Lettuce growers know Sclerotinia or leaf drop can destroy a crop, but University of Arizona plant pathologist Mike Matheron told the 14th Desert Crops Workshop in Yuma recently that a one-two punch of fungicides in the growing season and biological control in the off-season can bring the disease under control in the desert.
That is good news since the disease can be insidious by laying in wait in the soils for several years, waiting to attack young lettuce plants. Once a soil is infected with the sclerotia can lay dormant for several years to infect another lettuce crop. It has become such a problem that it is one reason given for a shift in lettuce acreage from Yuma to California's Imperial Valley.
The workshop was sponsored by the Cooperative Extensions at the University of Arizona and the University of California and Western Farm Press.
Two soil-borne pathogenic fungi cause leaf drop. One, Sclerotinia minor is a bit more benign than Sclerotinia sclerotiorum, but neither is a good actor.
Matheron has looked at a variety of fungicides to control the disease over the years. While most offer some control, is it often not sufficient. In some years his tests have shown complete failures of products that were effective just a year earlier.
One of the more promising products he is testing is the fungicide Omega. It is not registered yet on lettuce, but residue testing is under way on the Syngenta product in hopes of getting registration. Matheron said it is registered on peanuts and potatoes for Sclerotinia control.
Endura, a relatively new fungicide from BASF, is registered for control of leaf drop. Matheron said it is more effective on Sclerotinia minor than on sclerotiorum.
Rovral and Botran also are available for leaf drop control. Ronilan is no longer available, added Matheron. There are also biological products available. One is Serenade. Fungicide application on Matheron's trials came at thinning at about two to three weeks after that. Fungicides must be directed to the base of the young plants and be applied before plants become too large because the fungi are most devastating within an inch of the plant.
Since some of the fungicides are more effective on one of the fungi than the other, Matheron suggests tank mixes may be advisable for maximum control.
Fungicides are used to inhibit fungi germination. Matheron said the fungi attacks the letter in cool, wet weather, the type desert lettuce growers experienced this winter.
Sclerotinia sclerotiorum is the more dangerous of the two fungi since it can germinate and spread spores aerially. “When this happens, you can lose an entire field of lettuce,” he said.
Lettuce producers would prefer to destroy the sclerotia in the soil and Matheron has discovered they can do that with a combination of water and heat.
In field trials in the late spring and early summer, Matheron said sclerotia levels were reduced from 90 percent in the soil to 5 percent for minor and 12 percent for sclerotiorum with three weeks of fallow field flooding.
“Using fungicides when you need to during the growing season and using cultural controls in the spring and summer should lead to maximum control of leaf drop in desert lettuce,” said Matheron.