Apple trees are a great addition to any garden since they produce a lot of apples in the fall and early winter.
However, things can go wrong from time to time, robbing you of the pleasure of plucking those picture-perfect red, luscious apples from your tree. Apple tree black rot is one such issue, which can wipe out multiple apple trees in orchards at once, especially in warm, humid conditions. This is a fungal disease, common in pome fruits, such as apples and pears.
Continue reading and you’ll learn all about the disease, what causes it and what actions to take if your apple tree is infected with it.
Reader Poll: What online courses would interest you?
What Causes Apple Tree Black Rot?
Botryosphaeria obtusa is the fungus responsible for this disease. It feeds on the dead tissue of many host plants besides apples, but causes disease in only pome fruits. The bacteria causes three different types of disease, depending on the part of the tree it infects:
- Black rot cankers – when branches are infected
- Fruit rot – infection of the fruit
- Frogeye Leaf Spot – causes symptoms on the leaves
The bacteria is drawn to decaying apple tissue in particular.
Trees afflicted with fire blight, those that are not hardy to the region in which they grow, and those that are stressed by unfavourable environmental circumstances, such as drought, are more vulnerable to infection by the fungus.
If the fungus is present, apple trees with open wounds, such as pruning cuts, are more likely to be affected.
Subscribe to our newsletter!
How To Identify Apple Tree Rot
Symptoms differ with the part of the tree that gets infected with the fungus. Here’s how you identify the symptoms of black rot on different parts of the tree.
Frog-eye leaf spot occurs when the fungus affects the leaves. The first indicators appear as purple patches on the front side of the leaves in early April. The spots grow to roughly a quarter of an inch in diameter, with purple rims and light brown centres, like a frog’s eye as the condition progresses. Further chlorosis and leaf drop will occur if the illness persists. Excessive defoliation will result in stress to the tree.
Cankers form on infected branches, which appear as reddish-brown sunken regions.
When black bumps appear on these cankers, you should be aware that they are containing fresh fungus spores that will infect more healthy tissue once the weather conditions are favourable.
During the dormant season, prune out the diseased branches to eliminate the cankers and prevent the illness from spreading.
Large rotting patches on apples are difficult to miss. The rotting regions of black rot on fruit usually occur on the blossom end of the fruit, which separates it from other causes of fruit rot. Though both black rot and bitter rot emerge at the fruit’s blossom end, black rot produces a single huge rotten area, and bitter rot produces several smaller spots.
Infected fruits show as red spots that turn purple later in the season and are encircled by a crimson halo if they are infected early in the season. If not removed by pruning, some of the fruits mummify on the tree, causing further infestations.
How To Control Apple Tree Black Rot
The most important form of control is removing all the sources from where the fungal infection may spread. Wait until the temperature drops below 32°F to prune the trees. Pruning in the season of active growth risks spreading the infection even faster. When pruning the branches with cankers, make the cut at least 15 inches into the healthy tissue to make sure no part of the canker remains on the tree. Destroy all the infected tissue by burning it.
Since fungal spores also overwinter on fallen plant debris and mummified fruit, keep the area around the tree clean and free of all debris. Rake the garden and throw away all the plant debris to prevent re-infection.
Copper-based fungicides and lime sulfur can also be used to manage the problem but removing all the sources of fungal spores is much more effective. If you must use chemical fungicides, make sure you read and follow the instructions on the package on how to apply it.
Can Apple Tree Black Rot Be Prevented?
Though curing black rot once it occurs may be hard, some simple prevention techniques can keep your healthy apple tree shielded against Botryosphaeria obtusa and all other pathogens lurking in the surroundings. Here are some handy tips to prevent black rot in apple trees:
- Good sanitation practices, such as keeping the garden clean and free of fallen plant debris can help prevent your tree from being infected by the disease in the first place.
- Keep the tree well-watered and fed because stressed plants are even more vulnerable to attack by fungal pathogens.
- Prune out any limbs that are killed by fire blight because these regions are especially susceptible to black rot.
- Only grow cultivars that are hardy to your region.
Grow apple trees in fertile, well-drained spots and keep their health in check by ensuring optimal moisture and nutrient content, regular pruning and cleaning up plant debris during the fall. Not only will these good gardening practices help keep black rot at bay, it will also minimize the damage to your tree if it’s already infected by the disease. Besides black rot, good sanitation practices also keep your garden free of several other diseases and pests.