LOVELOCK - Whether you call it a fruit or vegetable, the tomato is the most popular of all home-grown foods in the United States. Nevada is no exception. A survey by the National Garden Association found 93 percent of the American gardeners surveyed grew tomatoes. Unfortunately, there are more production problems growing tomatoes than growing many other garden vegetables.
Harsh climatic conditions, nutrient deficiencies, poor cultural practices and disease all bring on disorders that limit tomato production. Common problems include blossom drop, blossom end rot, curling leaves, sunscald, cracking and poor overall growth. Gardeners can minimize these problems by using good cultural practices and common sense.
The most often asked question by Nevada tomato growers is, "Why do my tomato blossoms drop?" Blossom drop may be caused by one a more of the following:
1) Too low or too high temperatures. 2) Low humidity. 3) Unusually heavy fruit set. 4) Improper watering. 5) Poor fertility practices.
Extreme temperatures or an abrupt change to hot weather can do a number on tomato plants.
Flower drop tends to be the first problem to emerge - especially following an abrupt temperature rise. This blossom drop becomes worse as temperatures go above 95 degrees, particularly if accompanied by hot, dry winds. Plants that are excessively lush or over-fertilized are most at risk. Each flower that falls from the vine equals the loss of one tomato.
There is not much you can do to prevent it, although some varieties are more prone to blossom drop than others. If you can keep the plants alive and healthy, however, they'll put out new flowers that produce fruit when cooler weather returns.
Heat's immediate threat to the plants themselves is severe stress. Mulch helps keep the soil from warming up and drying out quite so quickly. Still, garden tomatoes need a thorough watering at least once a week to encourage deep root growth down to the cooler soil zones. Container-grown tomatoes may need watering once or more per day, so long as the weather remains hot.
Tomatoes grow best if temperatures range between 70 and 90 degrees Fahrenheit, but can tolerate temperatures lower than 55° F and higher than 100° F for short periods. Temperatures lower than 55° F for four or more consecutive nights cause blossoms to drop. Temperatures that are higher than 90° F during the day are the main cause of blossom drop. In fact, temperatures over 104° F for four hours or more cause the flowers to abort. Night temperatures higher than 75° F will also cause blossom abortion. These sustained, hot temperatures at night burn up the food reserves the tomato produced during the day and the flowers abort.
Relative humidity plays a major role in pollen transfer. Tomatoes set best when humidity is between 40 and 70 percent. Nevada has long, dry periods with the humidity below 20 percent. As a result, pollen does not stick to the dry stigma. Calm, windless summer days also contribute to poor pollination.
Directing a jet of water at the blossoms twice during the heat of the day will improve blossom set when daytime temperatures range between 90° and 100° F and below 75° F at night. If daytime temperatures exceed 100 F and night temperatures stay above 75° F, this technique is not effective.
Excessive irrigations and water stress also cause blossom drop. The root zone should be kept uniformly moist throughout the growing season to develop a large, healthy root system. Tomato roots have a potential to grow to a depth of 3 to 5 feet. However, shallow watering, heavy, wet, poorly-drained soils, caliche or hard pan interfere with deep rooting. Every effort should be made to avoid or remedy these adverse conditions.
Two or three deep irrigations during the first month after planting will help establish the plants. After that, deep irrigations should be continued throughout the growing season.
High or low application rates of fertilizer will also cause blossom drop. Over-fertilization may cause water stress resulting in tissue damage inside the tomatoes. Under-fertilization leads to spindly vines with low food reserves that cannot support a crop.
To determine the tomatoes' nitrogen needs, stems should be examined 6 inches back from the end of the major branches. Ideally, the stems should be the thickness of your small finger. If smaller, nitrogen fertilizer should be applied. If stems are larger and growth is rank, fertilizer, especially nitrogen, should not be added.
Blossom end rot
Blossom end rot is a large, dark, sunken, leathery spot found on the bottom or blossom end of tomato fruits. Disfigured fruits usually appear in late spring and early summer, particularly following a heavy fruit set or a sustained dry period. To avoid continued development of blossom end rot, fruit should be thinned and adequate irrigation provided.
A lack of calcium also contributes to blossom end rot. Light, infrequent irrigations cause more blossom end rot than the lack of calcium in the soil. Light, frequent irrigations also contribute to blossom end rot because an adequate root system is not developed to allow enough calcium to be removed from the soil. Deep, infrequent irrigations are best.
Curling leaves
Curling leaves on healthy tomatoes is very common. Some varieties naturally curl more than others. Bright, sunny days, staking and excessive pruning also can cause curl.
A virus disease called curly top also causes leaves to curl. Leaves of infected plants roll upward with some twisting. Veins on the underside of the leaf turn purple and the foliage becomes dull yellow. The plant stops growing. The fruit on the vine ripens prematurely without increasing in size and the plant eventually dies. Sugar beet leafhoppers transmit the curly top virus to the tomatoes. It will not spread from plant to plant within the garden without the insects present.
Sunscald
Large green and near-ripe tomatoes sunscald easily when exposed to extreme heat and bright sun. The tissue develops a blistered, water-soaked appearance. Rapid desiccation leads to sunken areas that are white or gray in green fruits and have a yellow cast in red fruits.
To prevent sunscald, plant heavy foliage tomato varieties - Patio, Better Bush, etc. - should be used or fruit should be shaded. Staking and selectively pruning can encourage more foliage, which further shades the fruit.
Fruit cracking
Cracking occurs when temperatures climb above 90° F and irrigation, rain or high relative humidity follows a long, dry period. The moisture promotes extremely rapid growth and excessive cell enlargement within the fruit and cracks result. An even supply of moisture to the plant should prevent the problem. One way to maintain uniform soil moisture is to cover the ground under the tomatoes with 2 to 4-inches of mulch.
It helps gardeners if they understand symptoms and causes of common disorders associated with growing tomatoes in Nevada. Taking appropriate actions to avoid problems makes growing the most popular vegetable in the American garden a rewarding, productive experience.
Source: Factsheet 88-60, Linn Mills, Horticulturist, Las Vegas Valley Water District Wayne Johnson, State Horticulture Specialist Plant Science Department.
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