It is often easy to tell if a plant is sick when it is covered in spots or leaves and fruits are rotting away, but it can be much more difficult to tell when a plant has an infection in its roots or stems. These lower portions of the plant are essential for health and productivity, but are often attacked by pathogens that reside in the soil. Below are some of the most important soilborne diseases of tomatoes and methods for managing them.
How to identify soilborne diseases of tomato
V-shaped necrosis and yellowing of leaves characteristic of Verticillium wilt.
In general, some clues that indicate if a tomato has a soilborne disease includes stunting, wilting, yellowing, dieback and reduced yield. These are also symptoms of nutrient deficiencies and some viruses, so it is important to rule out these possibilities.
Verticillium wilt: Distinctive V-shaped lesions form on the edges of leaves, with V-shaped dead tissue surrounded by a yellow halo. Plants wilt and have yellowing and dieback. Plants may wilt during the day and recover overnight. The inside of the stem has brown discoloration.
Tomato plant with Fusarium wilt. Note the yellow discoloration on only half of the plant.
Fusarium wilt: Plants have yellowing, dieback, and wilting. Sometimes only half a leaf or leaves on only one half of the plant turn yellow and die. The inside of the stem has brown discoloration near the soil line and discoloration may continue up the stem. Roots may look brown and rotten.
Corky root rot: Plants may appear slightly yellow and have weakened growth. Roots appear to be dry, brown, and cracked and have a similar appearance to tree bark. Cracked areas usually occur in distinctive bands and may be swollen. Dark brown cracking may occur on the crown and taproot of the plant.
Black dot root rot: Roots are discolored, usually a honey-brown to grayish-brown, and are speckled with black dots.
Root knot: Roots are misshapen with small to large nematode-induced galls. Galls may range in size from pin-head to finger-sized. Golden-brown dots (egg masses) may appear on the outside of galls. Plants may appear stunted and weak.
Roots with severe corky root rot. Note the cracked, corky, bark-like roots.
Roots with sclerotia of Colletotrichum coccodes, the causal agent of black dot root rot. The honey brown discoloration is also characteristic of this disease.
Tomato roots with severe root knot nematode galling.
What causes these diseases?
Verticillium wilt is caused by the fungus Verticillium dahliae, which has an extremely broad host range. There are two races of V. dahliae that infect tomatoes.
Fusarium wilt is caused by the fungus Fusarium oxysporum f. sp. lycopersici and there are three races that infect tomatoes. The pathogen is an excellent soil survivor.
Corky root rot is caused by the fungus Pyrenochaeta lycopersici and survives in soil via microsclerotia that form on roots.
Black dot root rot is caused by the fungus Colletotrichum coccodes, which also causes anthracnose on tomato fruits. The pathogen is capable of surviving in soil by microsclerotia that form on infected fruit and roots (the black dots).
Root knot nematodes belong to the genus Meloidogyne. Both the northern root knot nematode (Meloidogyne hapla) and southern root knot nematode (Meloidogyne incognita) are present in Ohio tomatoes. In general, M. hapla forms smaller, distinct galls on tomato roots, while M. incognita tends to form larger, fused and malformed galls. Both species of nematodes have extremely broad host ranges.
How can I manage these diseases?
When managing soilborne diseases, growers should combine management practices that prevent existing soilborne pathogen populations from increasing with practices that actively reduce pathogen populations in the soil.
- Prevention: For transplant production, always use clean planting materials including seed, soilless media (recommended) and well or “city” water for irrigation. Ensure that seedlings are healthy before transplanting. Maintain proper fertility and watering to ensure healthy seedling development and maintain adequate nutrient and water levels throughout crop development.
- Sanitation: Remove diseased plants and diseased plant parts. Clean soil from boots and equipment between fields and high tunnels. Do not move from soilborne disease-affected fields to non-affected fields.
- Rotation: Rotate out of the same plant family when possible. For pathogens with extremely wide host ranges, such as Verticillium sp. and Meloidogyne spp., it is difficult to rotate to a suitable non-host crop. Since most soilborne pathogens are excellent soil survivors, rotations of 3-5 years are usually necessary to reduce pathogen populations adequately.
- Host resistance and grafting: Resistant varieties should be selected whenever possible and resistance to Verticillium wilt and Fusarium wilt is incorporated into most modern tomato varieties. Grafting a disease susceptible scion onto a disease resistant rootstock can reduce damage due to soilborne diseases. Many commonly used rootstocks have resistance to Verticillium wilt, Fusarium wilt, corky root rot, and some resistance to root knot nematode.
- Soil disinfestation: Several soil disinfestation options are available that vary in cost, efficacy, and environmental impact. Chemical fumigation and steam sterilization are two options that have been used historically, but are often not feasible for use on vegetable farms. Anaerobic soil disinfestation is a newer method of soil disinfestation that involves amending, saturating, and tarping soil. Soil solarization uses solar-generated heat trapped under plastic sheeting to kill soilborne pathogens, but this technique is not often effective under Midwestern conditions. Soils can be flooded or left fallow to kill pathogens over a period of time, but these methods are often ineffective due to the survival structures of most soilborne pathogens.
- Chemical or biological control: Few options are available and many biological control options are still experimental.
Article contributed by Anna Testen and Sally Miller, The Ohio State University Department of Plant Pathology