 There are two very different types of infectious plant diseases known as mildews, namely powdery mildews and downy mildews. They are different diseases, caused by different types of organisms, and with different disease cycles and control strategies. It is important diagnostically to discriminate between powdery mildews and downy mildews since often downy mildews are more serious to plant health and since different pesticides are effective against these very different organisms.With this article, we’ll focus on powdery mildews. First though, here is one perpetual reminder about the nature of infectious plant diseases and their management: the disease triangle. The disease triangle is the simple concept that infectious disease is a process, involving three facets: a virulent pathogen (microscopic organisms such as certain fungi and bacteria), a susceptible host plant, and an environment conducive to disease. Pathogens differ with each disease. If the disease is powdery mildew of lilac, the pathogen is Microsphaera alni. If the disease is powdery mildew of rose there is a different pathogen, Sphaerotheca pannosa. All aspects of the disease triangle are unique for each disease; for rose powdery mildew you need S. pannosa, a rose susceptible to this pathogen, and particular conditions of relative humidity. The environment conducive to disease varies for each type of infectious disease as well and is as crucial as the other components of the disease triangle. For example, let’s look at a non-mildew disease, rose black spot. The rose black spot pathogen is Diplocarpon rosae, and the environmental conditions conducive to spore germination and infection are the number of hours of leaf wetness. With powdery mildew of rose, spores do not germinate properly in free water on the leaves, so leaf wetness is not a key component of the environment conducive to powdery mildew of rose. Hosts Powdery mildews are some of the most familiar and ubiquitous diseases in the landscape. Familiar hosts abound: lilac, rose, zinnia, phlox, dogwood, ninebark, sycamore and turf. There are more than 7,000 host plants for powdery mildew diseases worldwide. One of the first things to realize is that these diseases are caused by different powdery mildew fungi. The powdery mildew of rose fungus will not infect lilac. You can cause quite a sensation by going to a meeting of rosarians and rubbing lilac leaves with powdery mildew on a champion mildew-free rose. Though you still may be banned from the meeting, it will not cross-infect. The powdery mildew of zinnia fungus will not infect phlox. The powdery mildew of apple fungus will not infect dogwood. There are exceptions, but these make sense. For example, the fungus that causes powdery mildew of the American planetree or sycamore (Platanus occidentalis) will also infect London planetree. When you realize that London planetrees (P. × acerifolia) are hybrids of the American planetree and the Oriental planetree (P. orientalis), this cross-infection makes more sense. Signs and symptoms  Severe powdery mildew infestation on Norway maple. Photos by Jim ChatfieldPlant diagnosticians discriminate between signs and symptoms. When some part of the pathogen is observed it is a “sign” of the pathogen. With powdery mildews, the familiar sign we see is the whitish to grayish masses of threads of the powdery mildew fungus (the mycelia) and spore-bearing structures (sporophores) on leaves and stems. Another less-often noticed sign is tiny dark round sexual fruiting bodies of these powdery mildew fungi (cleistothecia), which are one of the ways the fungus overwinters. This sign of the fungus is about the size of a tiny pepper grain when seen by the naked eye, but ornamentations on the cleistothecia help with identification when using a dissecting microscope. The key diagnostic sign to look for is the familiar powdery growth. Symptoms are the results of the interaction with the plant and the pathogen. With powdery mildews symptoms include: curling and twisting of young shoot growth, flowers and leaves; yellowing and reddening of leaves; and drying and browning of leaf tissue in serious infestations. Also, the results of the fungal growth on leaf tissue result in impaired photosynthesis by the plant leaves. It is often said that powdery mildews are not a serious disease, since these signs and symptoms do not result in plant death. But down-playing the importance of powdery mildew diseases for ornamental plants may be a mistake. With ornamentals, these symptoms may make the plant unattractive. Lilac powdery mildew will not kill a lilac in the landscape, but it might kill sales in production or retail. Overall effects may be significant down the road, for instance poorer flowering of roses or an extra year to produce a sellable flowering dogwood for nursery growers. Control methods Powdery mildew fungal spores do not germinate in free water, and the level of relative humidity that favor different powdery mildew diseases is variable. But one general key for control is site plants in environments with good air movement. Whenever possible, do not plant in damp, humid environments, and space plants to encourage air movement. Use good sanitation practices by pruning out badly infested old wood and the raking up of infested leaves. Powdery mildew fungi infect plants superficially, not penetrating deep into the cell layers of a leaf. Fungicides can be quite effective if applied according to label directions and in a timely manner. A wide range of fungicides are available for various powdery mildew diseases, from old stand-by multisite products to newer highly specific products, as well as a number of biological control alternatives. A good listing of labeled fungicides is available in the powdery mildew fact sheet in the diseases of landscape plants series from Purdue University. Joe Boggs, Ohio State University, Department of Entomology, contributed to this story. |
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