These stresses have sent many befuddled gardeners looking for a pest or disease. Tomato experts call the stresses physiological problems, meaning the cause has to do with the functioning of the plant itself, not with any outside agent.Blossom-End Rot
What it looks like: Brown-black sunken areas appear on the blossom end of green or ripening fruit.
Causes: Insufficient calcium levels in the developing fruit cause the cells in the blossom end to break down. Though insufficient levels of calcium in the soil may be the cause, it is more likely fluctuating moisture levels. This is why it is important to apply a mulch. Water transports calcium through the plant. With insufficient water, calcium doesn't move quickly enough to the fruits. As little as 30 minutes of water deficiency at any time can cause blossom-end rot.
These other factors contribute but are all ultimately connected to calcium availability in the developing fruit: excess nitrogen fertilization, high soil salinity, waterlogged soils, root damage during cultivation, and soil pH that's too low or too high. Blossom-end rot occurs most often on the first fruit clusters, when the plant grows quickly and demands calcium for leaf growth.
What to do: Pick and destroy rotten fruits, keep the soil pH around 6.5, reduce nitrogen fertilization, and apply a complete fertilizer such as 5-10-10 (1/2 cup per 10-foot row) once early in the growing season. Also, mulch early in the season with a 4- to 6-inch layer of hay or straw. Apply at least 1 1/2 inches of water a week, and avoid growing susceptible older indeterminates (vining tomatoes) such as 'Beefsteak' and both determinate and indeterminate varieties of plum tomatoes ('Roma' is one example).
What it looks like: First you see light green or clear blotches on green fruit. These patches gradually turn yellow, and the tomato doesn't ripen evenly. Often, the tomato is rotten inside.
Causes: Graywall is most often caused by shade and cool temperatures followed by bright, sunny weather. Excessive foliage growth also causes it, so the most vigorous indeterminate tomato varieties are the most susceptible. But other factors promote graywall. For instance, it occurs more often in plants growing in soggy soils, and diseases like tobacco mosaic virus may indirectly cause this condition.
What to do: Grow determinate bush varieties that allow sunlight through the leaf canopy. Decrease nitrogen fertilizer, and increase potassium. Stake or cage plants to allow sufficient light to reach the fruit.
What it looks like: Misshapen fruit has black scarred areas on the blossom end that often rot.
Causes: Catfacing happens when flowers don't develop properly. The most common cause is low temperatures (below 65° F during the day and 55° F at night) three weeks before flowers open. High wind on plants with little foliage can also damage blossoms. Although a common problem on the first fruit clusters, it disappears when temperatures rise. But it may recur if the plants are still setting fruit as temperatures drop in the autumn. Larger and older varieties such as 'Beefsteak' are more susceptible to catfacing.
What to do: Grow the plants when temperatures are high during pollination by planting later in the season and protecting transplants from cold and wind with plastic cloches or floating row covers.
What to do: To avoid catfacing next year, grow modern hybrid varieties that are much less likely than older ones to be bothered by low temperatures and don't prune off the foliage.
What it looks like: When ripening, fruits crack around the stem end and along the sides, and the fruit rots. Cracks may be radial or concentric.
Causes: Abruptly alternating wet and dry periods cause cracking. When the plant takes up deep drinks of moisture after a dry spell, the fruit cells expand too fast and burst, and the skin cracks. (Heavy dew worsens cracking because the fruit can take water in through the skin.) The soft-fruited 'Celebrity' and cherry tomato 'Sweet 100' are particularly prone to cracking. Too much nitrogen in the soil also contributes to the problem. Green fruits usually don't crack because they're harder and can't expand as fast, and their skin cells are stronger.
What to do: Keep the soil evenly moist, especially during ripening, with a 4- to 6-inch mulch of hay or straw. Don't overfertilize. If maintaining soil moisture is difficult in your area, plant less-susceptible varieties next year. 'Mountain Spring' and 'Mountain Belle' (a cherry tomato) are good options.
What it looks like: The "shoulders" on the tomato's stem end stay green (or yellow) and hard as the rest of the fruit ripens.
Cause: Normally chlorophyll breaks down as the fruit ripens. However, in some varieties, during periods of high temperatures and direct sun exposure, the chlorophyll does not break down, or does so too slowly.
What to do: This problem is most common in heirloom varieties that happen to lack the gene for uniform ripening. Most modern hybrids have this gene and rarely develop green shoulders. However, if you want to grow the older, susceptible varieties, minimize green shoulders by maintaining good foliage cover and picking the tomatoes when they're entirely green to ripen indoors, away from exposure to direct sun.
What it looks like: Fruits lack internal jelly and have a hollow or "puffy" feel.
Causes: Puffiness is caused by incomplete pollination, which in turn is most commonly caused by temperatures that are too high or too low. (Tomatoes are almost completely self-pollinating, so pollinating insects aren't a factor.) Whenever nighttime temperatures drop below 55° F or daytime temperatures rise above 100° F, seed doesn't set properly. Improper pollination prevents the jelly of the inside fruit chamber from developing. Also, too much nitrogen or too little potassium in the soil causes poor pollen formation, leading to puffiness.
What to do: Set plants out once nighttime temperatures are consistently above 55° F, or protect young transplants from cold with a floating row cover. In hot climates, use shadecloth to keep plants cool. Protect them from hot winds. Reduce nitrogen fertilization, and test soil to check potassium levels. Next year, plant resistant varieties, such as 'Celebrity' and 'Better Boy', which are less likely to develop the problem under any circumstances.
What it looks like: The fruit has lighter-colored leathery patches, and fruit usually rots.
Cause: This discoloration is like a sunburn. Fruits exposed either suddenly or continually to hot sun develop sunscald, which is most likely to occur on varieties that don't produce enough leaves.
What to do: Avoid pruning leaves or stems while the fruit is ripening, and consider shading the fruit. A small section of shadecloth or row cover would suffice. Finally, try to reduce the severity of leaf diseases such as early and late blight, common fungal diseases. If you live where summers are sunny and hot, grow indeterminate varieties that produce a thick cover of shading leaves. Examples are 'Jet Star', and 'Big Beef'.
What it looks like: This problem is aptly named because it looks just like a zipper running from top to bottom on the skin of the fruit. It's disfiguring, but the fruit can still develop properly.
Cause: Temperatures below 55° F when flowers are dropping off the young fruit cause this condition. Zippering can occur at higher temperatures, too. Some varieties, such as 'Mountain Spring' and 'Mountain Pride', are genetically predisposed to the problem. Others, such as 'Big Beef', are not.
What to do: Cover young transplants with floating row covers to reduce the chances of cold damage to young fruit. Next year, plant a resistant variety such as 'Big Beef'.
Charlie Nardozzi is a senior horticulturist at National Gardening.
Photo courtesy of University of Wisconsin-Madison, Dept. of Plant Pathology