Yellowing in tomato plants may be symptomatic of a number of causes. Some are environmental, some are nutrient related and some may be based on insect or disease threats. Prompt investigation by concerned gardeners can quickly and effectively determine the specific problem is so that it can be dealt with accordingly.
Yellow leaves may be caused by both overwatering and underwatering tomato plants. Tomatoes are heavy feeders and are quite thirsty. However, overwatering can cause nutrients to leach out of the soil, in addition to drowning the roots. Underwatering causes problems with nutrient uptake as well, even if the necessary nutrients are available in the soil.
Tomatoes are heavy nitrogen feeders. If enough nitrogen is not present in the soil, their leaves can begin to yellow in response. They may also look weak and sickly. Iron deficiency may be another nutrient-related cause. Soil testing is advisable if nutrient deficiencies are suspected.
A number of insects can cause tomato plants to turn yellow. Microscopic root knot nematodes operate underground, laying eggs on the root tips. When larvae hatch, they burrow into the roots to eat. Their digestive juices cause swelling, which leads to knots (also called galls) on the roots. This inhibits nutrient uptake, which causes stunted growth and yellow leaves. Tomato or tobacco hornworms (related insects with different markings) chew leaves and can cause yellow spots to form. Whiteflies and aphids also chew leaves, leaving a sticky substance on and tiny holes in them, as well as turning them yellow. Mites of various species may also suck juices out of tomato leaves to turn them yellow. Mites usually congregate on the undersides of leaves.
Early blight, fusarium wilt and tobacco tomato mosaic virus are all diseases that can cause yellowing or patterns featuring yellow to form on tomato plants. Fusarium wilt results in all-over yellowing, as well as wilt. Early blight shows up as big spots with brown, orange and yellow rings on leaves; these spots almost look like rust spots on a car. Mosaic virus is so named because the foliage damage patterns most resemble mosaics.
With the exception of environmental and nutrient concerns, crop rotation every year is an essential means of preventing these issues from arising with tomato plants. Watering soil rather than plants is a good way to prevent diseases. Application of a tomato-specific fertilizer according to the timetable presented by on the package by the manufacturer will prevent nutrient deficiencies.