Poor coloring of red tomatoes is caused by high temperatures. Lycopene, the red pigment in tomatoes, will not develop in certain varieties when the temperature of the fruit is over 86 degrees Fahrenheit. Therefore, leaf cover is important for development of a uniform red color on the fruit. Other causes include severe whitefly infestations and picking fruit while immature and too green.
Several tomato viruses are known to occur in Imperial Valley. Once plants are infected with a virus, there is no cure, but resistant varieties are available for some viruses. All of these viruses cause distortions of the leaves and some affect fruit set as well as size, color and form of the fruit.
Tomato bushy stunt virus-infected plants have a dense canopy and are severely stunted. This virus survives in the soil and is moved on equipment and with water.
Alfalfa mosaic virus causes distortions and irregular brown patterns that form on the fruit. Aphids carry the virus from nearby alfalfa plants that have the virus to tomato plants, and approximately 150 other plant species. If the tomatoes are more than several hundred yards from an alfalfa field, it is unlikely AMV will be a problem
Tobacco mosaic virus causes fruit to be reduced in size and number and ripen unevenly. ToMV can be moved from a source, such as tobacco products, plant debris or equipment, to a tomato plant just by touch. Use treated seed and wash hands with soap and water before and during handling plants will help prevent spread. In addition, varieties such as Big Beef, Carnival and Celebrity are resistant to ToMV.
Powdery mildew is the most common fungal disease attacking tomato leaves in the Imperial Valley. Disease symptoms begin as a light green to bright yellow blotch on the leaf. As the disease progresses, the blotch turns brown and dies. A gray-white growth may appear on these lesions. This growth is the spores of the fungus, which can be carried long distances on air currents. Sulfur applications at label rates can control this disease.
Fusarium wilt is caused by the fungus Fusarium oxysporum f. sp. lycopersici. This fungus enters the plant through the roots, gets into the water conducting vessels of the plant and blocks water movement. The symptoms include yellowing and death of leaves beginning with the margins of the older leaves, wilting of the plant during the day and possibly the death of the plant.
Nematodes are microscopic round worms that live in the soil and damage roots, which reduces nutrient and water uptake. The above-ground effect of nematode damage is general poor growth and low yield, so this problem can be difficult to recognize since there are no striking symptoms visible unless you inspect the roots.
There are several types of nematodes that can affect tomato plants. Root-knot nematodes live in the roots and produce a substance that causes conspicuous galling on the roots. Needle nematode lives freely in the soil and feeds on the growing tips of the roots; the result of this feeding is a swelling at the tips of stunted roots. Other nematodes may cause damage that does not produce visible symptoms on the roots. Nematode damage tends to be more severe in sandy soils.
Irrigating and covering the area with clear plastic during the summer, which is called solarization, will reduce the nematode populations. Some researchers have shown that mixing cabbage, broccoli or cabbage crop residues into the soil before covering with plastic causes the nematode populations to decline even further.
>> General recommendations
It is likely that if proper amounts of fertilizer and water were applied, some problems were avoided. Tomatoes are "full sun" plants, so avoid planting them under a tree or near a structure. They have deep roots, so if possible plant where the soil is deep and well-drained. Keep the roots moist throughout the growing season, but over-irrigation on poorly drained soils kills plants by drowning roots or causing root rot.