Rain Causing Leaves on Tomato Plants to Turn Yellow

Overwatering

• Overwatering is among the most common problems tomato growers face. Long periods of rain may cause plants and their surrounding soil to be waterlogged, which may cause leaf yellowing. Too much water, particularly if soil is not draining well, prevents proper air flow to roots, essentially drowning them and eliminating the nutrient supply. As roots die, leaves will begin to turn yellow before they die and drop. Though the leaf yellowing is not reversible, correcting the soil to eliminate standing water will aid in returning the plant to a healthy condition. Underwatering may also cause problems with leaf yellowing and premature dropping. If growers don't keep soil moist, particularly during the hottest months of summer, growth and fruit production may be reduced.

Iron Chlorosis

• The nutritional balance in soil is somewhat fragile and may be negatively affected by excessive rain. Decreased iron is among the leading causes of leaf yellowing in most growing zones. There are 16 elements required to keep plants healthy, and iron is perhaps the most important, aiding in making chlorophyll in the leaves, a key component of photosynthesis. Iron deficiency reduces the amount of chlorophyll in the leaves, which causes the leaves to turn yellow-green. This discoloration begins at the edges and eventually overtakes the whole leaf. Growers should treat the soil as soon as possible to correct the problem. Treatment includes aerating the soil and applying iron chelate. Once this is done, growers should apply a generous layer of well decomposed organic compost around the base of the plant.

Nitrogen Deficiency

• Nitrogen deficiency, unlike iron chlorosis, causes a uniform yellowing of leaves and, like iron deficiency, may be made worse in times of excess rain. This discoloration begins at the lower parts of the plant, and progresses upward if left untreated. Treating nitrogen deficiency may be especially difficult for tomato plants, as treatment involves adding a nitrogen-rich fertilizer. If too much is added too fast, leaf scorching may occur and more greenery will develop. This excess foliage provides hiding spots for aphids and other pests, which may cause further health problems. For a natural, though slower treatment, growers may add a layer of compost around the plant base. Compost will create more steady treatment, while avoiding negative side effects.

Fungal and Bacterial Growth

• Bacterial and fungal growth, which tend to become more prevalent in times of excess rain and when standing water is present, are among the leading causes of leaf yellowing in tomato plants. Root rot is among the most serious and turns roots into an unusable mush, making plants unable to get food or water. Additional problems may come from Pseudomonas cichorii, which will cause leaf yellowing and eventual death of the plant and may be spread by wind or splashing rain. Treatment involves correcting soil conditions to eliminate standing water and improving air flow. Growers should remove infected debris from around plants and cut away infected leaves and twigs to prevent spreading the bacteria.