In California, increasing numbers of cases of black root rot of lettuce have been documented in 2017 and especially in 2018. This continuing trend shows that this disease is becoming more important to lettuce growers on the coast. Because black root rot of lettuce can result in symptoms similar to those caused by other diseases, poor soil preparation, uneven fertilizer application, and other issues, growers and PCAs should be aware of this disease and have suspect plants examined by a lab.
Lettuce plants can become infected at any stage, resulting in plants showing varying degrees of stunted, delayed growth. An affected field will therefore look uneven, with healthy full-sized plants growing adjacent to infected smaller plants. Note that in many cases, leaves of infected lettuce plants remain green and normal looking. In other cases, only the lower leaves may turn yellow. However, black root rot by itself does not cause plant dieback and collapse as seen with Pythium wilt, Fusarium wilt, Verticillium wilt, lettuce drop, or INSV.
Roots of plants with black root rot initially have small, orange-brown to black discolored bands on the roots. The small secondary feeder roots typically show the most symptoms. In advanced stages of disease, taproots can also have these discolored bands. If severe, black root rot can cause much of the feeder root surface to be discolored. However, these root infections do not cause the entire root to become brown, decayed, and soft (compare this with Pythium wilt of lettuce, in which most of the affected roots are very dark and mushy in texture). Black root rot does not cause the central core of the roots to turn brown or black, as seen with Fusarium and Verticillium wilts.
Black root rot is caused by the soilborne fungus Thielaviopsis basicola. The fungus makes two types of spores, with one type being a thick-walled spore that resists weathering and enables the fungus to survive for long periods in the soil. This pathogen is reported to have a broad host range that includes some of the vegetable crops rotated with lettuce such as bean, beet, carrot, garlic, onion, pea, pepper, and tomato. Thielaviopsis has not been found to infect coastal crops such as broccoli, cauliflower, celery, cilantro, parsley, rappini, spinach, or strawberry.
Typical symptoms of black root rot of lettuce consist of orange-brown to black bands on small feeder roots.
In severe cases, much of the feeder root surface can be affected.
Black root rot causes plants to be stunted and delayed in development.
Thielaviopsis basicola produces thick walled structures that enable it to be spread in contaminated soil and to survive for long periods in the field. These structures can grow on the root surface and even inside root hairs.