Downy and powdery share the same last name — mildew — but they are far from identical twins. However, pest control advisers and growers occasionally confuse them, according to Frank Laemmlen, University of California Extension plant pathologist.
“You can have both on the same plant at the same time,” acknowledged Laemmlen at a recent vegetable consultants meeting in San Diego, Calif., sponsored by Bayer CropScience.
Laemmlen, who recently retired as a Santa Barbara County farm advisor/county director, says they should not be easily confusing. Failure to identify either one correctly could costly since it takes different chemistry to control downy and powdery in vegetable crops. If a PCA makes a misdiagnosis, it could be costly.
Laemmlen said it is relatively easy to tell the different between the two mildews. Fold a lettuce leaf over and look along the ridge line; powdery mildew will look like single chain-like stalks; downy mildew looks like a forest of trees with leaves.
Laemmlen was plant pathology farm advisor in Imperial County before moving to the California central coast. In the desert, powdery mildew is more common than downy mildew. Downy is more common in cool, wet coastal climates. However, they both are found in all vegetable producing areas.
Downy mildew has spread rapidly through the vegetable producing areas of the California and Arizona, Laemmlen believes, because lettuce is grown year-around in both areas by mostly the same producers. They move crews, boxes, trucks, irrigation pipe and other hardware from infected Downy mildew sites to other areas.
Downy mildew attacks all stages of disease development and can go systemic in the plant.
Here are the visible symptoms of the powdery and downy mildew:
Powdery: Spots are circular, not limited to leaf veins; There is yellowing, but only after powdery mildew fungus has been presentg for some time; fungus grows on leaf surface (infected wrapper leaves can be removed for a marketable head); spores produced in a single chain on a single stalk and grows on either leaf surface.
Downy mildew spots tend to be angular and limited by leaf veins. Yellowing develops with downy mildew infections sometimes before fungus is evident. Fungus grows inside the leaf and produces spores on the surface. Spores are produced singly on the end of branched stalks. The disease tends to be more visible on the underside of leaf surface as fungus sporulates through stomata in the leaf.
Downy mildew can attack at any state of lettuce development and can go systemic in the plant.
Resistance management has become the law with many crop protection products. The wake up call for vegetable producers to this fact was when downy mildew became resistant to Ridomil, one a popular vegetable fungicide, in the late 1990s.
That is when Laemmlen began DM fungicide trials in what he calls “mildew alley,” the cool, wet summer growing area around Oso Flaco near Santa Maria, Calif.
Maneb remains the standard against which all other downy mildew fungicides are evaluated. Laemmlen calls Reason, a new fungicide from Bayer, a “winner,” but urged PCAs not to abandon Maneb because of this new, effective product. He suggests alternating Reason and Maneb to avoid resistance.
Overall, Laemmlen said vegetable producers have a good arsenal of products to control downy mildew. “We had nine treatments with a dozen materials and half the chemicals gave satisfactory control. This is what we need for a good IPM program,” he said.
However, sometimes Maneb will “drop out of a trial” and not provide good control. He said he can get similar, “crazy results” from other fungicides in trials where there control mysteriously not adequate.
“Don't panic,” he says. “It may not be resistance,” but a situation where pathotypes are changing.
“This is what plant breeders are going crazy trying to breed for resistance,” he said. Many of the older pathotypes are disappearing and new “novel types” are replacing them. “We do not always know what they are,” he added. Fungus pathotypes have “radically changed” over the past decade. “These pathotypes are continually in flux.”
Control may differ with the same chemical in different locations. That is the reason, he said, PCAs and producers must constantly reassess chemicals. Downy mildew is a variable organism, he reiterated.
For example, there are not eight pathotypes of downy mildew in spinach and all are different than those identified as causing downy mildew in spinach just five years ago.