Updated: September 16, 2021
By Gerald (Jerry) Brust , Ben Beale , and Karen Rane

Corky Root Disease in Tomatoes

Corky root disease on tomato roots showing dark brown banded lesions (arrows).
Fig. 1 Corky root disease on tomato roots showing dark brown banded lesions (arrows).

Corky root is caused by the fungus Pyrenochaeta lycopersici. Tomato plants affected with corky root may appear stunted and generally lack vigor. Later the infected plants may wilt during the day and recover at night, along with a slight yellowing and senescence of older leaves. Branches on mature plants may die back from the tips. Roots show dark brown, banded lesions (fig. 1) with barklike cracking and a loss of fine roots and cortex tissue.  As the disease develops even the larger roots become infected and develop extensive brown lesions that are somewhat swollen and cracked along their length, giving them a corky appearance (fig. 2). The disease is most commonly seen in tomato, but host range studies indicate that other vegetable crops, such as pepper, eggplant cucumber and melons, are susceptible.

Corky root disease on tomato root showing dark lesion that is cracked and swollen.
Fig. 2 Corky root disease on tomato root showing dark lesion that is cracked and swollen. 

The corky root pathogen produces microsclerotia on the roots of host plants that can survive in the soil for up to 15 years. In the presence of a susceptible host the microsclerotia will germinate and infect host roots. Optimum soil temperatures for root infection are between 60-68°F, which means that most initial infections occur early in the growing season. Aboveground symptoms may not develop until later in the season when temperatures increase and the compromised root system cannot absorb enough water to meet plant needs. Research has shown that reduced incidence of corky root disease is associated with use of plant -based composts (as opposed to composted manure) as well as lower concentrations of NH4-N and higher concentrations of calcium in the soil. Crop rotation out of solanaceous and cucurbit crops, and delaying planting until soils warm to 68°F, may also help in managing this disease, but these two cultural practices are not practical for most growers. For tomato, grafting scions onto corky root resistant rootstocks has been used effectively - there are several root stocks that give complete or high resistance to corky root disease listed at the vegetablegrafting.org website. For high tunnels the best management for the disease is probably either grafting or steam sterilization; the latter also gives the added benefits of reducing other soilborne diseases and weeds.

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This article appears on September 9, 2021, Volume 12, Issue 6 of the Vegetable and Fruit News

Vegetable and Fruit News, September 2021, Vol. 12, Issue 6

Vegetable and Fruit News is a statewide publication for the commercial vegetable and fruit industries and is published monthly during the growing season (April through October). Subscribers will receive an email with the latest edition.