Figure 1: PM on Tomato |
Disease and Development
There are many different species of (Powdery Mildew) fungi. Most species
only attack specific plants in the same or closely related plant families. For
example, powdery mildew on roses does not spread to grapes.
Powdery Mildew fungi overwinter in buds, twigs, bark, and plant debris. Their spores are carried by wind to new hosts. Low relative humidity during the day favors spore dispersal.
Fig 2: PM on grapes |
Management
The disease triangle is a useful tool to understand and manage diseases like Powdery Mildew (Fig. 3). The severity of Powdery Mildew infections is a function of the presence of the
pathogen, the host susceptibility, and the environment. Powdery Mildew can be managed by good cultural practices.
Fig 3: The Disease Triangle |
Another good control method is making environmental
conditions less favorable for the disease. Avoid dense canopies by proper plant
spacing and pruning to provide good air circulation, reduce humidity and increase
sunlight penetration. Plants should not be planted in poorly drained soils or
shady damp areas.
Overhead watering of the leaves during the day and when
humidity is low can reduce infection. Water washes spores off the plant and
inhibits its germination. This control method is not recommended for vegetables
since it may contribute to other pest problems.
Powdery Mildew seldom needs chemical control in residential plantings, but it may be warranted in commercial plantings of susceptible crops like grapes and cucurbits. Chemicals are most effective when combined with cultural control. They should be applied before the disease symptoms appear. Further spread may be stopped or slowed with chemical treatment. Materials like horticultural oil or neem oils have a low environmental impact. Apply every 7 to 14 days throughout the season. Do not apply to drought-stress plants when the air temperature is above 85° nor within 2 weeks of a sulfur spray. Wettable sulfur is very effective in preventing infection. Like oil application, it should not be applied when the air temperature is over 90° nor within 2 weeks of an oil application.