Early fungicide application is crucial to reduce downy mildew and lettuce drop disease in winter head lettuce grown in the low desert of Yuma County, Ariz., and Imperial Valley, California.

“Fungicides should be applied before growers actually see downy mildew and lettuce drop on young plants,” according to Mike Matheron, University of Arizona (UA) Extension plant pathologist. He is based at the UA's Yuma Agricultural Center (YAC), Yuma, Ariz.

Fungicides help prevent and destroy sclerotia caused by the fungal pathogens Sclerotinia minor and S. sclerotiorum which cause lettuce drop, Matheron says.

Matheron shared his latest findings on the diseases and recent test trial results during an August preseason vegetable workshop in Yuma sponsored by UA Cooperative Extension, Yuma County.

“We all aspire to have perfectly healthy lettuce,” Matheron grinned as he showed a PowerPoint photo of a flawless field to the 70 growers, pest control advisers, and industry members in attendance. In the real world Matheron said fungal pathogens challenge lettuce growers in irrigated agriculture and efforts to control production costs.

Downy mildew

Downy mildew in winter lettuce is caused by the pathogen Bremia lactucae. Symptoms include light green to yellow angular lesions on the leaf top and sporulation on the under leaf side.

The organism grows in cool, moist conditions found in irrigated lettuce fields. Older leaves are attacked first. The infection of the cotyledon (embryo leaf) of young seedlings can result in plant death.

The initial inoculum is spread by oospores in plant debris, sporangia from nearby lettuce fields, inoculum in wild lettuce plants, and rarely by lettuce seed.

The secondary spread of inoculum is airborne. Sporangia are produced at night after a dark, dry period combined with several hours of 100 percent relative humidity and low wind speed.

Ideal temperatures for sporangia production range from 40 F to 75 F; 68 F is optimal. Sporangia are released at sunrise; the peak period is 10 a.m. to noon. Downy mildew infection occurs within several hours in a 23 F to 85 F environment.

Matheron cited studies conducted in the Salinas Valley where downy mildew impacts summer lettuce. Infection occurs when the leaves are wet until 10 a.m. or later. No infection occurs if the leaves are dry by 8 a.m. The same is true in desert production.

“Wet leaves past 10 over several days is favorable weather for downy mildew development,” Matheron said. “Growers should consider fungicide applications to protect the crop.”

The best management options include resistant cultivars and fungicides. Some resistant cultivars are grown in the Salinas Valley, the world's largest salad vegetable-growing region. Matheron recommends supplemental fungicide use even with resistant cultivars.

Fungicides are generally applied on lettuce plant leaves for downy mildew control.

Matheron conducted a downy mildew-fungicide trial at the YAC during the 2008-2009 winter lettuce season. The Nov. 3-planted crop was sprayed Jan. 22 and again Feb. 2, 10, and 16. Downy mildew was first observed Feb. 4.

“We had two applications of fungicides on the lettuce before we saw the first evidence of downy mildew,” Matheron said. “That's the way it should be done.”

The fungicides providing the best downy mildew control in the trial were a combination of Blockade, Manex, and Revus. Each plant averaged about 3.5 lesions.

Matheron said, “Generally every product we tested performed well either by itself and when combined with other products. Some performed better than others. The key is to apply fungicide early for the best results.”

Similar trials conducted in the Salinas Valley have generated similar results, Matheron said.

The incidence of downy mildew incidence is variable in the low desert since the weather that provides moist conditions needed for severe disease development is relatively uncommon. The incidence rate can range from 1 percent to an uncommon 50 percent. Last season downy mildew was found in about 10 percent to 15 percent of all Yuma County fields over the entire season.

Lettuce drop disease

Lettuce drop is caused by two fungi — Sclerotinia minor and S. sclerotiorum. Both create problems in desert lettuce. Sclerotinia minor has sclerotia one-quarter to one-half inch smaller than S. sclerotiorum yet more Sclerotinia minor sclerotia are found on the lettuce.

Conditions conducive to lettuce drop development include moist soil with a high sclerotia population, the multi-year survival of both pathogen species, plus favorable growing temperatures comparable to downy mildew development.

Sclerotinia minor infects stems and leaves in contact with the soil. The fungus causes a brown, soft decay that destroys the plant crown tissue. Older leaves then wilt. The entire plant eventually wilts and collapses. The lettuce is unmarketable.

S. sclerotiorum also infects lower leaves and stems causing symptoms similar to S. minor. S. sclerotiorum has an aerial spore that attacks the upper leaves. Spores usually infect damaged or senescent tissue in cool and moist weather. Infection causes a watery, soft rot accompanied by white mycelial growth and sclerotia formation.

“My suspicion is both pathogens are less likely to survive as long in the desert with the hot temperatures and crop rotation than in the Salinas Valley,” Matheron said.

A field with S. sclerotiorum or S. minor disked up after the season's final lettuce harvest harbors the pathogen until the next year's lettuce crop.

“To successfully control lettuce drop, sclerotia of the fungal pathogens must not germinate or else be destroyed,” Matheron emphasized.

Sclerotinia lettuce drop is found in about 10 percent to 15 percent of fields on an annual basis. Financial losses from the disease can cost growers an average of 1 percent to 2 percent income annually but more in “wet” years.

In separate YAC lettuce field trials with Sclerotinia minor and S. sclerotiorum, six fungicides were tested for lettuce drop control. Each included four trials with two soil-surface applications.

“Each fungicide currently on the market statistically performed about the same and provided good control,” Matheron said.

“The product that performed the best overall was fluazinam which is not currently registered in Arizona and California. Fluazinam, if registered, will be a great tool in the grower's fungicide arsenal,” Matheron said.

Matheron also tested how fungicides are applied. In separate trials the products Endura and Rovral were applied to the soil surface and incorporated 2 inches under the soil. No significant difference was found.

In another trial Matheron applied Contans and Botran at seeding versus thinning. The fungicides applied at seeding provided equivalent control (about 40 percent) of Sclerotinia minor. However the fungicides at seeding achieved 90 percent control of S. sclerotiorum. The trials will be repeated.

Current fungicides provide about a 50 percent control of lettuce drop depending on the year. Matheron believes current chemistries and new compounds under development could provide 70 percent control.

Yuma County farmers planted about 45,000 acres of head lettuce during the 2006-2007 growing season. The crop value totaled about $305 million or about $6,800 per acre.

Imperial County farmers harvested almost 24,000 acres of head lettuce during the 2008-2009 season valued at about $121 million. Riverside County production included about 900 acres for the same period.