Jennifer Schultz Nelson
Extension Educator, Horticulture
Home vegetable gardening continues its rise in popularity. A survey by the National Gardening Association showed a 19% increase in home vegetable gardening from 2008 to 2009. Tomatoes are a great beginner plant for those new to gardening, and offer incredible diversity in shape, size, color and flavor for the seasoned gardener looking for variety.
At some point, every gardener faces challenges in growing their favorite plants. Tomatoes are no exception. A significant portion of questions received at the Master Gardener Help Desk each summer are concerning tomato problems. These problems can be divided into the broad categories of fungal, bacterial, viral, or cultural causes. For further description of the problems described in this article, please call the Master Gardener Help Desk at (217) 877-6872 or e-mail me at firstname.lastname@example.org. We are happy to provide further information to help diagnose problems with tomatoes, or other garden plants.
· Fungal Diseases: Very common during periods of wet weather or improper watering. Disease causing fungus can survive in the soil and be splashed onto the plant through rain or watering. Tomatoes should be watered at the base to avoid wetting the foliage, which promotes fungal growth. Typically fungal diseases start with the lower leaves and spread upward by splashing from rain or watering. Removing affected leaves as soon as possible is recommended. Fungicides labeled for use on vegetables may be used to slow the progress of the disease, but will not reverse damage already done.
o Septoria Leaf Spot: 1/8 inch diameter grayish white spots with dark edges. Heavy infection results in leaf loss. Loves warm, wet weather. Usually develops after plants have set fruit.
o Early Blight (also called Alternaria or Target Leaf Spot): ¼ to ½ inch diameter brown/black spots with dark edges. Spots may merge to produce irregular blotches. Spots may contain concentric rings that make them look like a target. Loves warm, wet weather. Loss of lower leaves quite common with this disease. Often occurs alongside Septoria Leaf Spot. Can also infect potato.
o Anthracnose: Commonly found infecting overripe fruits. Small, circular indented spots on the fruit develop a sunken appearance with dark centers or specks.
o Fusarium Wilt: fungus invades the vascular system of the plant, causing widespread or one-sided wilting of the plant. Wilting begins at the base of the affected stem. Leaves may yellow and drop. Vascular tissue in affected stems displays brown discoloration when split open.
o Verticillium Wilt: Plants show brown discoloration in affected stems like in Fusarium Wilt, but is most pronounced at the soil line, extending up about 12 inches. Lower leaves develop yellow splotches, then wither and drop. Plants may survive infection but are stunted and produce less fruit.
o Late Blight: May show itself in lower (older) leaves or upper (younger) leaves first. First symptoms appear as water-soaked areas of leaves that grow rapidly into greenish-black, irregular blotches. Plants appear as if damaged by frost. Infected fruit shows brown blotches and have foul odor. Infections typically emerge in mid- to late-August during cool, wet weather, particularly at night.
· Bacterial Diseases: Disease causing bacteria can survive in soil until splashed onto the plant by rain or watering. Wounded plants, from pruning or insect feeding provide a way for bacteria to enter the plant. Some bacterial diseases may be brought into the garden on infected plants or seeds.
o Bacterial Spot: Symptoms occur on both leaves and fruit; easiest to observe on fruit. Dark, greasy, 1/8 inch diameter lesions on leaves surrounded by a yellow halo enlarge, can grow together and cause leaf to die. Symptoms on fruit are black, pimple like dots surrounded by water-soaked areas that enlarge to ¼ to ½ inch diameter scabby, sunken gray-brown lesions.
o Bacterial Speck: As with Bacterial Spot, symptoms on fruit easier to observe than symptoms on leaves. Tiny 1/16 inch diameter black spots with yellow halos appear on leaves. Tiny 1/16 inch diameter spots also appear on fruits, but do not enlarge, and can be scraped off with a fingernail. Does not reduce yields, but reduces fruit quality. Typically develops during cool, wet weather, especially following rainstorms that damage leaves, creating a way for the bacteria to enter the leaf.
· Viral Diseases: Viruses may cause overall plant stunting, distorted or discolored leaves. Infected plants may not set fruit. Distorted leaves often mistaken for herbicide injury. Insects carry viruses from plant to plant via their feeding. There is no cure for an infected plant. It should be removed before insect feeding infects neighboring plants.
· Cultural Issues: Some tomato problems are not due to disease-causing organisms, but are caused by the environment in which the plant is growing. Changing the environment where possible will help reduce or eliminate the problem.
o Blossom End Rot: End of fruit opposite stem develops dark sunken region that enlarges and allows fungus to enter and rot the fruit. Caused by calcium deficiency related to lack of moisture. Keep plants consistently watered, avoiding wide fluctuations between too dry and too wet. Excess nitrogen in the form of ammonia-based fertilizer may make problem worse. Maintaining a soil pH of 6.5 will help make calcium more available to the plant.
o Catfacing: Misshapen fruits develop bands of dark leathery tissue. Usually a problem when blossoms develop during cool weather. Portions of the bloom that are supposed to develop into fruit die due to the cold temperatures, and odd-shaped fruit is the result.
o Cracking: Fruits beginning to ripen typically develop cracking after large amounts of water and high temperatures occur following a dry period.
o Sunscald: Occurs on green tomatoes exposed to direct sunlight. White, blistered area gradually becomes yellowish, sunken and wrinkled. Entire fruit may eventually rot due to invading fungal organisms. Commonly found on plants that have lost leaves due to fungal infections.
o Leafroll: Leaves may roll inward and upward during times of water stress, either too much or too little water. Herbicide damage may induce downward leaf curling and distortion of the growing points. Parts of the plant may look twisted.
o Failure to Set Fruit: High day and night temperatures inhibit fruit set. Nighttime temperatures above 70°F reduce flower and pollen production; temperatures below 55°F may cause flower drop. High daytime temperatures along with dry conditions may also cause flower drop. All result in no fruit set. Some viral infections may also result in no fruit set.
Many of the diseases and problems described can be minimized or prevented altogether through good cultural practices. To maximize your tomato crop and minimize problems, consider adding the following to your gardening routine:
· Choose disease-free plants and seeds from reliable sources. If a plant looks like it might have a disease when you buy it, there's a good chance it's not going to improve.
· When choosing what to plant, choose cultivars that are resistant to common diseases. These are typically listed in catalog descriptions in a string of letters following the cultivar name. A little work before you buy plants or seed will save a lot of grief in the long run.
· Space plants appropriately in the garden. About three feet apart in all directions is appropriate for most tomatoes. This allows for good air circulation which minimizes fungal growth, also minimizes plants touching each other, reducing plant-to-plant transmission of disease.
· Water plants at the base in the morning, to minimize how much and how long the leaves are wet. Avoid working with plants while they are wet to reduce the chance of spreading disease between plants.
· Remove diseased plant debris from the garden, both during and after the gardening season. Many disease-causing organisms can overwinter in plant material left in the garden.
· Rotate crops regularly in the garden to reduce the buildup of disease-causing organisms in the soil.
A good rule of thumb to keep in mind when growing tomatoes, or any other plant for that matter, is that a healthy plant is far less likely to develop disease than a stressed-out, weak plant. Proper watering, fertilizing, and sun-exposure will go a long way in producing a healthy garden. Above all, enjoy your garden and if your plants develop problems, think of it as a learning opportunity, not failure. There is always room for improvement, no matter if you are a beginner or expert gardener. Just enjoy the journey.