Reasons for the sudden appearance of widespread dieback have not been determined. Thus far we have records of dieback occurring in at least twenty counties of California and throughout most of the range of Q. In California, this disease has been observed mainly in Quercus agrifolia, Q.
In nature, infection apparently occurs mainly in early spring, but symptoms are often most pronounced during hot periods in late summer. Within 24 hours after branches are wetted, spores are produced that can be spread by rain splash and probably by insects. The result is branch flagging and eventual stagheading in tree crowns, sometimes sufficiently severe as to result in the death of the tree.įungus fruiting bodies are produced on dead tissues. Slosson Foundation and the International Society of Arboriculture, cause, pathology, and control are being investigated.ĭieback 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. This twig blight has continued to be present and has by now affected large numbers of oaks. At about the same time (1981), an extensive dieback of twigs was observed. In 1985 it reappeared again after a relatively dry season. This dieback became increasingly severe until 1981, when it began to decline. In 1977, dieback of large branches up to six inches in diameter was first noted. 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.
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