We often get worried calls in regards to a white powdery substance on tree and shrub leaves. Commonly affected species include caragana shrubs, flowering crabapple trees and on occasion turf grass. The problem in question is a fungal disease called powdery mildew, the symptomatic white substance covering the leaves is the fungus and its spores.
In most circumstances powdery mildew is a cosmetic affliction of plants meaning that it will not kill the plant despite reducing its aesthetics and appearance. Fungicides offer little control over curing the disease because fungicides work more as a preventative than a cure for fungal diseases. A fungicide may reduce the spread of the fungus but affected tissues will remain symptomatic.
A better means of control would be to implement preventative cultural practices such as thoroughly cleaning up infected debris in the fall and improving airflow around the infected plant. Another cultural method would be plant health care, ensure that the infected plant is receiving adequate water while being careful not to over water so that the tree can build up energy reserves to naturally combat the disease. Improving root health by using wood chip mulch as a ground cover is also another method you can use to improve the health and growing conditions of the root system.
If the affected tree or shrub has a nutrient deficiency or imbalance it could be more susceptible to disease such as powdery mildew. Nitrogen is a food source for most tree and shrub pests, adding a high nitrogen fertilizer can actually make the problem worse. If you are using fertilizer we recommend a balanced fertilizer containing micro nutrients that is also high in potassium such as tomato and vegetable food. Potassium is the last number of 3 seen on product labels and nitrogen is the first. Prior to using any fertilizer or products it is wise to have a detailed soil or leaf tissue analysis. Samples should be conducted by an agricultural laboratory to determine if there are any nutrient deficiencies or specific soil conditions that could lead to nutrient imbalances.
On a final note please remember that powdery mildew is a cosmetic affliction that will not cause the death of your tree or shrub, try to work on the site conditions leading to the problem rather than trying to find a cure.