Originally Published In The Deseret News -- Wednesday, June 7, 1995
Anthracnose Affects Sycamores, Oaks, Ashes And Maples And Is Most Severe During Wet Periods.
While I'm never one to complain about rain in the desert, I'll be the first to admit that recent weather has affected many plants. Most have grown well with little or no stress. Spring flowers have been so luxuriant, one hates to remove them for summer annuals. In spite of this, there are some drawbacks to rain in our desert.
Perhaps the biggest in terms of size if not in terms of severity is a malady that has befallen the sycamore trees and may affect oaks, ashes and maples. Anthracnose is most severe during periods of wet, rainy weather. On sycamores, the casual organism is Apiognomonia veneta. While this pest is not exactly a household word, it nevertheless causes severe disease symptoms on the large and stately sycamore trees.
Apiognomonia veneta is a fungus that propagates by spores. It overwinters in the infected twigs. In the spring, spores are produced and spread to the new growth by splashing or windborne rain. When moisture is abundant, as new growth is produced, spores germinate and infect new twigs and foliage. New infections produce more spores that readily spread by rain to nearby foliage.
Anthracnose damage on sycamore, ash and maple generally shows an unusual symptom. Large, dead lesions occur along the leaf mid-rib, causing extensive die back. Young, infected leaves become curled and distorted and portions of the leaves die back. They often appear to have been damaged by frost. Right now, a more visible symptom is severe die back of the small twigs. Numerous small shoots have completely died, and in some cases huge trees are almost leafless. As the twigs die, cankers form at the base and girdle the young twigs, causing them to die. The resulting regrowth on these branches forces many small shoots to appear and gives the trees a rather unusual appearance.
The most common control recommendation is to avoid planting susceptible species. Since the disease was almost unknown in Utah until the wet years in the early '80s, it is not practical to replant magnificent, old sycamore trees. American sycamores and London plane trees, especially the Bloodgood variety, are most susceptible, so avoid planting them if this disease is a concern.
Other control measures are usually not practical for large trees. One recommended strategy is to prune out and dispose of infected twigs during fall and winter. This removes overwintering fungi and reduces problems in the spring, but try doing it on a tree that stretches more than 50 feet in height.
Fungicides are of limited effectiveness and in all cases must be applied prior to the time the infection occurs. Recommended fungicidal treatments must be repeated prior to the time the infection occurs. Such treatments are sprays of fixed copper or chlorothalonil at bud break. Sprays must be repeated every two weeks as long as wet, rainy weather continues. This means three or four sprays would have been applied already this year. Because of the size of sycamore trees and the labor and expense of spraying, these controls are usually reserved for nursery stock and other high-value trees. Spraying now is a waste of time and money.
The good news is that sycamores are very resilient. As the weather gets warmer and drier, they will send out a new set of leaves and hopefully appear reasonably normal by the end of the growing season.
Two other tree diseases are also producing extensive symptoms. Aspen leaf spot shows up as black spots on the leaves that grow together and cause the entire leaf to go black. The causal organism is different, but control recommendations are very similar to those used for anthracnose.
Coryneum blight affects all of the stone fruit trees. There are three symptoms you are likely to see right now. The most likely is purple spots on the leaves or holes where the spots have fallen out, giving a shot-hole appearance.
In many cases, this symptom is accompanied by numerous dead buds that are producing balls of gum. Apricot fruits, if you are lucky enough to have any, will likely be covered with small purple spots. Spraying now for Coryneum blight is useless. It must be sprayed in the fall after the leaves fall to protect the trees.
Codling moth controls to prevent wormy apples need to start right now. Use Imidan every 14-21 days, diazinon every 7-10 days, malathion every 3-5 days. Organic gardeners can spray Dipel or Thuricide every 3-5 days. Cherry fruit fly sprays are not needed until the cherries change from green to yellow. Detailed control measures will be in next week's column.
Don't let the weather get you or your trees down. Deserts remain deserts in spite of occasional wet years. Trees may need an occasional boost to overcome diseases, but long-term problems should be minimal.
- IT'S AN AGGIE FAMILY GARDEN PARTY, sponsored by the Salt Lake Chapter of the Utah State University Alumni Association. Bring a picnic lunch and a blanket on Saturday, June 17, at 2 p.m. The event will be at the Utah State Botanical Gardens, 1817 N. Main, Farmington.
There will be a gardening and landscape question-and-answer session hosted by Larry Sagers, Sammy T. Clown for the kids, tours of the Botanical Gardens, a drawing for family tickets to the USU/Weber State basketball game at the Delta Center on Dec. 16, and free Aggie ice cream.
For more information, call 468-3170 (daytime) or 254-6588.
© 1998 Deseret News Publishing Co.