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Brown rot

brown-rot

Brown rot on 'Bali' cherry

Thaddeus McCamant, Central Lakes College

brown-rot

Fruit with brown rot and healthy fruit on sand cherries

Thaddeus McCamant, Central Lakes College

brown-rot

Mummy plum touching a small branch. The branch shows the early stages of infection

Thaddeus McCamant, Central Lakes College

Brown rot, caused by the fungi Monolinia fructicola and Monolinia laxa, is the most serious disease in plums, tart cherries and apricots in Minnesota and damages shoots, twigs, and fruit. During ripening and in storage after harvest, brown rot can spread quickly from one fruit to another until most of the fruit are inedible. Brown rot can also cause twig blight, which occurs when cankers form on small branches. Twig blight caused by brown rot is particularly common in apricot trees. Twig blight gradually weakens the trees, which may become vulnerable to winter injury or wood rotting fungi.

Identification

Blossom Blight - In spring, brown rot can cause some blossoms to turn brown and die. Blossoms that die from brown rot typically stay attached to the branch with a sticky gum like droplet. In contrast, blossoms killed by frost fall to the ground.

Twig blight - The infection can progress from the infected flower to the spur holding the flower and into branch below the blossom, causing a canker in the branch. Cankers on twigs are discolored and often have drops of sticky gum between the diseased and healthy area of the branch. When these cankers encircle the twig, all leaves beyond the canker turn brown and wilt. These leaves remain attached to the branch. Of the stone fruits grown in Minnesota, apricot is most susceptible to twig blight.

Fruit Rot - Brown rot on the fruit starts out as a small, firm, brown spot on ripe or ripening fruit. The brown spot quickly grows to encompass large portions of the fruit. Infected fruit remain attached to the tree. With time these fruit become dry, shriveled mummy fruit that may remain attached to the tree well into the following growing season.

Fungal spores - All infected plant parts produce tan to gray powdery clumps of spores in response to the presence of moisture at temperatures above 41°F.

Important biology

Brown rot fungi overwinter in mummified fruit, infected twigs and cankers in the bark. The primary source of spores in the spring is mummified fruit on the trees or on the ground. In the spring, the fungus starts growing under cool, wet conditions and produces spores. Spores spread through wind or rain to the blossoms. The amount of time the flowers need to be wet in order to infect blossoms depends on the temperature. If the temperature is at 70°F, the blossom only needs to be wet for 3 hours in order for the infection to occur. At cooler temperatures, the blossom must be wet longer. At 45°F, the blossom must be wet for at least 6 hours in order for an infection to occur. Frequently, the infection spreads from the blossom through the stem and into the bark, where a small canker can form beneath the dead flower. Fungi in the cankers below the dead blossom can then produce secondary spores for much of the summer. In early summer, the disease spreads slowly, because developing fruit tends to be fairly resistant to infection unless damaged by insects, hail or stem rub. As the fruits ripen and change color, they become susceptible to infection. Infected fruit produce many fluffy, tan to gray spores that can easily spread to other fruit on the same tree and to other trees. Once one ripe fruit is infected, then the infection can spread to neighboring fruit until all the fruit in a cluster are infected.

Management

Sanitation

Keeping the area around the trees clean of fallen fruit and other plant debris will lower the number of spores produced in the orchard and make brown rot easier to control. Pick up all fallen fruit during the summer. Harvest and either consume or dispose of all fruit as it ripens. Before bud break in early spring, remove all mummy fruit that remain on the trees. When pruning in winter, cut out all branches with cankers and remove all twigs that have died from brown rot.

Fungicides

If the brown rot fungi has caused severe twig blight or fruit rot in previous years and sanitation and pruning have not reduced disease, gardeners may consider using a fungicide to protect the tree from infection. To effectively protect the tree, fungicides must be applied at two distinct times. To protect trees from blossom blight and twig blight begin fungicide applications when blossoms first begin to open. Repeat sprays according to label instructions until petal fall. To protect trees from fruit rot, begin fungicide sprays 2 - 3 weeks prior to harvest as fruit is ripening. Repeat sprays according to label instructions until harvest. Take care to wait the complete 'Post Harvest Interval' (PHI) listed on the label. The PHI is the number of days a gardener must wait after applying a fungicide before harvest is allowed. This time period allows fungicide residue to break down to a safe level. Fungicides need not be applied while green fruit are on the tree as these immature fruit are relatively resistant to infection. In all cases fungicides are most effective when combined with sanitation.

The name of the plant being treated MUST BE LISTED on the fungicide label or the product cannot be used! Some products are registered for use on ornamental Prunus species but are not safe to use on stone fruit intended for human consumption. Always completely read and follow all instructions on the fungicide label.

Active ingredient Example trade names* Comments
Myclobutanil Spectracide Immunox Multipurpose fungicide
Propiconazole Bonide Infuse Systemic fungicide
Chlorothalonil Many products available Apply only for blossom blight.
Do not use for fruit rot control
Captan Many products available
Sulfur Many products available Some formulations acceptable in organic agriculture

*Trade names are for demonstration purposes only and do not imply endorsement by UMN Extension. Trade names may change over time. Products with the same active ingredient but different trade names should offer disease control as well.


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