Summer Tree Care: Identifying Common Tree Diseases in Summer, Part 6
This is the sixth part of a series on summer tree diseases. This article examines dogwood anthracnose, and Dothistroma needle blight.
During the summer months, plants are in the midst of their development. While many plants flourish due to the warmer climate, others can be subject to infections from a slew of disease pathogens. The following describes some of the most common diseases to affect plants in summer, and how they impact their hosts.
Dogwood Anthracnose (Discula destructiva)
Dogwood Anthracnose is a fungal disease of flowering and Pacific dogwood. An anthracnose fungus, Discula destructiva, has been identified as the causal agent. Prolonged infection from anthracnose can decimate dogwoods. Consecutive years of heavy infection often results in extensive mortality in both Pacific and flowering dogwood.
Dogwood anthracnose infects flowering and Pacific dogwood.
Symptoms of Infection
Infection of dogwoods is favored by cool, wet conditions in spring. Drought stress and winter injury can exacerbate the disease. Initial foliar symptoms develop in the lower crown in May and June. Brown spots or lesions up to ¼ inch in diameter form on both the upper and lower leaf surfaces. These lesions may be circular or irregular in shape, with distinctive purple-brown margins. As the infection progresses, leaf lesions tend to become more numerous, eventually coalescing. This leads to the development of large, necrotic areas on the infected leaf. Black fruiting structures may sometimes form within the centers of foliar lesions. If an entire leaf becomes infected, the fungus will proceed into the petiole, followed by the twig. This causes cankers to form on the infected leaf. The cankers that form are often tan, with a slightly sunken appearance.
Flower bracts are also susceptible to infection. When infected, they develop reddish-brown spots or blotches. These are most prevalent when rainy conditions prevail during flowering. Anthracnose fungi can directly infect shoots during spring and fall. These infections develop into small cankers, which gradually increase in size. As the cankers become larger, they create a girdling effect on healthy tissue. This often results in significant foliar dieback. When severely defoliated, some trees will attempt to compensate for the loss of growth by producing succulent shoots on the lower trunk and main branches. These branches, also called epicormic sprouts, are highly susceptible to infection. Sprout infections tend to spread quickly to the trunk, causing large cankers and splits to form in the bark.
- When planting, consider plant resistant species or cultivars. Kousa dogwood is less susceptible to infection than flowering dogwood.
- Prune and dispose of dead wood or cankered limbs. Dead wood and cankered limbs are susceptible to infection from anthracnose fungi. Removing them helps reduce the ability of the fungus to grow into the main trunk.
- Rake and remove fallen leaves to remove potential sources of overwintering fungi.
- Maintain tree vigor by following sound cultural practices.
- Applying a balanced fertilizer in early spring can bolster tree vigor.
- Follow proper pruning practices to improve air circulation throughout the tree.
- Ensure that trees are sufficiently watered, especially during dry periods.
- Applying a layer of organic mulch around a tree can help maintain soil moisture, moderate soil temperatures, improve soil quality, and minimize the potential for mechanical injuries.
- Treat insect infestations through biological or chemical control practices.
- Fungicides can assist in disease management. The initial application should be made just prior to budbreak, before new leaves and shoots expand. Two or three additional applications can be made during leaf expansion, at ten to fourteen day intervals. Further applications may be necessary during wet or prolonged spring conditions.
- Some of the more effective compounds available for use include chlorothalonil, copper hydroxide, mancozeb, and thiophanatemethyl. Organic alternatives include copper products, sulfur, Serenade, and potassium bicarbonate. When using any of these, consult the label for dosage rates and safety precautions.
Dothistroma Needle Blight (Dothistroma pini)
Dothistroma needle blight, often referred to as red band disease in the western United States, is a devastating foliar disease of pines. The causal agent of the disease is the fungus, Dothistroma pini. The fungus infects, and kills susceptible pine needles. Severe infections can defoliate host trees, hindering their growth, and rendering them prone to failure.
Dothistroma needle blight affects twenty nine pine species and hosts. Austrian, lodgepole, and ponderosa pines are the most commonly infected. Scots pine, and mugo are also infected, albeit with less frequency.
Symptoms of Infection
Dark green bands, and yellow to tan spots appear on infected needles. The dark green bands fade as the disease progresses, making them increasingly difficult to detect. The spots and bands later turn brown to reddish-brown. In California, Oregon, Washington, and Idaho, the bands tend to exhibit a brighter shade of red. The black fruiting bodies may be visible on the surface of the infected needles within the spots and bands. The ends of the infected needles become discolored, initially appearing light green before turning tan, and then brown. The base of the infected needles remains green. The needles may develop extensive necrosis two to three weeks after they have been infected. Infected needles are often shed from the tree, with second year needles dropping first. Needles that become infected the year they emerge often remain on the tree until late summer of the following year. Infection is generally most severe in the lower part of the crown. Successive years of infection can culminate in tree mortality.
- Registered fungicides may be utilized to prevent infections. Two applications should be performed during the growing season: one in late spring, to protect the older needles, and a second in early summer to protect the current year’s needles.
- When planting in urban settings, select resistant varieties, such as Mugo and Scots pine. Allow for ample spacing between trees.
- If a tree is planted in a landscape setting that has an irrigation system installed, ensure that they water does not splash onto the needles. Damp foliage is more prone to infection.
- Thin out dense canopies to promote air circulation, and sustain tree vigor.
- Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during periods of extended drought. Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and retain soil moisture.
Photo courtesy of NatureServe CC-by-2.0