Oaks ‘n’ Folks – Volume 9, Issue 1 – April 1994
In spite of abundant rainfall in the winter and spring of last year, the foliage of many oaks in the Sacramento Valley developed a brown appearance in 1993 and many trees lost leaves much earlier than normal. Since the drought was over and populations of defoliating insects were low, shouldn’t the trees have produced the healthiest growth seen in years? Yes, except that too much rain in the spring can promote a number of fungal diseases of leaves, one of which is known as anthracnose. The late rains in 1993 were unusual and so too was the widespread occurrence of anthracnose that resulted.
Leaf Phenology, Weather, and Infection
Oaks in the Sacramento Valley typically break bud in late February to early in march. It is not unusual for cool wet weather to extend through these months. By April, however, rainfall typically decreases and temperatures are noticeably warmer.
An extremely wet spring occurred in 1993 as illustrated by precipitation records for Redding, located at the northern end of the Sacramento Valley. The wet weather most likely contributed to infection by anthracnose fungi in a couple of ways. First, the cool wet weather in early spring slowed down leaf development, thus extending the period of time during which the leaves remained succulent and presumably most susceptible to infection. Second, the wet weather continued as temperatures increased, thus providing the ideal climatic conditions for infection to take place. These conditions may very well have produced the most spectacular outbreak of oak anthracnose in recent history.
Hosts and Damage
Home owners and land managers in the Redding area began reporting problems with their oak trees in June, although damage was evident sometime before then. “The leaves are all turning brown” or “The leaves are turning brown and falling off” were common refrains. Black oaks (Quercus kelloggi), valley oaks (Q. lobata), and blue oaks (Q. douglasii) were all affected. Brown spots or splotches of dead tissue developed on leaves, typically bounded by the leaf’s minute veins. Interior live oaks (Q. wislenzii) showed no symptoms.
Heavily diseased black oaks underwent partial defoliation in June. Blue oaks also exhibited some premature defoliation, but mostly later in the season in response to heat stress. For the most part, 1993’s summer temperatures were relatively mild, but a spate of hot days in late July/early August caused damaged leaves on blue oaks to fall, leaving their crowns thin for the remainder of the season. Valley oaks also experienced defoliation, but it was less apparent.
Many diseased oaks exhibited recovery by producing an additional flush of leaves, noticeable in the latter part of June. This was most pronounced on valley oaks; trees that were very brown in early June had full green crowns a month or so later, with the older, damaged leaves being masked by the abundant new growth. In contrast, production of additional foliage occurred.
This past fall, Jeff Stephens, CDF Service Forester for the Shasta-Trinity Ranger Unit, and I conducted a survey of oaks in the northern Sacramento Valley. Areas from Shasta Dam, at the head of the valley, south to Cottonwood and northern Tehama County were surveyed. Anthracnose was present on blue, valley, and black oaks everywhere that these trees grew, while there was no evidence of anthracnose on interior live oak. In Whiskeytown National Recreation Area, just west of Redding, anthracnose was quite prevalent on black oak.
At the time of the survey, near the end of October, the three oak species damaged by anthracnose varied considerably in their appearance. Blue oaks had lost most of their infected leaves during the latter part of the summer. Their crowns were quite thin with small amounts of green foliage on the very ends of branches. Valley oaks had mostly green, relatively full crowns due to a substantial second flush of foliage. Black oaks had relatively full crowns, but anthracnose was very prominent, giving the crowns an overall brown appearance.
Oak anthracnose was by no means limited to the area surveyed. Douglas McCreary, Area Natural Resources Specialist with the University of California, confirmed the presence of oak anthracnose through to the southern end of the Sacramento Valley. The severity of the disease, however, was greatest at the northern end of the valley and decreased to the south. Anthracnose and other foliage diseases were also reported form black and Oregon white oaks in other areas of Northern California, particularly areas of relatively high rainfall to the west and north of the Sacramento Valley.
Oak anthracnose has been attributed to infection by the fungus Apiognomonia errabuna (= Discula umbrinella). However, Diana Fogle, agricultural biologist with the California Department of Agriculture, found that the samples of oak anthracnose sent to her in 1993 were often due to other, undescribed, fungi. All of the fungi from leaf spots produced conidia (asexual spores) in fruiting structures called acervuli on the lower sides of infected leaves. It is apparent that more than one fungus was the cause of anthracnose symptoms observed on oaks in 1993.
In some situations, anthracnose is characterized by twig dieback in addition to the typical leaf symptoms. Thus far there is no evidence that twig dieback has occurred in association with the oak anthracnose outbreak in the Sacramento Valley, although it is something to watch for.
Another concern is that D. umbrinella and some related fungi are capable of causing twig dieback in the absence of anthracnose symptoms on the leaves. In this situation, the disease is not referred to as anthracnose, but simply twig blight. This disease has been reported from a number of oak species, but the most common host is coast live oak, Q. agrifolia. Some stands of coast live oak extend in to the Central Valley east of the San Francisco Bay Area, but the tree primarily occurs in coastal counties from Baja California north to southern Mendocino Co. Because twig blight is promoted by wet weather, it may have been a problem in some parts of the state in 1993. More information on this disease can be found in the University of California Cooperative Extension publication Twig Blight and Branch Dieback of Oaks in California, Leaflet 21462.
Looking at the Past and Future
Unusual wet spring weather was the primary cause of the 1993 outbreak of oak anthracnose. While the fungi that cause anthracnose are generally present, the environmental conditions that promote leaf infection and disease are not. Scanning Redding’s weather records for the past 40 years indicates that spring weather similar to 1993’s is rare. For example, there were only three other years during which the months of march, April and May all had above average precipitation; 1983, 1963, and 1958.
Reports of past occurrences of oak foliage diseases can be found in the annual summary “Forest Pest Conditions in California,” published by the California Forest Pest Council. It is apparent from these reports that foliage diseases of oaks do appear from time to time in association with wet spring weather. About one quarter of the annual summaries list some kind of oak foliage disease in some part of the state, but most of these reports do not suggest serious or widespread problems. The year 1958, however, stands out as having severe outbreaks of oak foliage diseases due to exceptionally wet spring weather that extended through June and into July.
Control of oak anthracnose is neither practical nor recommended. Spraying a tree with the fungicide benomyl might be effective at preventing anthracnose, but it will not cure a tree of the disease once infection has occurred. Preventative spray treatments are typically applied every year in anticipation of the disease occurring. This may be warranted for some tree species that are particularly prone to anthracnose, but it is not warranted for the rare occurrence of anthracnose on California oaks.
Because unusual weather patterns are needed for outbreaks of oak anthracnose and similar foliage diseases, outbreaks will typically be limited to one growing season and infrequently repeated. The immediate effects of these outbreaks may be startling, but there should be little long-term impact on tree health.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc