Insects and Diseases

The spruce gall adelgid causes a cone-like growth on spruce trees that is green during the summer, but darkens as fall approaches. These can be unsightly and a real nuisance. Spring is the best time to take care of this problem. Spray the hosts (Douglas-fir & spruce) when the new growth is about 1 - 2 inches long and still tender. The crawler stage of the adelgid is moving around to establish itself at this time and is vulnerable to treatment. If you wait until the galls start to form (on spruce) or the insect has developed a white woolly covering (on Douglas-fir), the insects are protected and treatment will do little if any good. Registered products that can be applied at the correct time include Carbaryl (Sevin) and permethrin.  Horticultural oils can also be effective, but can cause a temporary discoloration of the needles.  Marathon and Merit (systemic insecticides with imidacloprid as the active ingredient) can be applied to the soil in fall so that it is present in the foliage in spring. 

Several bark beetles that attack pines can be prevented from making their attacks through the use of preventive sprays. Generally this is only done when there are very high value trees that are being threatened. Trees under stress are more susceptible and when there are high populations of bark beetles in the area, preventive treatment may be warranted. The principal pesticide for these preventive treatments is Sevin, a general action carbamate pesticide containing carbaryl as the active ingredient. The spray needs to be applied to the bole (trunk) of the tree from ground level up to where the tree is only about 6 -8 inches in diameter. Since this is often 80 or more feet up the tree, it usually requires hiring a pest control operator who would have equipment of adequate force to get the pesticide that high. The treatment is generally done in the spring as this is the best time to assure that protection is provided against all beetles with the potential to attack pines. When done properly, this treatment provides two years of protection.

Tree diseases that we often see in spring are:

Needle Casts on lodgepole pine and Douglas-fir. The red or brown spotting of leaves is caused by a fungus. There is no control, but it might be comforting to know that the affected needles will fall off and the looks of your tree will improve.

Powdery Mildew is a common disease in many kinds of trees and landscape plants.  Powdery mildew as the name implies, is a fungi that appears as a grayish or white powdery growth on leaves and other succulent tissue.  Patches of the disease may enlarge until they cover the entire leaf on one or both sides. Extended cool spring weather provides the perfect environmental conditions for the disease to proliferate and it can be very visible.  The fungi spores are spread by the wind and over-winter on plant tissue, dormant buds and fallen leaves. Many landscape plants are susceptible to one or more species of powdery mildew.  Each species of powdery mildew has a very limited host range.  Infection of one type of plant does not necessarily mean that others are susceptible.  For example, the fungus that causes powdery mildew on tree leaves is not the same powdery mildew that can infect garden plants. Powdery mildews seldom seriously harm trees and moderate amounts of infection can generally be ignored.  However, this disease can be quite unsightly.  Infected leaves display white patches which can cover a large portion of the canopy, leaves often drop prematurely and shoots may become distorted. Fungicides can be applied at the earliest signs of infection, but are too late once the growth has become extensive.  Refer to instructions on product packages to determine methods and timing of applications.  Temperature and humidity are important factors when determining when to apply fungicides. The following are the best steps you can take to reducing the effects of powdery mildew:Provide sunshine and good air circulation within the plant.  Pruning may help, but don't prune during dry weather.  Winter is the best time to prune.Avoid excessive fertilization and irrigation, which stimulates the growth of succulent tissue that the powdery mildew infects.Overhead irrigation in mid-afternoon may reduce powdery mildew because spores cannot germinate, and some are killed, when plants are wet. (mid-afternoon is when the most spores are formed).Rake up and dispose of the leaf litter off-site this fall.  Do not use infected leaves for compost.Plant disease resistant cultivars.