Powdery mildew

The powdery mildew fungus is easily spread through the air and, unlike many other foliar diseases, does not require wet conditions for infection to occur. Powdery mildews thrive when the temperature is cool and relative humidity is high. Severely infected leaves become brittle and wither and die.

Many types of powdery mildew fungi have extremely narrow host ranges. They have absolute specificity in what they can attack.

“One of the aspects of powdery mildew that makes it unique is that many of the species that cause powdery mildew may infect only certain crop species,” says Allan Dufoe, Turf & Ornamental key account manager for FMC. “For example, if you have a particular fungus infecting your rose or your dogwood, it may not necessarily infect your viburnum bush. The species can be host-specific. But once you have it and you see the telltale white powdery mass on the leaf surface, then you have to take some sort of corrective action.”

One aspect that lends these pathogens to be extremely virulent is the fact that the spores are very easily spread by wind. The spores also can overwinter and cause infections the following spring.

“Sanitation is critically important, no matter if you’re in the nursery or greenhouse setting, because the more you can dispose of in terms of dead tissue, the the more the population and its potential for inoculation and infection is reduced,” Dufoe says.

Allan Dufoe is Turf & Ornamental key account manager for FMC, and he provides support to the nursery/greenhouse market. Reach him at allan.dufoe@fmc.com.
 

The facts about powdery mildew

Life cycle

All powdery mildew fungi require living plant tissue to grow. On perennial hosts such as roses, powdery mildew survives from one season to the next as vegetative strands in buds or as spherical fruiting bodies, called chasmothecia, on the bark of branches and stems. Most powdery mildew fungi grow as thin layers of mycelium on the surface of the affected plant parts. Spores, which you can see with a hand lens, are part of the white, powdery appearance of this fungi and are produced in chains on upper or lower leaf surfaces or on flowers, fruits, or herbaceous stems. In contrast, downy mildew, another fungal disease that produces visible powdery growth, has spores that grow on branched stalks and look like tiny trees. Also, downy mildew spores occur mostly on the lower leaf surface. Environmental conditions that favor the growth of downy mildew are different from those that favor powdery mildew and include low temperatures of 50°-70°F, a relative humidity of 90 percent or higher, and free moisture.
 

Favored crops

Many popular ornamentals, notably roses, hydrangeas, dogwood, rhododendron, viburnum and azalea, as well as many herbaceous perennials like dahlia, phlox and zinnia.

Appearance

You can recognize this disease by the white, powdery mycelial and spore growth that forms on leaf surfaces and shoots and sometimes on flowers and fruits. Powdery mildews may infect new or old foliage. This disease can be serious on woody species such as rose, crape myrtle and sycamore where it attacks new growth including buds, shoots, flowers and leaves. New growth may be dwarfed, distorted, and covered with a white, powdery growth. Infected leaves generally die and drop from the plant earlier than healthy leaves.
 

Locations

Powdery mildew is one of the most widespread plant fungal diseases. It can grow in any environment with high humidity and moderate temperatures. From vegetable gardens to rose gardens, ornamental trees and shrubs, almost no type of plant is immune.
 

Spread

Wind carries powdery mildew spores to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, water on plant surfaces for extended periods inhibits germination and kills the spores of most powdery mildew fungi. Moderate temperatures of 60°-80°F and shady conditions generally are the most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and leaf temperatures above 95°F may kill the fungus.

The guide

Follow these best practices in your nursery to control powdery mildew.

Smart spacing
As you place plants in pots in the field, keep in mind their size at maturity. Don’t overcrowd them — that diminishes canopy ventilation. Remember, powdery mildew thrives in humidity. By creating a spacing scenario in which airflow is strong between the plants, you can limit the incidents of sporulation and inoculation on the plants with powdery mildew.

Scout it out
Most fungicides are not curative. Growers should scout for diseases, especially when conditions are conducive, spot it as early as possible, and apply fungicide at the first sign of disease to prevent any further impact. Watch for the telltale leaf spots.

Find new options
Growers have choices. Powdery mildew-resistant cultivars of some ornamental plants have been developed. Look for these on the marketplace to help diminish occurrences of the disease.

Rotate MOAs
For chemical control, it’s vital that growers know the mode of action of their fungicide. Rotate the MOA at every other application to manage resistance potential.

Don’t keep it around
Good pruning is another key to keep continuous airflow and maintain a low humidity level. Be careful when excising severely infected plant parts and disposing of them. These plant parts can act as storage reservoirs for overwintering fungi for later growth.
 

If you find it: Your local university extension or fungicide distributor can be used as a source of guidance.

Source: University of California Agriculture and Natural Resources IPM Program