Black Oaks Lose Leaves in El Nino Winter
BERKELEY - This summer, people living in the Sierra Nevada and CoastRanges have reported seeing the leaves on California black oaks coveredwith brown spots, curling at the edges and even turning completely brownand falling off. In some cases, entire hillsides are blanketed with browntree crowns. While black oaks are deciduous - changing color and droppingtheir leaves in the fall - they are normally fully leafed out with dark-greenfoliage in early and mid-summer.
Many landowners, concerned their trees are dying, have contacted their University of California Cooperative Extension county office, the CaliforniaDepartment of Forestry and Fire Protection or local arborists and landscapersto find out what is causing the problem and whether their trees are in jeopardy.
While several organisms may be contributing to the symptoms observed,two fungi -- Septoria quercicola and Cylindrosporium kelloggii -- have beenidentifieed from leaf samples sent to the California Department of Foodand Agriculture, according to Doug McCreary, natural resources specialistwith UC's Integrated Hardwood Range Management Program. While these foliagediseases were present on black oaks last year, they are much more prevalentand widespread this year, he says. Last spring's extremely wet conditions are responsible, providing an ideal environment for the fungi to infectthe leaves.
According to Don Owen, pest specialist with the California Departmentof Forestry and Fire Protection, more damage is unlikely, since the rainshave ended. However, trees may continue to lose their leaves throughoutthe summer, he says. The level of leaf loss relates directly to the severityof infection: If leaf loss occurred early in the year, the trees will beable to refoliate. Tree vigor also helps determine the amount of refoliation,since trees with more energy reserves are better able to refoliate thanweakened trees. Trees with pre-existing stress, or infected trees that losetheir foliage relatively late in the season, may not refoliate as fully.They may also experience some dieback in the crown.
While there are fungicides available to prevent the infection, they mustbe applied in the spring when the leaves are susceptible to the fungi. Timingis critical; if conditions favor infection over an extended period of time,multiple treatments may be necessary.
The leaf loss resulting from these diseases can be harmful to the trees,since it reduces their ability to manufacture food through photosynthesis."Over time, repeated defoliations can seriously weaken trees,"McCreary says. "We would also expect reduced acorn production on treeswhere damage is severe, which in turn could adversely affect the many wildlifespecies dependent upon acorns as a food source."
While spotting and brown leaves on oaks in late spring and early summerare certainly a cause for concern, the vast majority of the affected treesshould recover and leaf out normally next year, McCreary says. However,in rare cases, severely weakened trees could be killed by the defoliation.For more information about oaks and oak management and the names of expertsworking with oaks by region, visit the home page of the Integrated HardwoodRange Management Program at http://danr.ucop.edu/ihrmp/.
prepared and edited by Richard B. Standiford