A Long Drawn-out Spring
In May, the abundant rain and cool weather during spring leaf expansion, created conditions ripe for various foliar diseases. Anthracnose, oak leaf blister and apple scab are already showing themselves and Rhizosphaera, Dothistroma and the pine tipblights soon will be. While none of these diseases are fatal, the damage can be severe and worrisome.
Anthracnose is a descriptive term rather than a specific fungus. There are as many if not more fungal species that cause anthracnose, or leaf blotch diseases than there are host species. Ash, oak and maple are among the hosts we get the most calls about. But walnut and elm are also affected, as well as several other tree species.
This year, the white and bur oaks look particularly bad. Besides the typical brown leaf blotches of anthracnose caused by Apiognomonia quercina, we are seeing another fungus causing a disease called oak leaf blister. The disease agent is Taphrina caerulescens. Rather than brown irregular blotches, oak leaf blister causes large rounded 'blisters', or puckers scattered across the leaves and along leaf margins. These 'blisters' start out light green in color and then turn brown as the affected leaf tissue dies. Because affected areas pucker so, affected leaves can become badly distorted. While the samples that have come into us have been mostly on bur oaks, oak leaf blister is also commonly found on red and pin oaks. So watch for signs of damage on those species as well.
Unlike anthracnose diseases, Taphrina has one spore type and thus only one infection period. So while the light green puckers will continue to brown, you won't get any more puckers than you currently have. That may be small consolation, but at least there won't be any additional damage. Anthracnose, on the other hand produces conidia, or asexual spore types that can continue to infect new leaf tissue as long as there is sufficient moisture for infection. That means we may see additional damage IF the weather continues to stay wet.
We normally do not recommend treatment with either disease other than sanitation, i.e. rake and destroy fallen leaves now and in the fall. This removes overwintering fungi and thus limits infections next spring. Rarely do these foliar diseases cause any long-term health damage, unless the tree in already under some form of other stress, like a new transplant or an older declining tree. In those cases, the additional defoliation can reduce natural defenses and leave the tree susceptible to secondary attack by boring insects. However, that is not very common, since foliar diseases are so dependent on weather conditions.
Apple scab is in the very early stages, so the leaf spots are relatively smooth and light olive green in color. They are pretty inconspicuous and can only be readily seen when looking through the leaf up into the light. But these spots will begin to expand soon and discolor the leaves. The leaves will gradually turn more and more yellow and eventually fall off the tree. Because the apple scab fungus, Ventura inaequalis, produces abundant spores whenever the weather turns rainy, new infections continue to show up all summer. Thus defoliation can be quite severe. Last year was another ideal year for apple scab and many trees were nearly if not completed defoliated. In those cases, natural defenses are likely to be affected. While control is recommended in commercial situations, it is rarely recommended in ornamental settings. However, this year, control may be advised in those cases where trees were defoliated last year. Check with your local garden supply center for a fungicide labeled for foliar diseases and apply as directed on the label. As in cases of anthracnose or oak leaf blister, rake and destroy all fallen leaves to limit infections next spring.
Dutch elm disease
The incidence of Dutch elm disease continues to climb, dramatically increasing the number of tree removals across the area. Disease incidence had been up last year as well and weather-related stresses have contributed to the outbreak. While city budgets are stretched keeping up with the workload and homeowners are saddened by the loss, District Energy has benefited by the increased supply of wood chips.
Spring defoliators and sucking insects have been abundant. The snow cover this last winter provided ample protection for overwintering insects and the increased tree stress (inhibiting natural defenses) has maintained those initial population numbers. While the damage is unsightly, it is not likely to cause long-term health problems.
Winter injury to pines
Winter injury due to the overly dry soils this last winter (even under the snow) is common among pines through out the state. As expected, it is particularly noticeable along roadsides. The base of the needle usually remains green on trees with winter injury but, in some cases, the entire needle has turned brown. The bud normally is healthy and new growth is now occurring on the trees. With adequate rainfall the trees should recover. Additionally, the rain in May seems to have kept most of the bark beetles at bay, at least for now. We'll have to see how the rest of the season goes.
Heavy seed production on maples
There has been very heavy seed production on red maples this year, especially noticeable in parts of Itasca, Cass and Crow Wing Counties. Some red maples appear to have more seeds than leaves. Once the seeds fall, the trees are likely to look as if they have been defoliated.
Due to the effect of a combination of weather patterns, we are seeing a range of odd symptoms in trees scattered across the landscape. While it might sound like a cliche, blaming the weather is appropriate for most of the current symptoms. Unfortunately, which weather pattern did what and when is harder to determine. Here are some of the symptoms we are seeing in the central region and possible causes.
Black leaf tips - The cold spell mid May nipped a number of species as they began to leaf out. The damage produced blackened leaf tips and shoots on several hardwoods, most notably ash. Healthy trees should reflush and resume normal growth.
Partial bud break- A number of trees began to leaf out as usual, only to stop mid-stream. Then individual branches and/or whole trees died. In a few cases the few remaining leaves are very small, sparse and/or discolored even though the trees looked fine last year.
These symptoms are indications of severe stress, most likely due to the weather extremes of the last two seasons. Until this May, we'd been under prolonged drought conditions for most of the last 2 years (with the exception of June 03). There was also no snow cover during the winter of '02-'03 and several harsh cold spells. The dry conditions and the cold temperatures produced excessive stress in '03. Many trees were very late to bud out and in many cases, the first flush of growth was small and undersized. Summer '03 was also very dry, so trees remained stressed going into this last winter. While we received a good amount of snow, there was little moisture in it and the soils beneath the snow stayed dry for most of the winter. This spring was also very dry (until May) with a couple of unusual warm spells. So many trees began to leaf out this spring with very little moisture in reserve.
Vulnerable trees, older trees or those under other forms of stress, were just not able to persevere. The level of decline and the chance of recovery vary with the tree and the site, but are not promising.
Root dysfunction - Early in the season, the soils remained cold for so long that normal nutrient uptake was inhibited. This induced temporary nutrient deficiencies and chlorotic growth. If the trees were otherwise healthy, normal color should develop as soon as the soils warm and root function resumes. Since May, roots in overly saturated soils have been starved of oxygen and may not be able to function properly. When the weather quickly warms, tree canopies respire faster than dysfunctional roots can maintain. The result can be partial to complete wilting. Trees may recover from minor wilting. But branch dieback and/or complete tree death may result from prolonged oxygen starvation. Where possible, temporarily divert surface drainage away from saturated root systems. But beware - roots damaged by flooding will be overly sensitive to dry spells later on. As the weather heats up and dries out, be sure to monitor soil moisture levels and water when necessary.
Still droughty in NC counties
While many areas of the state are well above average in seasonal and annual rainfall, the north central counties remain quite dry. Even the "average" amount of rainfall received in May did little to ameliorate the droughty conditions that have persisted for over two years.