Abstracts

Mircetich, S.M., W.R. Schreader, W.J. Moller, and W.C. Micke. 1976. Root and crown rot of cherry trees. California Agric. 30(8): 10-11.

Decline and death of cherry trees resulted in an estimated loss of 22 percent of all sweet cherry trees in San Joaquin County during 1973-75. In 1975 some orchards were almost 100 percent affected with crown and root rots. Such occurrences of cherry tree decline in the past have been variously attributed to “wet feet,” “soursap,” or occasionally root-infecting fungi, but evidence for a clear-cut cause has often been lacking. The water mold fungus Phytophthora frequently has been suspected but never directly proved as a causal agent of cherry tree decline. Our field observations and results from the greenhouse tests strongly indicated that the water regime in orchard soil infested with Phytophthora and the type of rootstock are very important factors that determine severity and incidence of Phytophthora roo t and crown rot in commercial orchards. In the light of this new information, further research is now necessary to develop better management practices.

Moller, W.J. and M.H. Schroth. 1976. Biological control of crown gall. California Agric. 30(8): 8-9.

Spectacular biological control of crown gall was achieved last year in an experiment carried out on young almond, peach, plum, and apricot trees in a California nursery. Crown gall is a bacterial (Agrobacterium tumefaciens) disease of worldwide importance on many woody plants; it can be especially serious in deciduous fruit nurseries. An Australian researcher, Dr. Allen Kerr, recently reported on the effectiveness of a non-disease-producing form of the crown gall bacterium for protecting seeds and seedlings against the gall-forming strain. Kerr found that, by dipping seeds or seedlings in a suspension of this biological agent before planting, healthy trees could be gorwn, even in crown-gall-infested soil. Dr. Larry Moore of Oregon State University tested this approach in the field and obtained spectacular control for a number of plant varieties. Control achieved with the biological treatment was excellent, especially considering the drastic nature of the test conditions. Further tests are in progress to test the biological control agent with other plant species such as walnut, cherry, and grape.