Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 3 of 3

Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 3 of 3

“Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 3 of 3 (Pests and Diseases)”

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See Part 1 of this series for an introduction to the topic and common causes of tomato-leaf yellowing that result from issues management and maintenance.

See Part 2 of this series for common causes of tomato-leaf yellowing that result from abiotic, environmental, and soil problems.

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Below are common causes of yellowing tomato leaves that are caused by pests and diseases, with advice for diagnosing and resolving each problem. These are written for California gardeners but also apply to tomato-leaf-yellowing in other areas.

Downy Mildew: Downy mildew generally appears in tomatoes as yellowing leaf spots that are often angular and limited by leaf veins. Yellowing may spread and/or leaves may develop dead spots, which may cause downy mildew to be mistaken for other problems, such as nutrient deficiency. Downy mildew tends to be a problem during cool-to-cold, humid or wet weather, but can occur year-round. Overhead irrigation that wets plants and leaves may encourage infestations of downy mildew. Powdery Mildew: Powdery mildew appears as white or whitish circles or whitish layers on the upper or lower surfaces of leaves. Leaves slowly yellow and/or develop dead spots, which may cause powdery mildew to be mistaken for other problems, such as nutrient deficiency. Powdery mildew tends to become a problem in warm-to-hot, dry weather. How do you distinguish powdery mildew from downy mildew? Damage from downy mildew tends to appear initially as yellowing spots that are often angular and limited by leaf veins with no whitish mildew layer or spots on the tops of leaves.

Powdery mildew on tomato plant leaves

     Advice: Some mildews can be discouraged by using a nontoxic homemade spray of one tablespoon each of baking soda and tea tree oil per quart of water, shaken vigorously and regularly as applied via spray bottle. Mildews are often difficult to control and are best managed through prevention and good cultural practices, such as planting resistant varieties, planting in areas with good air circulation, allowing extra space between plants, keeping foliage dry, less-frequent irrigation, encouraging biologically active “living soil” that will support strong plant immune systems, and rotating crops. Mildews are often not fatal. Tomatoes may produce reasonable yields and fresh uninfected leaves despite ongoing infections on older leaves.

Curly Top Virus is spread by the beet leafhopper. It uncommonly affects tomato plants in home gardens. The virus is spread only by leafhoppers and does not spread by human contact or wind from plant to plant. Affected plants are stunted if infected when young. Leaves curl inward or downward, and as infection progresses plant leaves may turn yellow and/or die.

     Advice: Planting resistant varieties is the only known way of managing Curly Top Virus in affected areas.

Tomato Mosaic Virus causes symptoms on tomatoes that may be mistaken for herbicide damage, and may also be mistaken for chlorosis or nutrient deficiencies. It causes a general mottled lightening in color of leaves to light-green, yellow, or white. It is often accompanied by sparser, stringier foliage than on healthy plants. The virus is most commonly spread by people using infected tobacco products, and can also be spread via infected seeds. There is no known cure. Infected plants may produce yield, with fruit size and quantity usually reduced.

Tobacco mosaic virus on tomato plant leaves

     Advice: Plant resistant varieties. Remove infected plants to avoid infection spreading to healthy plants. Avoid smoking or using tobacco products around tomato plants, and use good sanitation, including washing hands and changing clothes, after smoking and before working around tomato plants. Rotate crops; plant vegetable or other varieties that aren’t susceptible to Tomato Mosaic Virus for a few seasons after an area has been affected.

Root Diseases and Soil-Borne Diseases: Various soil-borne and root diseases cause yellowing leaves in tomato plants. Fusarium wilt is unique in that it causes yellowing on a single stem or on one part or side of a tomato plant before spreading to the rest of the plant. Verticilium wilt and fusarium wilt both cause a browning of the inner tissues inside stems, and can sometimes be diagnosed by cutting a stem and comparing the cross section at the cut with white-and-green tissues inside of a healthy stem cross-section.

     Advice: Fusarium and verticilium wilt are generally considered incurable once plants are infected and showing symptoms. Plant resistant varieties, provide suitable environmental conditions for growing tomatoes, encourage thriving soil ecosystem, and maintain even soil moisture without overwatering.

Late Blight (pictured at top) is a wind-borne fungal disease that spreads rapidly during cool, wet or humid weather, especially in California coastal and coastal-influence areas. It is uncommon in home gardens in California inland areas, and rarely occurs during warm, dry weather. It usually affects established plants after blooming and while fruiting. Late blight produces leaf lesions or discolored, water-soaked leaf areas that spread and turn brown or black, often with yelllowing of leaves at the margins of lesions. White fungal growth may be apparent on undersides of leaf lesions. As infection progresses, fruits become discolored and decayed. Late blight usually causes a partial or complete loss of harvest, but depending upon timing and severity of infection, tomato plants may still produce uninfected fruits. Early blight causes black or brown spotting on tomato leaves, stems, and fruits but does not cause generalized leaf yellowing.

     Advice: Plant resistant and early tomato varieties. Allow sufficient space between plants for air circulation to encourage dry leaves. Avoid wetting tomato leaves, such as with sprinklers or overhead irrigation. Serenade is an organic-approved anti-fungal spray that slows and helps to control Late Blight.

Sucking Insects: Common sucking insects on tomato leaves include aphids, spider mites, and whiteflies. Yellowing of tomato leaves may occur in spots or stippled patterns as sucking insects feed on them from the undersides of leaves. Aphids are soft-bodied, oval or pear-shaped insects that are often visible with the naked eye. Spider mites are usually too small to see easily with the naked eye; inspect the undersides of leaves with a hand lens or shake plants over a blank piece of white paper to see more clearly. Webbing or web tents that may be mistaken for spiderwebs are often present with spider-mite infestations. Whiteflies are tiny moth-like flies that typically cause the greatest damage to tomato plants in the larval or nymphal stages, which are small and flat and may be mistaken for scale insects.

     Advice: Plant resistant varieties, provide suitable environmental conditions for growing tomatoes, encourage thriving soil ecosystem, maintain even watering without overwatering. Provide habitat and conditions to encourage beneficial and predatory insects. A strong stream of water can often resolve minor infestations. For severe infestations, use horticultural oils or insecticidal soaps that are labeled for the pest in Pesticides and insecticides often kill predators of sucking insects and encourage repeated infestations.

Root Knot Nematodes are tiny worms too small to be seen with the naked eye that infest the roots of many plant species. Aboveground symptoms on tomato and other plants are usually general and include yellowing of leaves, slow growth or lack of vigor, poor fruiting or blooming, wilting even when sufficiently watered, and unresponsiveness to management and treatment. To diagnose, confirm galls on tomato roots. Root-knot nematodes cause galls or what appear to be round or lumpy growths on sections of roots, and may infect multiple species in a given area, including shrubs and trees.

     Advice: Prevent infestations by preventing transfer of soil into your garden. Root-knot nematodes are often transported to new areas in soil, such as with infected transplants or on muddy shoes or equipment from an infected site by labor gardeners. Resistant tomato varieties are available for some species of nematodes. Degree of damage to plants and harvest depend upon level of infestation. Root-knot nematodes may persist on numerous plant species including trees and weeds, and are extremely difficult to control or eliminate once an area is infested. Soil solarization may reduce nematode populations in the upper several inches of soil long enough to succeed with annual vegetable crops. A combination of planting resistant and non-host plant varieties, soil solarization, and fallowing (leaving soil unplanted) for several months to a year at a time may also help reduce nematode populations. Fallowing soil for 4 to 5 years or longer with no host plants may be required to eliminate root-know nematodes. Encouraging a thriving soil ecosystem that will abound with predators of nematodes requires effort but may be the most effective long-term solution for skilled gardeners.

See customized advice and information for growing tomatoes in your Southern California zipcode
Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 1 of 3 (Management and Maintenance Issues)
Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 2 of 3 (Abiotic, Environmental, and Soil Problems)

Other articles of interest:

GardenZeus Tips for Fertilizing Tomatoes During the Growing Season

The GardenZeus Guide to Staking, Supporting and Trellising Tomato Plants

GardenZeus Solutions to Common Abiotic Problems With Garden Tomatoes

GardenZeus Calfornia climate zones

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