Pythium wilt disease has become the main root rot problem on coastal California lettuce. First found in the Salinas Valley in 2011 on only two ranches, this disease is now present in numerous fields throughout this coastal region. In contrast to Pythium root rots of spinach and other vegetables, this lettuce pathogen does not cause damping-off on newly emerged, young lettuce seedlings. Instead, Pythium wilt mostly develops on lettuce that is at the rosette stage or older. Infected plants will be stunted. Outer leaves will start to wilt and eventually turn yellow before becoming brown and dead. As disease progresses, all leaves can wilt and the plant can collapse. Below ground, the pathogen first attacks the small feeder roots, making them soft and brown gray in color. Late in disease development the taproot can be darkly discolored and the entire root system can be rotted. This disease has been confirmed on iceberg, romaine, and greenleaf lettuce types.
Pythium wilt is caused by Pythium uncinulatum. The pathogen produces swimming zoospores that allows it to move in the soil and be spread via surface or sub-surface water flow. The pathogen also produces a sexual spore (oospore) that is encased within a spiny outer covering. Pythium uncinulatum is host specific to lettuce and apparently does not infect other vegetable crops such as broccoli, cabbage, carrot, onion, pepper, radish, spinach, or tomato.
Because Pythium wilt causes a general wilting and collapse of lettuce foliage, this disease can be confused with other problems. Sclerotinia and Botrytis infections both can result in plant wilt and collapse. However, in these cases the symptoms result from crown infections; neither Sclerotinia nor Botrytis infect lettuce roots. Verticillium and Fusarium wilts, which also cause lettuce to collapse, will exhibit the distinctive discolorations of the vascular tissues while not causing any root rot. The other root disease new to Salinas Valley lettuce growers, black root rot (pathogen: Thielaviopsis basicola), causes dark bands to form on roots but does not result in the extensive feeder and taproot decay as seen with Pythium.
Accurate diagnosis of Pythium wilt and these other lettuce diseases usually requires laboratory analysis. For assistance in diagnosing these problems contact our TriCal Diagnostics team.
Lettuce field severely infected with Pythium uncinulatum.
Severely stunted lettuce plants affected by Pythium wilt disease. A healthy plant is on the left.
Pythium uncinulatum can cause extensive rotting of lettuce roots.
The pathogen produces a spiny survival structure.