Portland Japanese Garden

Recently we have received several inquiries regarding the health of weeping cherry trees in the Portland area. These trees have difficulty coping with the climate and humidity in this region. Most particularly they are affected by Shot Hole Fungus which is triggered by warm days and cool nights coupled with excessive moisture. I have included below additional information regarding the disease and control methods.

Cherry leaf spot, or shot hole, caused by the fungus Blumeriella jaapii, reduces flowering and weakens the tree. Cherry leaf spot often defoliates the tree by midsummer. Repeated defoliation makes the tree more susceptible to winter injury and may eventually kill it.

Symptoms

Leaves show small purple to brown spots with definite boundaries in early summer. The leaves turn yellow and fall. In July, the centers of the infected spots frequently fall out, giving a shot-hole appearance.

Cause

The fungus overwinters on fallen leaves. In spring following wet weather, spores form and winds blow them to infect leaves. When temperatures are favorable (60 to 75 degrees F), infection requires a wet period of only a few hours. Leaves are susceptible when fully unfolded—this usually occurs near petal fall. The initial leaf infections form spots, and more spores are produced in the spots. These spores are rain splashed to infect other leaves. Secondary spread and infection by spores continues repeatedly, whenever wet warm weather occurs, until leaves fall in autumn.

Control

Rake and destroy fallen leaves before spring. Fungicides should be used if plants are prematurely defoliated. Time spray applications beginning at petal fall; continue every 10-14 days if rain is predicted during the period. If initial infection is controlled, subsequent spray applications are unwarranted, however, we do recommend applications throughout the rainy season.

I have also attached information regarding the fungicide mixture I prefer to use. Bordeaux Mix fits well within our Integrated Pest Management Program. An additional recommendation is to inoculate the soil around the tree with beneficial soil microbes. There are beneficial bacteria that will control the populations of this harmful fungus, other fungal microbes will strengthen the tree through a symbiotic relationship.

Bordeaux Fungicide Mixture—How to Make Your Own Supply

This product is effective not only for fungal diseases but is also active for certain bacterial diseases such as fire blight of pear and apple.
The mixture can be prepared by combining hydrated lime or any type of finely ground lime with powdered copper sulfate (sometimes referred to as “bluestone”). Both materials should be available at farm and garden supply stores. However, some checking about may be required to locate the copper sulfate.

While Bordeaux Mixture can be prepared in several strengths, the recommended for dormant season application is a 4-4-50 formulation. The numbers refer to the ratio of copper sulfate and lime in 50 gallons of water. For example, 4 pounds each of these materials would be combined in 50 gallons of water. Since this amount of spray material is considerably more than needed by most gardeners, the recipe can be whittled down for preparation of smaller amounts of spray mixture.

For example, to prepare a gallon amount of a 4-4-50 Bordeaux Mixture spray, measure out 6 ½ teaspoons of copper sulfate and 3 tablespoons of hydrated lime. The lime should be mixed with a pint of water to make a “milk of lime” suspension. The copper sulfate should also be dissolved in a pint of water. The latter process may take several minutes.

Before mixing the lime and copper components of the mixture, one additional step (but an important one) must be carried out. Each container of these materials should be strained through a cheesecloth filter. If cheesecloth isn’t available, use cloth of a similar loose weave. The filtering is necessary to remove small pieces of lime or copper sulfate that won’t dissolve; otherwise, you’ll find these tiny pieces in the end of your garden sprayer tip, clogging up the works!

To make the filter, simply place the cheesecloth loosely over the top of another container and fix securely in place with a string or rubber band.
The filtered copper sulfate solution is added to a one-gallon container, followed by addition of the filtered lime solution. Enough water (about 3 quarts) is then added to the container to bring the total volume up to one-gallon.

You now have one gallon of Bordeaux Mixture ready for use. For two gallons, simply double the proportion of materials and prepare as before.
For best results, use your Bordeaux Mixture the same day of preparation and keep the sprayer agitated. Insecticides should not be added with the Bordeaux Mixture without first checking the insecticide label for possible compatibility problems.

What are some dormant season uses for Bordeaux Mixture? For peaches, use the material as a dormant spray for peach leaf curl, on apples and pears to help with control of fire blight, on grapes for black rot control, and on roses for black spot and other fungus disease control. A single dormant season application of Bordeaux Mixture won’t provide total control of these and other diseases the next season, but used along with other in-season control procedures, the task becomes a lot easier.