Powdery Mildew - Why All the Confusion?
G. D. Lewis, Ph.D.
Colts Neck, New Jersey
Recent concern about powdery mildew on rhododendrons and azaleas has caused some people to wonder why simple answers to the problem have not been forthcoming. The reason is that the problem is not simple - it is complex and difficult to study.
The first fact that must be recognized is that there are many different powdery mildew diseases caused by many different powdery mildew fungi. At least nine different species of powdery mildew fungi have been reported on Rhododendron species and there are quite possibly more that have not yet been discovered. This does not mean that you have more than one powdery mildew disease in your collection of plants, but it is possible.
The powdery mildew fungi are somewhat unique in several ways. For one, they all infect some kind of plant or group of plants and can only live as active parasites attacking susceptible living plant cells. Thus they cannot be grown in the laboratory in pure culture on any known culture medium and this makes their study difficult. In addition, they all look pretty much alike so they are difficult to identify even by experts and this also makes their study difficult. It is no wonder that many plant pathologists prefer not to get deeply involved with a fungus that is impossible to grow and very difficult to identify.
Like many other fungi, this group produces two kinds of spores. Ascospores, produced by a sexual (but not very exciting) process, are produced in a spherical, totally enclosed body called a cleistothecium. Each genus is recognized by appendages protruding from the cleistothecium. The manner of branching of the appendages is used to identify the fungus to the genus, but some powdery mildew fungi rarely if ever produce the sexual stage and this makes their identification difficult. The ascospore stage is generally the overwintering or dormant stage in the life history of the organism.
The asexual stage spores, called conidia, are produced - usually in great profusion - during the growing season and can be produced from almost any cell in the organism's body. The vegetative strands of the fungus grow on the surface of the plant tissue sending feeding organs called haustoria into the epidermal cells of the host plant while at the same time producing countless numbers of conidia which look like chains of single cells reaching upward from the plant surface. It is this massive production of spores that gives the infected leaf its dusty white appearance. The single celled conidia break off and drift on the wind, often for many miles and are the cause of the many secondary infections that can occur during one growing season. Unfortunately, the conidia of most powdery mildew species look pretty much alike so it is very difficult to separate or identify the species on the basis of the asexual stage - the stage that we usually have to work with.
These contrary creatures also have different environmental requirements for spore germination and infection of the host plant. Most fungus spores germinate in droplets or films of water. The powdery mildew spores germinate poorly, if at all, in free water. They germinate in air when the relative humidity is high - usually at around 95 percent or higher. Thus, it is periods of high humidity, not rain, that encourage epidemics. Rhododendrons grown in the greenhouse will probably benefit from a ventilation system that will lower the humidity.
In the past, most fungicides effective for the control of many other plant pathogenic fungi did not control the powdery mildews, but in recent years we have some newer fungicides, many of them systemic, that show activity against members of this group. None of them should be expected to be effective against all powdery mildew species. Carefully conducted field tests are needed to determine which fungicide will control the powdery mildew or mildews infecting your rhododendrons. When you find an effective fungicide, use it no more frequently than is necessary for adequate control. The powdery mildews have shown a remarkable capability to develop resistance to effective fungicides when repeatedly exposed to them.
Although fungicides may control the disease, disease resistance is a much more satisfactory and long range solution. For this reason, I use no fungicides (or insecticides) on any of the hybrid seedlings that are the results of my rhododendron breeding efforts. We must look for more than just a better or different flower. We need pest resistance as well. Let the susceptible plants die! We don't need to add more weaklings to the list of new plants.
Dr. G. David Lewis, a member of the Princeton Chapter, is a professor of plant pathology at Rutgers. He serves as a technical reviewer for the Journal.