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As landscape trees, cherries can puzzle and exasperate growers. Perfection of flowering may be elusive, with a common problem called "brown rot" affecting flowers and leaves. It appears on both edible and ornamental cherries growing on the wet side of the Cascades, caused by a fungus that results in blight.
The infection begins when fungal spores attack the flower buds of the cherry or plum as they begin to open in spring. Infected flowers wilt, turn brown, and die before unfolding fully. The dying flowers look crushed and have a grayish or brownish hue. Damaged blooms remain on the twigs after infection, and become covered with a grayish-brown fungal growth that is worsened by wet spring weather. If we have less rain than usual during April, this common fungal problem may be lessened, but it will spread with only a few wet days and presence of fungal material to initiate the infection. The fungus organism, Monilinia fructicola, grows best when the temperature is above 40 degrees and weather is damp. Another brown rot fungus, Monilinia laxa, appears at temperatures above 55 degrees. Often both types will exist on affected trees.
The fungal infection perpetuates itself by reproduction from affected blooms, leaves, and fruit hanging in the tree over the winter. Good clues to this disease are the crumbled old blossoms, dead twigs, and dead leaves that have remained on the plant through winter
Once established in the bloom, the infection may spread to twigs and shoots. Infected twigs develop sunken cankers that may have gumming at the margins. Leaves on the twig will die but will stay attached to the twig.
If the plant is a fruiting cherry, infection may not appear until after the fruit begins to ripen. Soft brownish spots on the fruit will gradually expand until the fruit is covered with gray-brown fungal spores. A cherry grower who has succeeded in preventing birds from eating the crop can certainly become discouraged when brown rot attacks the ripening fruit.
Now, just as cherry blooms are swelling and beginning to open, check the trees for dead looking areas and mummified fruit left from previous infections. Prune out as much of the affected and damaged parts as possible. It's easier to see them when the trees just begin bloom when it is easier to distinguish dead from living areas. Remove and destroy diseased twigs as you spot them, continuing to do this throughout the summer.
Don't compost this diseased material. (Cleaning out the dead material is obviously easier on a small tree than on a large 30-foot cherry. Many infections simply grow beyond the capacity of the gardener to deal with them. A professional pruner can help.) In addition to pruning to remove dead material, be sure to prune to open the tree to good air circulation, which will help it dry from rains.
A few other good management tips can help with brown rot control. Rake and clean up under the tree during the summer to remove all fallen blossoms, leaves, and fruit. Keeping the ground raked of litter under fruit trees helps with all disease management. (rake up fallen apple leaves that show scab, for instance.).
Be sure to keep high nitrogen fertilizer away from cherry trees; WSU research reports that brown rot infection can be worse if the trees are heavily fertilized with nitrogen. A cherry tree located in a lawn that received a high nitrogen application (such as a 31-0-0) could be made more susceptible to brown rot. Use only moderate amounts of nitrogen fertilizer (such as one application per year of a 5-10-10 or none if the tree is growing normally.)
Some trees may coexist with symptoms and continue to bloom and fruit sufficiently. Gardeners can often avoid using fungicide on brown rot by simply keeping damage pruned out and garden litter cleaned up.
But if the tree is a prominent landscape specimen and damage is severe, and you choose to use fungicides, apply them at the correct time and in the correct sequence.
Spray the tree three times during the bloom period: suggested intervals are 'early, when you see red or pink showing on the buds'; one more spray when the tree is in full bloom, and a final spray after the petals have dropped.
Chemicals registered for home use include Daconil , Captan, and Spectracide Immunox. Check the label carefully for all instructions and be sure that cherries are listed on the label. If the cherry has edible fruit, it's necessary to find a product listing 'fruiting cherry.'
Copper fungicides are also registered for brown rot, but they aren't recommended for use in western Washington on the third spray after petals have fallen. Sulfur is not listed for use on brown rot in western Washington.
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