ISSUE 8 June 21, 2001
Branches on many trees in western North Dakota and other parts of the state failed to leaf out this spring and others leafed out only to die soon after. Canker diseases, caused by fungi that can girdle limbs, are often identified as the agents that result in branch or tree death. Dead branches with no leaves or flags are often the first canker symptoms noticed. Flags are dead branches with yellow or brown dead leaves remaining attached. A flag is caused by rapid death of a branch as it is girdled by a canker causing fungus. Canker fungi will eventually leave a sunken area in the bark. The presence of a canker can be confirmed by this sunken area or by pealing back the bark, revealing a darkened discoloration of the sapwood. After the canker margin (line between infected and noninfected tissue) is identified, a canker fungus can be removed by pruning. Most canker fungi will not normally kill healthy trees.
Canker diseases can kill trees weakened by factors such as herbicide injury, defoliation by insects, weed competition, mechanical damage, and extremes in temperature. Climatic extremes are the most important damaging factors. The moderate temperatures last fall followed by fluctuations in temperature in winter are probably important in the number of cankers we are seeing in trees this year. Drought conditions late last year in some parts of the state are also contributing to the increased incidence of canker diseases in those areas. Although climatic extremes cannot be controlled, many of the other factors predisposing trees to damage from cankers can often be prevented or reduced in severity and can add years to the useful life of a tree.
Apple scab is nearly a perennial disease on Malus sp., things in the apple family like apple trees and crabapples. Mountain ash can also be infected with the apple scab pathogen, Venturia inaequalis. Under the right environmental conditions (cool and wet), the fungus will produces one type of spore called primary spores (ascospores) that infect young leaves and fruit. After this initial infection, secondary spores (conidia) are produced on the diseased plant material. Conidia may continue to be produced as long as environmental conditions remain favorable.
The disease first appears as olive green, velvety spots on the leaves, and eventually on the fruit. Disease lesions on the fruit may become corky and brown, and may cause some disfigurement of the fruit. Diseased areas may be cut out and the apples eaten, but early disease may cause premature fruit drop and significant yield reductions.
Management for apple scab should start with good sanitation. All plant debris, leaves, broken branches, and dropped fruit should be raked up and destroyed or taken away in the fall. Then in the spring, if the weather is favorable for disease development, and especially if there are other trees in the area that may be a source of infection, fungicides can provide good protection from disease. Chlorothalonil, myclobutanil, and captan all provide protection from infection. Once an infection is established, according to a New England study, only thiophanate-methyl (Topsin-M) provides very good disease management; however, fenarimol (Rubigan) and myclobutanil (Spectracide Immunox), and triflumizole (Procure) all show fair to good disease management results. Since products and formulation will vary from location to location, look for products with these active ingredients and follow label recommendations carefully. Some of the products may require a 14 day window before harvest so plan spray treatments carefully so as not to contaminate the fruit.