Bacterial Canker of Sweet Cherry

Agdex#:	213/634
Publication Date:	04/88
Order#:	88-086
Last Reviewed:	04/88
History:	Revision of, "Bacterial Canker of Sweet Cherry," July 1986
Written by:	W.R. Allen - Agriculture Canada Research Station

Table of Contents 

1. Introduction
2. Detection
3. Control
4. Suggestions for Mixing Spray
5. Bordeaux Mixture Safened with Vegetable Oils
6. Pruning
7. For more information...

Introduction

Bacterial canker or bacterial gummosis of sweet cherry is caused by a Pseudomonas bacterium. The economic importance of this disease is difficult to determine because it not only reduces yields by killing fruit spurs and branches, but also seriously injures the tree, causing setbacks in growth. Reduced yields due to bud killing alone result in losses of 10 to 20% in an average year. During years of heavy infection, which may be frequent, the losses may exceed 50%. The real potential for losses can most easily be seen in young plantings, where the disease can kill 10% of the trees or more and can cause substantial yield losses in the surviving trees.

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Detection

The most obvious symptom of the disease in older trees is the bare-branch or leggy appearance caused by extensive killing of leaf and flower buds and spurs on wood 3 years of age or older. Closer inspection may reveal cankers or dark, sunken areas around dead buds and spurs and in the crotch angles. These cankers, when active, usually exude the amber gum so characteristic of the disease.

Signs of past infections are the retention of blossom stems that were killed by a canker at their base. Dead tip and side shoots also may be numerous.

Several symptoms of the disease may appear in the spring. If weather is wet and cool, leaf-spotting occurs, followed by shot-holing. A more serious condition that occurs in warm, dry springs is wilting and collapsing of opening flower and leaf buds. If wood damage is severe enough, entire branches will wilt and die.

Figure 1. Mature sweet cherry tree with bare-branch condition due to bud and spur killing by bacteria.

Figure 2. One year old twig with extensive bud kill.

Figure 3. Two year old twig with two spurs infected by bacterial canker. Note lack of leaf or bud development on twig on right.

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Control

Cankers get started mainly in the autumn after most of the leaves have fallen and the trees have begun dormancy. The bacteria that start these cankers are found on the surfaces of mature leaves and other green tissues, and do not come from existing cankers. The only effective way to control the disease is to reduce the number of bacteria before the trees enter their susceptible period. Successful control has been obtained with repeated sprays of Bordeaux mixture.

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Suggestions for Mixing Spray

Bordeaux should be used immediately after mixing. The standard tank-mix is best prepared as follows:

1. Start water flowing into the spray tank;

2. When the tank is about one-third full and the mechanical agitator is in operation, start washing the powdered copper sulfate into the tank through a screen with water from the supply hose. A wooden spoon is helpful in working the powder through the screen;

3. By the time the tank is half to two-thirds full, all of the copper sulfate should be in the tank. Next, add the lime (hydrated or builders') through the screen, using the water-supply hose. The lime should be dilute as possible before it mixes with the copper sulfate; therefore, presoaking the lime to make a slurry is preferred to washing the powdered lime directly through the screen;

4. Better leaf coverage can be achieved if a non-ionic (neutral) spreader is added to the tank mix. The spreader may be used at the rate of 250 mL per 1000 L of mix;

5. Keep the agitator running continuously and apply the Bordeaux mixture immediately. Do not allow the mixture to settle out.

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Bordeaux Mixture Safened With Vegetable Oils

Bordeaux mixture may be sprayed on cherry trees two or three weeks earlier than is indicated in the Spray Schedule if a vegetable oil safener and spreader are added to the tank mix. An oil such as crude, canola (rape seed) oil, or cotton seed oil, used at the rate of 7 L per 1000 L of mix, has been shown to reduce early leaf fall caused by Bordeaux mixture alone. The advantage of starting the biweekly sprays earlier than is recommended with straight Bordeaux mixture is that bacterial populations can be reduced and held at low levels prior to the onset of dormancy which is the period of maximum susceptibility to the bacterium.

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Pruning

Cankers that have not healed smoothly by late summer, but remain sunken with open cracks, should be pruned out. Cankers of this type usually have developed into fungal cankers that may cause wood rot and the loss of major bearing surfaces. Pruning can be done through the summer up until October, and later in the winter. Prune during dry weather to prevent infection of pruning cuts.

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