suggests that the problem might be stink bug damage. Stink bugs adults are shield-shaped and colored brown or green. They have mouthparts that pierce the skin of the tomato fruit and suck juices from it. This leaves a small "pinprick" on the tomato that ripens to yellow instead of red. Hundreds of feeding sites located in close proximity to one another would tend to result in a patchwork appearance of the fruit. Fortunately, the damage is superficial and does not affect the eating quality of the tomato fruit.

Stink bug control begins with strict sanitation and the removal of weeds both in and surrounding the garden. If damage is severe, chemical control might be warranted.

Irregular yellow spots on the skin of a tomato often are the result of stink bug damage. (Credit: Cornell University)