Identify and Control Diplodia Shoot Blight, Collar Rot, and Canker of Conifers
The fungus Diplodia pinea is worldwide in distribution and importance. In North America, Diplodia causes shoot blight and stem canker of conifers in plantations, windbreaks, and ornamental plantings. Most conifers are susceptible to infection, especially exotic species such as Austrian pine. In the north-central United States, the most common hosts are red, jack, Scotch, and Austrian pine. Diplodia usually attacks trees stressed by one or more of the following factors: poor site, drought, hail damage, snow damage, mechanical wounds, and insect activity.
Diplodia kills trees by killing the new foliage year after year or by initiating girdling stem cankers on stressed trees. Nursery seedlings are usually killed in the first year of infection.
Diplodia-infected read pine windbreak
Infected 1-0 red pine seedlings Branch mortality on red pine
Diplodia fruiting bodies on needles Diplodia fruiting bodies on cone
Diplodia overwinters as mycelium or fruiting bodies (pycnidia) in conifer shoots, bark, cones, or litter. Spores (conidia) are disseminated during wet weather in spring through fall. Peak spore production and infection usually coincide with host bud break and shoot and needle elongation. The fungus invades and kills succulent shoot tissue, and forms pycnidia on dead tissues. Spores from current-year infections may be disseminated in the same or following year. Infection can also occur through wounds caused by various agents. The fungus invades woody tissue under such conditions.
Shoot blight and branch and stem cankers are the most common symptoms of Diplodia. Shoot blight is common on trees of all ages, but cankers are more prevalent in saplings and pole-sized trees. Collar rot is frequently observed in nursery seedlings. All symptoms are generally characterized by conspicuous amounts of resin.
Resinous canker (bark removed) on red pine
Look for: Brown, stunted, or curled current-year shoots in trees of all ages. Black fruiting bodies may be present in dead needles or shoot tissue. If shoots are solid, Diplodia may be the primary cause of mortality. If shoots are hollow, a shoot insect may be the primary cause of shoot death. Other shoot blight fungi, such as Sirococcus, may cause symptoms similar to Diplodia. You may need to have a pest specialist determine the causal agent.
Look for: Elongated, depressed areas on branches or stems, often with resin flow on outer bark. When bark is removed, olive-green streaking and brown pitch-soaked wood are visible. Older infections may have pronounced callus growth around canker edges. The top of the tree above girdling cankers is dead.
Look for: Dead or dying nursery seedlings with fully elongated needles. Symptoms first appear in mid-summer. Pycnidia are found at the root collar.
Shoot mortality on red pine
Diplodia left to right: Two weeks and 6 days old incubated at
20º C in the dark on potato dextrose agar
Any standard mycological media, such as malt or potato dextrose agar, may be used to isolate Diplodia. Incubate in dark or light at 20-25ºC for 2-4 days. The fungus grows rapidly.
Look for: Grey-green fluffy mycelium. Cultures are first white and then they darken with age. Diplodia does not sporulate readily in culture. To produce spores for positive identification, place sterile pine needles over actively growing cultures and incubate in light. Pycnidia and spores are usually produced in about 1 week.
Look for: Brown spores, sometimes with one septation, rough-walled and elliptical in shape. They range in size from 22.5-44.0 x 9.5-18.5 µ.
Tree top mortality on red pine caused by girdling Diplodia cankers
Prevention is the best way to control Diplodia infection. Avoid planting highly susceptible exotic conifer species where Diplodia has been a problem. Native species are more susceptible to infection when planted on poor sites or subjected to other stresses. Logging wounds should be avoided particularly during peak periods of spore dispersal. Trees planted on the best sites will be more vigorous and their wounds will heal more quickly, reducing their susceptibility to Diplodia infection. If possible, avoid planting or growing susceptible pine seedlings in beds or near windbreaks bearing infected cones or shoots. Do not ship infected nursery seedlings. To avoid transferring Diplodia spores to healthy trees, do not shear infected Christmas trees while wet.
In nurseries, remove infected seedlings to reduce chance of infecting adjacent seedlings.
Chemical control may be practical for nursery stock, Christmas trees, and ornamentals. Fungicides containing benomyl as the active ingredient or 4-4-50 Bordeaux mixture can be used to control Diplodia shoot blight. Control of shoot blight infections on 2-year-old and older trees is most important from the onset of bud break until the current year growth is completed. Rising 1-year-old stock should be protected through August or until the buds set. The chemical used will determine application rate and frequency. Follow label directions and make sure that the chemical is registered for the intended use.
Apply pesticides so that they do not endanger humans, livestock, crops, beneficial insects, fish, and wildlife. Do not apply pesticides when there is danger of drift, when honey bees or other pollinating insects are visiting plants, or in ways that may contaminate water or leave illegal residues.
Store pesticides in original containers under lock and keyout of the reach of children and animalsand away from food and feed. Follow the directions and heed all precautions on the labels.
Avoid prolonged inhalation of pesticide sprays or dusts; wear protective clothing and equipment if specified on the container.
Authors: Marguerita A. Palmer
Research Plant Pathologist
North Central Research Station
St. Paul, Minnesota
Thomas H. Nicholls
Principal Plant Pathologist
North Central Research Station
St. Paul, Minnesota
Copies available from: North Central Research Station
1992 Folwell Avenue
St. Paul, MN 55108
U.S. GOVERNMENT PRINTING OFFICE: 1985-567-508
Pesticides used improperly can be injurious to human beings, animals, and plants. Follow the directions and heed all precautions on labels. Store pesticides in original containers under lock and key out of the reach of children and animals and away from food and feed.
Apply pesticides so that they do not endanger humans, livestock, crops, beneficial insects, fish, and wildlife. Do not apply pesticides where there is danger of drift when honey bees or other pollinating insects are visiting plants, or in ways that may contaminate water or leave illegal residues.
Avoid prolonged inhalation of pesticide sprays or dusts, wear protective clothing and equipment, if specified on the label.
If your hands become contaminated with a pesticide, do not eat or drink until you have washed. In case a pesticide is swallowed or gets in the eyes, follow the first aid treatment given on the label, and get prompt medical attention. If a pesticide is spilled on your skin or clothing remove clothing immediately and wash skin thoroughly.
NOTE: Some States have restrictions on the use of certain pesticides. Check your State and local regulations. Also, because registrations of pesticides are under constant review by the U.S. Environmental Protection Agency, consult your local forest pathologist, county agriculture agent, or State extension specialist to be sure the intended use is still registered.