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Dothistroma Needle Blight of Pines
by Paige Thrush, Nancy J. Taylor & Francesca Peduto Hand
Dothistroma needle blight, also called red band disease, is a foliar disease of pines and other conifers. In Ohio and throughout the Midwest, the disease is a significant problem on Austrian, red and Scots pine in the landscape. Other conifers that are reported to be susceptible hosts include Douglas-fir, Sitka spruce and European larch. The disease is found worldwide, with extensive damage occurring in exotic pine plantations in the Southern Hemisphere (e.g. New Zealand, East and South Africa, Chile), in areas where cool and moist weather are persistent. The disease has also caused considerable damage in North America, especially to ornamentals in the landscape, shelterbelt, and Christmas tree plantings.
Dothistroma needle blight is caused by two fungal pathogens, Dothistroma septosporum and Dothistroma pini. Dothistroma causes needle blight and premature defoliation, greatly reducing the vigor, aesthetic quality, and market value of its host. When favorable weather persists for several years, the disease may reach epidemic levels. Recent epidemics may be attributed to climate change, which has altered traditional rainfall patterns, resulting in increased summer precipitation in some areas.
Infection by Dothistroma typically begins in spring when the environment is cool and wet, although infection can take place anytime throughout the growing season. On average, symptoms become visible 3-6 months after infection. Symptoms typically begin in the lower canopy of trees less than 15 years old, although trees of any age may become infected. If conditions are conducive in the following years, the disease will begin to spread up the tree (Figure 1). Older needles, close to the trunk of the tree, are more severely affected than newer needles at branch tips. Needles formed after bud break in spring (current or first year needles) do not become susceptible to infection until they have matured completely in early to mid-summer.
Yellow to tan spots develop on diseased needles in late summer or early fall. Typically, clusters of needles on a single shoot will be uniformly affected. Throughout the autumn months, spots will turn a brown to reddish-brown color, gradually enlarge, and form bands around the needles (Figure 2). This characteristic symptom is where the disease gets its alternate name, “red band disease”. Once a band has girdled a needle, the needle will die above the band (Figure 3). Typically, the needle tip will be tan to brown, while the base of the needle appears healthy and green. The transition between healthy and dead needle tissue is often abrupt.
Black fruiting bodies, called stromata, form in the dead areas of the needle (Figure 4). In most locations throughout the U.S., stromata will mature and rupture the needle’s epidermis in early spring the year following infection. A hand lens can be used to observe the fruiting bodies, which will appear as tiny black specks on the diseased needles.
Dead needles will be cast prematurely, often during the spring and summer months, the year after infection. Needle cast can sometimes occur the same year of infection. When a significant percentage of foliage has been killed, the tree will experience an overall reduction of growth. Severe and repeated infections may result in tree death and/or make the host more susceptible to other diseases.
Symptoms of Dothistroma needle blight can easily be confused with those of brown spot disease or other fungal needle cast diseases, such as Lophodermium and Cyclaneusma needle casts. These diseases can be distinguished by microscopic examination of fruiting bodies and spores. If disease is suspected, samples can be sent to a diagnostic laboratory for confirmation. Information on The Ohio State University laboratory can be found on the C. Wayne Ellett Plant and Pest Diagnostic Clinic website.
From top left to bottom right: Figure 1. Dothistroma needle blightspreading up an Austrian pine (A. Steven Munson, USDA Forest Service, Bugwood.org); Figure 2. Transverse bands with dark border on Austrian pine needles (Andrej Kunca, National Forest Centre – Slovakia, Bugwood.org ); Figure 3. Once a band has girdled a needle, the needle dies above the band (USDA Forest Service, Northern and Intermountain Region , USDA Forest Service, Bugwood.org ); Figure 4. Close up of red bands and fruiting bodies that have ruptured the dead epidermal tissue of Austrian pine needles (Petr Kapitola, Central Institute for Supervising and Testing in Agriculture, Bugwood.org).
Dothistroma spp. overwinter in infected needles. The pathogen produces asexual spores, called conidia, once stromata mature. This occurs when environmental conditions are conducive, typically in early spring the year after infection, although stromata can mature within the same year of infection in some locations. Dead needles that have been prematurely cast are capable of producing conidia for up to 4 months. Spores from diseased and recently dead needles are dispersed by rain and wind throughout the growing season to infect healthy needles. Newly forming and elongating needles are resistant to infection until they have fully developed in early to mid-summer.
Once conidia land on healthy tissue, several consecutive days of cool, wet weather are needed for successful infection. Conidia are able to remain dormant for many months until their germination requirements are met. Once spores germinate, the fungus enters the needle through pore-like openings, called stomata. As the fungus moves through the needle tissue, it releases a toxin called dothistromin, which destroys cells and results in the production of symptoms in late summer to early fall. High rainfall and humidity are conducive to disease development and increase symptom severity.
Favorable Environmental Conditions
- High humidity and abundant moisture
- Optimal temperature range for infection and disease development is between 12-24°C (54-75°F)
Integrated pest management (IPM) is an approach to plant health care and disease and pest control. IPM incorporates a wide range of strategies to prevent, minimize, and/or control abiotic and biotic diseases and pests. These strategies involve monitoring and scouting, learning behavior and life cycle of pests and pathogens, accurately identifying the source of disease, developing threshold levels, employing preventive measures and integrating cultural, mechanical, biological and chemical controls. All management decisions should be carried out based upon the specific requirements of the plant. Analysis of each strategy involves considering the impact on host plants, pests/pathogens, microbial interactions, the environment and society. The costs and benefits of each strategy should be evaluated before making management decisions.
- Plant selection:
- Purchase pest and pathogen-free stock from reputable sources.
- When purchasing plants from a nursery, inspect for signs and symptoms of disease. Avoid purchasing plants that look defoliated, discolored or wilted.
- Consider planting resistant species/cultivars if disease pressure is high in the landscape.
- Yugoslavian seed sources of Austrian pine have proven to be highly resistant and are currently being used for production in the Great Plains region.
- Quarantine incoming plants and inspect for signs and symptoms of disease. Remove and dispose of, or return, all infected plants to the original source.
- Do not ship infected trees, unless they are being returned to the original source where they were purchased. Movement of diseased plants and plant products in commercial trade can result in long distance dispersal of plant pathogens.
- Proper site selection:
- Avoid planting trees in low lying areas with poor drainage and air circulation.
- Proper planting practices:
- Proper plant spacing will minimize moisture retention on the plant surface and will decrease the likelihood of disease development. When planting new trees in the landscape, space them to allow for optimum air circulation and sunlight penetration into the tree’s interior. Plan for the mature size of the tree(s) when spacing plants, or plan to remove trees as they begin to intermingle lower branches.
- Prevent and minimize stress to plant:
- Conduct regular soil and plant tissue analysis tests to determine if plants require supplemental nutrients. Otherwise, do not fertilize woody plants.
- Water tree(s) during periods of drought.
- Inspect trees, especially those less than 15 years old, for brown needles or premature defoliation of lower branches anytime throughout the year.
- In fall, look for yellow to tan spots that enlarge to form brown to reddish-brown spots and bands that girdle needles. Look for dead tips beyond the band, while the base of the needle remains green.
- Use a hand lens to observe small black fruiting bodies in dead spots and bands on needles in the spring.
Cultural control practices:
- Severely diseased trees may need to be removed from the landscape. .
- Avoid pruning, shearing, and/or other operations that would facilitate spore dispersal and infection during wet periods.
- To avoid transferring diseases, prune or shear healthy blocks in plantations or healthy trees in the landscape before pruning or shearing unhealthy blocks or trees.
- Sterilize tools after pruning or shearing trees known to be infected.
- For high value trees, dislodge and destroy fallen needles that collect in branch crotches and on the ground. These needles may harbor large amounts of pathogen inoculum.
- Keep weeds and other vegetation from encroaching around the base of the tree to encourage air circulation among and light penetration to the interior of the tree.
- Prune the lowest whorl or two or branches to encourage air circulation.
- Severely diseased trees may need to be removed from the landscape.
- Annual fungicide treatment for prevention and control of Dothistroma needle blight is recommended in Christmas tree plantations, where the market value of the crop is high and the risk of financial loss is significant. Annual preventive treatment is unnecessary in certain types of settings, such as residential neighborhoods or parks. Ornamental conifers in these settings are usually not treated unless the disease has been observed and is spreading throughout the tree and/or to other trees.
- Timely applications of copper based fungicides are effective for disease control. The treatment period will vary depending on geographic location, weather, and tree species.
- The first application should occur right before bud break to protect older needles, often in mid to late-May.
- The second application should occur once considerable growth of new foliage has taken place, often in mid-June to mid-July.
- Protective/preventive fungicide application may be required for multiple years to restore a severely diseased tree to an aesthetically acceptable form.
- Read each fungicide label thoroughly for safety precautions and information on how to prepare, apply, store, and dispose of the product.
- Check with your county or state extension specialist for an updated list of registered fungicides that are available.