By Erin Marissa Russell
Brown rot, sometimes known as stone fruit disease, is a fungal infection caused by the pathogen Monilinia fructicola. Brown rot can impact apricots, cherries, nectarines, peaches, and plums. As the season draws to a close, apple trees become vulnerable to brown rot, particularly if the apple trees are near a stone fruit orchard where plants struggle with brown rot.
How to Identify Brown Rot / Stone Fruit Disease
The first sign of brown rot is normally when small brown specks appear on affected fruit. When the weather becomes wet and humid, a fungus develops on the skin of the fruit that can be brown or tan and resembles ashes. As the fruit on the tree continues to ripen, the brown spots on its skin spread through the fruit and cause decay. Rotted fruits eventually dry out, making them completely mummified. These mummified fruits that remain attached to the tree or lie on the surface of the soil in the orchard are one overwintering location for Monilinia fructicola. When the circumstances are right, an entire fruit can rot completely with astonishing speed. The progression from infection to complete decay can take just two days when the environment is humid and wet.
The tree’s blossoms may be impacted, and infection in the blossoms can spread through the twigs, first leading to necrosis and eventually to girdling. As long as the weather remains wet and humid, the infection in the fruit can pass along to the tree’s twigs and cause a canker. The canker leads to girdling, which can result in the death of the girdled twig. These cankers are another location where Monilinia fructicola can spend the winter. Infected trees can secrete a gummy substance.
How to Prevent and Treat Brown Rot (Stone Fruit Disease)
- Any type of wounds on the skins of vulnerable fruits provide an easy way for the pathogen to enter, leading to infection. Therefore, any measure you can take to reduce wounding of the fruits in your orchard will help prevent brown rot (or at least reduce the severity of the disease).
- Don’t contribute to keeping brown rot in your orchard by inadvertently providing the fungus with safe places to spend the winter. Clean the orchard and remove all rotten or decaying fruit and twigs or other debris from infected trees after the harvest is over. If Monilinia fructicola is permitted to spend the winter in the orchard, tucked safely inside decaying fruit or plant debris all winter, it will begin generating spores again near the time the blossoms start to open.
- Prune your susceptible trees regularly, simultaneously performing a visual inspection to check for brown rot symptoms, such as cankers and girdling. Any wood that shows these signs should be pruned off the tree, along with dead wood, as the wood can carry the brown rot fungus through the winter and allow it to reappear in spring.
Make sure to schedule one pruning session during the summer. This maintenance will improve air circulation around the fruit in your trees, permitting them to dry off quicker after rainfall or dew and leading to reduced risk of brown rot. If you choose to use a fungicide or other spray treatment on your trees, the summertime pruning will help make sure the treatment reaches into the interior wood where it is needed.
- Whenever you work in the garden, use clean, sterilized shears. You should clean and sterilize your shears before you begin working, before you move from working on an infected plant to workin on a healthy plant, and when your work is done before putting away your tools. All you need to do to sterilize your gardening equipment is wipe it down with a cotton ball soaked in rubbing alcohol.
- When you remove debris from infected trees from the orchard, make sure to dispose of it properly. You should burn or bury the infected plant material, or at least make sure it is discarded in a trash receptacle. Twigs, branches, leaves, and other material from infected plants should never be composted, even in compost piles that get hot. It’s impossible to know that the compost has reached a high enough temperature to kill the pathogen even if you use a thermometer, as different portions of the compost heap will have different temperatures, and the temperature fluctuates all the time. Composting infected plant material means you run the risk of re-introducing the disease to your garden when you use the compost.
- If wild stone fruit trees exist in the vicinity of your orchard, you may need to destroy the trees. They can serve as a host for brown rot, carrying the disease and spreading it to your trees over and over. If you treat your plants successfully only to have the disease return again and again, you may wish to take a walk looking for wild stone fruit trees nearby to make sure an abandoned stand of trees aren’t harboring the disease nearby.
Some gardeners end up using fungicide sprays to alleviate brown rot in their stone fruit trees. Spray fungicide should be applied while flowers are blossoming as well as 18 days prior to harvest, nine days prior to harvest, and one day before harvest. However, if you use the environmental and cultural controls we’ve described in this article to help prevent brown rot and treat the disease if it does occur in your orchard, you should find it manageable. Most gardeners who rely on environmental and cultural prevention and treatment methods find they don’t need to turn to fungicide to keep brown rot at bay.