Bacterial Canker in Fruit Trees: Are your fruit trees showing signs of “shot-hole” damage, and you’re puzzled by it? Or perhaps you’ve noticed unexpected damage on your young trees as they emerge from winter? Organic farmer Jenna Rich will guide you through the process of identifying, preventing, and treating bacterial canker in fruit trees, ensuring your home orchard remains healthy and productive.
For many gardeners, cultivating a small orchard of stone fruits is a rewarding goal. Once established, these trees remain compact and can produce a generous supply of delicious and nutritious fruits year after year. However, fruit trees are susceptible to various pests and diseases that can harm their health and reduce fruit production.
Bacterial canker might seem to appear out of nowhere, but by understanding its causes and signs, you can take steps to prevent and manage it effectively. Knowing how to handle this disease is key to maintaining high yields.
In this guide, we’ll cover the causes and symptoms of bacterial canker, the fruit trees it commonly affects, and provide practical tips for identifying, preventing, and treating the disease.
What is Bacterial Canker?
Bacterial canker is a disease that targets stone fruit trees, leading to the development of small holes, dark lesions, and injuries on stems and young branches. In spring, you might notice cankers with an amber-colored, gummy substance, as well as branches that have died due to the constriction caused by these cankers. The disease can impact various parts of the tree.
Cause
Bacterial canker is caused by two types of bacteria: Pseudomonas syringae pv. syringae (Pss) and P. syringae pv. mors-prunorum (Psm). These bacteria spread rapidly via water droplets, wind, and insects, and can also be found in weeds. If not managed, the disease can quickly advance and affect the whole orchard.
Types of Plants Affected
Bacterial canker primarily targets stone fruits within the Prunus genus of the Rosaceae family. This includes almonds, cherries, peaches, nectarines, and plums. Among these, sweet cherries, plums, and prunes are the most frequently affected. The disease is particularly prevalent in young trees, typically those between two and eight years old.
Identification
If your fruit trees are showing signs of distress, such as dark, sunken areas in the bark and small, round holes in the leaves, they might be suffering from bacterial canker. This disease often manifests as dark spots on fruits, which transition from dark green to a deep brown or black.
How it Works
The “shot holes” you see are a result of infected tissue halting its growth, while the rest of the plant continues to develop. The bacteria responsible reside on the leaf surface, entering through the leaf pores during wet conditions. Infected tissue eventually dies off, forming holes and revealing the infection. When bacteria enter wounds or scars during fall, they can become dormant until spring, when they spread rapidly in wet weather.
Prevention
Maintaining a healthy garden environment is crucial for disease prevention. Keeping your yard clear of weeds and potential disease carriers is essential. Healthy plants are more resilient and can recover more effectively from any diseases that do arise.
Location
Position your fruit trees in areas that are less prone to frost and drought. When planting new trees, ensure they are placed away from known disease sources to prevent the spread. Even with precautions, vigilance is necessary to control the spread of disease.
Avoid Injury
Protect your trees by sealing any damage from weather, insects, or tools using white latex paint. Adding tree guards can shield trunks from animals like voles and rabbits. This helps to prevent further injury and disease entry.
Protect Wounds Before Winter
Before winter, address any wounds on your trees caused by environmental factors. Wrap young trees with protective material to minimize cracking. If deer are a problem, consider installing a fence to prevent damage.
Delay Pruning
Since the bacteria thrive in cool, wet conditions, delaying pruning until later in the summer can be beneficial. This timing helps to reduce the risk of spreading the bacteria during their most active period.
Proper Tool Management
Disinfect your pruning tools thoroughly between uses to avoid spreading diseases. Use a 70% rubbing alcohol or a 10% bleach solution, let it sit for at least 30 seconds, then dry the tools to prevent rust.
Proper Pruning
Conduct annual pruning in late winter, after the coldest part of the season, to improve light penetration and airflow. Avoid excessive pruning, which can delay fruiting and lead to excessive foliage. The open-center pruning method is effective in maintaining good airflow and a manageable tree size.
Provide Proper Nutrition and Optimal Soil Conditions
Ensure your fruit trees are planted in well-draining soil and meet their specific nutritional needs. Be cautious with late-season nitrogen, as excess can cause winter injury and increase the risk of bacterial infections.
Control Weeds
Maintaining a weed-free zone around your young fruit trees is crucial, especially during their initial years. Weeds, grasses, and certain cover crops like clover and vetch can harbor harmful bacteria such as Pseudomonas syringae. To protect your trees, consider using mulch or clearing the area around the base of each tree from grass and weeds. Aim to keep a clear area with a radius of at least one foot around the trunk.
Fumigate Against Nematodes
Ring nematodes can damage tree roots, making them more vulnerable to bacterial canker. Research suggests that pre-planting soil fumigation can effectively control these nematodes and help manage bacterial canker. Soil solarization, which involves covering the soil with a clear tarp, can also be an effective organic alternative for controlling nematodes.
Deep Water at the Base
Young fruit trees need about one to two inches of water weekly, whether from rainfall or irrigation. It’s better to water deeply and infrequently rather than shallowly and often. Deep watering encourages strong root development, which helps the trees withstand diseases and environmental stress. During hot and dry periods, increase irrigation and use mulch to retain soil moisture and reduce weed growth.
Grafting
Grafting your fruit trees onto certain rootstocks can help combat bacterial canker. The rootstock ‘Mazzard’ has shown some resistance to this disease and could be a viable option for orchardists looking to reduce their risk.
Plant Resistant Varieties
If grafted trees are not available, choose varieties known for their resistance to bacterial canker. Varieties such as ‘Merton Glory’ and ‘Merla’ for cherries, and ‘Marjorie’s Seedling’ and ‘Warwickshire Drooper’ for plums, are recommended. Always source plants from reputable suppliers to ensure you get quality, disease-resistant options.
Treatment
To manage bacterial canker, promptly remove any infected branches and prune back to healthy wood, ensuring you cut at least 12 inches below the infection site. Perform pruning only in the winter when trees are dormant to prevent spreading the disease. Dispose of infected material by burning or burying it, and apply paint to wounds to protect against further infection and damage from pests and weather.
While copper salt sprays have shown limited effectiveness in controlling bacterial canker, they are not ideal due to their inability to penetrate deeply into tissues and the presence of resistant strains. Unfortunately, fungicides specifically for bacterial canker are not available for home gardeners. For additional guidance on managing bacterial canker, consult your local Extension Office.