Sixty to 70 percent of lettuce grown in California and Arizona is treated for lettuce leaf drop caused by a pair of soil borne pathogenic fungi, Sclerotinia minor and Sclerotinia sclerotiorum.
Although both cause lettuce leaves to wild, shrivel and drop down quickly, Sclerotinia can cause a total loss in a field, according to University of Arizona plant pathologist Barry Pryor.
However, Pryor said the two fungi react quite different to control measures.
Pryor told the recent 14th annual Desert Crops Workshop that S. minor is relatively easy to control with a wide array of fungicides and plant breeders are advancing toward resistant cultivars. It can also be “controlled” by rotating it with broccoli.
However, it is a different story for S. sclerotiorum. It is tougher to control with existing pesticides and about the only way to bring it under control in lettuce now is through a fallow rotation.
However, there is a new biopesticide, Contans, Pryor and others have tested which has proven significantly control of leaf drop than a wide array of other products.
Contans WG consists of living spores of Coniothyrium minitans, a parasitic fungus that attacks and destroys Sclerotinia in the soil before conditions induce a mold outbreak.
Pryor told those at the Holtville conference that significant improved control of S. sclerotiorium would offer a breakthrough in resistant management as well as complementing other disease management strategies.
These fungi infect lettuce during cool, moist conditions causing a soft, watery decay of the plant tissue. Both fungi produce structures that enable them to persist in soils for long periods of time, especially under dry conditions. There are more than 1,500 Sclerotinia hosts.
Leaf drop can be managed by: avoiding excess irrigation, especially when the leaves cover the soil and can be in contact with germinating sclerotia; complete turning or deep plowing of the soil to bury and promote rotting of sclerotia; rotation with resistant crops such as corn and grasses and application of effective registered fungicides.
In his tests, Pryor found that widely used fungicides Rovral, Ronalin, Botran and Endura as well as the biopesticide Contans provide largely equal control of S. Minor. However, Contans did much better up against standard fungicides against S. sclerotiorum.
In plots infested with Sclerotinia minor, all materials tested at an appropriate rate significantly reduced disease.
The best treatments included an application of Contans followed by an application of Endura, as well as two applications of an experimental compound or the standard materials Ronilan and Rovral.
In plots infested with S. sclerotiorum, two applications of Contans provided the best level of disease reduction among tested materials.
Mild to moderate temperatures and moist soil conditions favor lettuce drop; therefore, the incidence of the disease normally is highest from December through March in western Arizona lettuce fields.
Apply after thinning
To minimize lettuce drop, a fungicide treatment can be applied to lettuce beds immediately after thinning when plants are very small. This fungicide application, which can be followed in about 3 weeks by another treatment, forms a chemical barrier between the soil and the developing leaf canopy of the lettuce plant. With this chemical barrier in place, the bottom leaves and stem of each lettuce plant will be protected from colonization by the germinating sclerotia of the pathogens.
Timely application is critical component when lettuce is planted in fields with a history of drop.