Tuesday, May 17, 2011

My plants have a little sugar coating :) Powdery Mildew Spotted in the Landscape

Powdery Mildew on Crape Myrtle
(Photo: H.D. Shew)
Powdery mildew has started to show up on many landscape plants. Powdery mildew is easily diagnosed on most plants by the characteristic coating on leaves that gives this disease its name. The “powder” will not rub off – it is actually the mycelium and spores of the disease-causing fungus. 

Powdery mildews frequently infect dogwoods, crape myrtles, roses, lilacs, tulip-trees, oaks, and numerous other shrubs and perennial plants.  In addition to the powdered appearance, typical symptoms include discolored and distorted leaves, shoots, and flowers. Although powdery mildews rarely cause serious damage to most hosts, severe cases can result in defoliation or poor growth. 

PM on fruit
(Photo: H.D. Shew)
PM on flower stem
(Photo: H.D. Shew)
While most powdery mildews look the same to the naked eye, hundreds of different species of fungi cause powdery mildews. Most are specific to a particular host. Under the microscope, different groups of powdery mildews can be identified by their highly distinctive cleistothecia (specialized spore-bearing structures).
PM micrograph
Note hooked appendages
(Photo: H.D. Shew)
PM micrograph
Note branched appendages
(Photo: H.D. Shew)

In the spring, cleistothecia from previous infections rupture and release wind-blown spores.  These spores land on susceptible plant tissue and begin to grow on the surface and into the upper layer on the leaf.   The fungus produces columns of asexual spores that are capable of causing new cycles of infection.  Late in the season, the cleistothecia appear as tiny dark specks embedded in the mildew.  These contain the sexual spores of the fungus and will overwinter to start new infections in the spring.
Black cleistothecia embedded in mycelial mat
on leaf surface (Photo: H.D. Shew)
Powdery mildew is most common in cool or warm and humid weather conditions, but it may occur and cause severe damage in warm and dry climates. Temperatures between 70-80 degrees F and a relative humidity between 85%-100% favor infection. Powdery mildew pathogens are some of the only fungal organisms that can germinate and infect in the absence of free water.  These fungi require high humidity but are not dependent on wet leaves for infection and spread.  Excess leaf wetness actually inhibits spore germination. 

Cultural Control:
  • Purchase mildew resistant varieties
  • Reduce humidity: Increase air circulation and light penetration by thinning and pruning plants to reduce overcrowding
  • Avoid planting susceptible plants in shaded areas
  • Sanitation: Prune out dead and diseased tissue and rake and remove any fallen tissue to reduce the chances of the fungus surviving until the following season
  • Overhead watering of the leaves: Water inhibits spore germination for most powdery mildews, but be careful because excess leaf wetness can increase chances of other foliar diseases.  
  • Avoid practices that stimulate succulent growth: Powdery mildews are obligate parasites that prefer fresh, succulent plant growth. Applying nitrogen fertilizer, pruning heavily, and watering excessively are not recommended because they promote succulent growth.  
Powdery mildew on dogwood (Photo: H.D. Shew)
Chemical Control: 
Chemical control is often not necessary in the landscape because this disease rarely kills the plant.  Spraying trees such as dogwoods and crape myrtles can be impractical. 

Fungicides may be used to control powdery mildew on highly susceptible varieties of roses and other plants that suffer severe symptoms and damage.

See Rose Diseases and Their Control in the Home Garden
Rose Diseases Link