PPDL Picture of the Week
October 15, 2018
Late season leaf diseases of grapes
Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture
There are two common leaf diseases that affect grapevines in the Midwest; downy mildew and powdery mildew. Although the names are similar, the causal agents are quite different from each other. Subsequently, environmental conditions that favor disease development, variety susceptibility, and control measures are different as well. Correct identification of the diseases is especially important for selecting appropriate control measures.
Downy mildew is usually common when we have hot, humid conditions with frequent rainfall. Powdery mildew, on the other hand, is generally more common under mild temperatures and dry conditions. Both diseases can infect grapes any time in the season, but we typically don't see them until late in the summer, near harvest. It is likely that fungicides used early in the season to control the common fruit pathogens are suppressing leaf disease development. As fruit become resistant to disease and fungicide applications reduced, the leaf diseases can get established. Both of these diseases can cause defoliation and reduce winter hardiness in vines. Thus it is important to manage these diseases after fruit harvest until frost.
Downy mildew is caused by the Oomycete Plasmopara viticola. This class of organisms is unique in that it has a motile zoospore stage. Infections occur when sporangia on a wet leaf surface release their zoospores into the film of water. The zoospores swim until they find a stomate. The zoospore encyst, and a germ tube forms to infect the leaf tissue through the stomate. Mycelium develops inside the leaf, causing yellow spots to appear on the upper leaf surface. When conditions are conducive, sporangiophores develop in the stomates and form a "downy" growth on the lower surface of the leaf, giving the disease its common name. Leaf wetness is required for zoospore dispersal thus downy mildew is most common under warm and humid conditions. Various downy mildews occur on a number of crops but each is very host specific. That is, cucurbit downy mildew will not infect grape and vice versa.
Powdery mildew is caused by the Ascomycete Uncinula necator. The Ascomycetes are a large class of fungi that cause many important plant diseases such as apple scab, grape black rot, etc. Powdery mildew is one of the most serious grape pathogens worldwide, perhaps because of its lack of dependence on rainfall for secondary spread. Powdery mildews are unique in that the fungal growth occurs on the outside surface of the plant. The only portion of the pathogen that invades the plant tissue are short peg-like haustoria that penetrate through the cuticle and plant cell wall to obtain nutrients from the plant. All of the mycelium of the pathogen is on the plant surface. Conidia are formed in short chains on the mycelium. These conidia and the mycelium can be seen on the plant surface as a white, powdery residue, giving the disease its common name. Powdery mildew occurs on many plant species and, like downy mildew, is very host specific. That is, the powdery mildew on pumpkins will not infect roses or grapes or lilacs and vice versa. Each host has its own specific pathogen.
Commercial growers have a number of options for fungicides for controlling these diseases. However, because the organisms that cause these diseases are so different, few fungicides will control both diseases. That's why it is critically important to properly identify the disease before choosing a control measure. Downy can be controlled after harvest (when PHIs are no longer a concern) with phosphorous acid products, mancozeb, captan or one of the newer products such as Presidio, Ranman or Zampro. However, none of those fungicides will control powdery mildew. Powdery mildew can be controlled with DMI fungicides such as Rally or Tebuzol, or one of the newer products such as Torino, Endura, Quintec, or Vivando, or even sulfur on sulfur-tolerant varieties. However, none of these products will control downy mildew. For the homeowner, fungicides with myclobutanil are very effective against powdery mildew. Captan is effective against downy mildew. But, again, neither fungicide is effective against the other disease. That's why correct identification is critically important.