Situation: During the latter part of the 2015 Salinas Valley season (August through October), significant crop losses have been caused by Pythium wilt disease of lettuce. This disease appeared to be only recently introduced to coastal California, and prior to 2014 seemed to be of minor importance. However, during 2014 and 2015 seasons, the problem spread to a number of locations in the valley and caused perhaps 30% or more losses in some fields.

Symptoms: Above ground symptoms of Pythium wilt develop on lettuce that is at the rosette stage or older. In contrast to Pythium root rots of spinach and other vegetables, this lettuce pathogen does not cause damping-off of newly emerged, young lettuce seedlings. Infected plants will be stunted and lag behind healthy lettuce. As disease progresses, outer leaves will start to wilt during the warmer times of the day and eventually turn yellow before becoming brown and dead. In advanced stages, the entire foliar canopy likewise can wilt and senesce; such plants clearly are not harvestable. Below ground, the pathogen first attacks the small feeder roots of the lettuce, making them soft and brown gray in color. Late in disease development the taproot will also be darkly discolored and the entire root system can be rotted. Pythium wilt does not cause a rot of the lettuce crown. Pythium wilt has so far been confirmed on iceberg (crisphead), romaine, and greenleaf cultivars. See photos below.

Diagnostic challenge: 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 result in distinctive discolorations of the lettuce vascular tissues while leaving the roots intact. 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 tap root decay as seen with Pythium. Finally, foliar symptoms caused by Impatiens necrotic spot virus or Lettuce necrotic stunt virus can add further confusion to the diagnostic task because of the yellowing and browning of infected lettuce leaves. Accurate diagnosis of these lettuce diseases usually requires laboratory analysis. For assistance in diagnosing these problems, submit samples to the UC Cooperative Extension diagnostic lab in Salinas. See Table 1 for a comparison of symptoms caused by soilborne pathogenic fungi of lettuce.

Causal pathogen: Pythium wilt is caused by Pythium uncinulatum. In addition to California, this lettuce pathogen has been reported from The Netherlands, Japan, and Arizona. California's first case of P. uncinulatum on lettuce was from the Coachella Valley in 1993. This pathogen was first found in the Salinas Valley in 2011. Pythium uncinulatum, like most Pythium species, produces swimming spores (zoospores) that are released and move within the water film in the soil. In addition to zoospores, the pathogen also produces a sexual spore (oospore) that is encased within a spiny outer covering (oogonium). It is the oospore that allows the pathogen to survive in the soil in the absence of susceptible plants. Pythium uncinulatum is host specific to lettuce and does not infect other vegetable crops such as broccoli, cabbage, carrot, onion, pepper, radish, spinach, or tomato.

Disease cycle: Pythium species are soil inhabitants and persist in most agricultural soils for extended periods of time, especially if soils are moist. Specific information on how Pythium uncinulatum might persist in coastal California soils is lacking; however, we assume this pathogen can persist in soil for a significant amount of time. The pathogen and its ability to infect lettuce are favored by wet soil conditions. Flowing water and movement of soil will spread the pathogen.

Disease management: The following management strategies should be considered when dealing with Pythium wilt disease. (1) Avoid planting lettuce into fields with a known history of the problem. (2) For infested fields, rotate to non-lettuce crops. Various research studies demonstrated that Pythium uncinulatum is host-specific to lettuce. (3) Implement field sanitation practices to minimize the movement of contaminated soil from infested to clean fields. (4) Be aware that surface water run-off from infested fields may contain the pathogen. Flooding events may also spread the pathogen to previously clean fields. (5) Prepare beds so that drainage of water is enhanced, since the pathogen is favored by wet soil conditions. (6) Manage the irrigation so that excessive soil moisture is avoided. (7) The effectiveness of fungicides for controlling Pythium wilt in California is currently unknown and will require field trials.

Photo 1. Romaine lettuce field severely infected with Pythium uncinulatum.

Photo 2. Greenleaf lettuce severely infected with Pythium uncinulatum.

Photo 3. Severely stunted lettuce plants affected by Pythium wilt disease. A healthy plant is on the left.

Photo 4. Dark, rotted feeder roots of lettuce infected with Pythium uncinulatum. Healthy roots are on the right.

Photo 5. In severe cases, Pythium uncinulatum will completely rot the lettuce taproot.

Photo 6. Pythium uncinulatum forms spiny structures called oospores that persist in the soil.