Oaks ‘n’ Folks – Volume 2, Issue 1 – August, 1987
Introduction
Oaks in California are valued as urban and park trees, as scenic adornment of Coast range and Sierra foothills, as food and cover for wildlife, and as shade on cattle ranges. In 1977, dieback of large branches up to six inches in diameter was first noted. This dieback became increasingly severe until 1981, when it began to decline. In 1985 it reappeared again after a relatively dry season. At about the same time (1981), an extensive dieback of twigs was observed. This twig blight has continued to be present and has by now affected large numbers of oaks. With support from the Elvenia J. Slosson Foundation and the International Society of Arboriculture, cause, pathology, and control are being investigated.
The Diseases
Dieback of oak branches of 0.5-6 inches in diameter is caused by Diplodia quercina, a fungus that attacks mainly through wounds and kills the sapwood, cambium, and phloem of branches. The result is branch flagging and eventual stagheading in tree crowns, sometimes sufficiently severe as to result in the death of the tree.
Fungus fruiting bodies are produced on dead tissues. Within 24 hours after branches are wetted, spores are produced that can be spread by rain splash and probably by insects. In nature, infection apparently occurs mainly in early spring, but symptoms are often most pronounced during hot periods in late summer.
In California, this disease has been observed mainly in Quercus agrifolia, Q. kelloggii, and Q. lobata, but inoculations show that Q. douglasii is also susceptible. Thus far we have records of dieback occurring in at least twenty counties of California and throughout most of the range of Q. agrifolia. Reasons for the sudden appearance of widespread dieback have not been determined. Since many such diebacks are associated with stress, it may be that the severe drought of 1976-1977 was instrumental in disease buildup. It may be worth noting that the disease had declined dramatically until the recent dry spell in 1985, when apparent increase in disease was observed. It appears likely that Diplodia will ìflare upî periodically in response to variations in rainfall.
Twig blight is associated with two fungi, Cryptocline cinereseens and Discula quercina. Both fungi behave similarly and can be discussed together under the general disease designation ìtwig blight.î Death of the current yearís twigs results from girdling and may involve a few or almost all twigs up to a diameter of 0.75 inch. In extreme cases, repeated twig dieback can result in death of trees.
Infection by water-borne spores occurs mainly during tree growth stages when twigs and leaves are immature and succulent, mainly in early spring. Wounding is apparently unnecessary for infection. Acervuli form readily on inoculated branches but mature spores are found only in late October or early November, just as the first rain storms occur. Spores germinate within 24 hours and acervuli continue to produce viable spores until March or April. Infected buds have been found in late fall.
Generally, only Q. agrifolia is severely affected, but Q. lobata, Q. douglasii, Q. wislizenii, and Q. chrysolepsiscan also be infected. Disease distribution has not been systematically determined, but so far the disease has been observed from Mendocino to Gaviota Pass in the Coast range, over most of the foothills of the westside Sierras, and in the Mt. Palomar Range. The conditions that triggered the current outbreak are unknown. Studies conducted in cooperation with C.S. Koehler and L.R. Costello showed that the amount of twig blight is correlated with the amount of oak pit scale (Asterolecanium minus).
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc