Fungus often to blame when cherry blossoms whither

Q: My cherry tree gets messy brownish blobs in the flowers; what's going on?

A: Cherry trees in bloom inspire poetry and seem to define the spring landscape along with rhododendrons and hardy spring bulbs. One of the loveliest spring sights is blue sky seen through cherry blossoms.

Many different species and types of cherries are planted in our landscapes, meaning that we experience a long blooming season in the maritime Northwest, often starting in November with the autumn cherry Prunus subhirtella `Autumnalis' and continuing through the late ornamental cherries such as `Shirofugen,' which provide bloom into May.

All cherries, both edible and ornamental, are members of the genus Prunus, as are flowering and fruiting plums, peaches, apricots, almonds and nectarines.

Diagnosing problems from just a few words is often difficult, but in this case when the difficulty is described as "messy brownish blobs," it's easy to infer what's happening. Cherry trees of all types, growing on the wet side of the Cascades, often show damage from a fungal problem appropriately nicknamed "brown rot blossom 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. Damaged blooms remain on the twigs after infection, and become covered with a grayish-brown fungal growth that is worsened by wet spring weather.

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. If these cankers encircle the stem, girdling it, 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.

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 remain on the plant through winter.

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 when the trees just begin bloom; if you wait until winter to prune, you have trouble distinguishing the dead areas from the dormant, leafless tree.

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.

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.) Prune trees to allow as much air circulation as possible.

You can also hope that the relatively dry spring we are experiencing continues. Sunny days that dry leaves and branches will help prevent brown-rot infections. The fungus organism, Monilinia fructicola, grows best when the temperature is above 40 degrees and weather is damp.

Be sure to keep high nitrogen fertilizer away from cherry trees; Washington State University 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).

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, use fungicides 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, again when the tree is in full bloom and a final spray after the petals drop. Chemicals registered for home use include Daconil 2787, 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 isn't listed for use in Western Washington.

If you want to enjoy cherry trees in bloom, especially those maintained by other people, check Arthur Lee Jacobson's book "Trees of Seattle" (Sasquatch Press, 1989) for locations of particularly spectacular cherry displays within the Seattle city limits.

Interlaken Park, Lincoln Park, Seward Park, the University of Washington campus, the Washington Park Arboretum and Evergreen Park Cemetery all have mature, elegant cherry trees. Outside Seattle, go to Marymoor Park, Point Defiance Park in Tacoma or McCollum Park in Everett.

Gardening runs Sunday in Home/Real Estate. It is prepared by Mary Robson, area horticulture agent, Washington State University/King County Cooperative Extension.