Tree Diseases: Diplodia Tip Blight of Pines
Diplodia tip blight, also known as Sphaeropsis tip blight, is a common fungal disease caused by two species of Diplodia: Diplodia pinea and Diplodia scrobiculata. Diplodia pinea is the more aggressive species, but both fungi are capable of causing disease. Diplodia tip blight is most common in pines that have two to three needles per bundle. Austrian pine is the most susceptible species. Other conifers, such as spruce, fir, larch and arbovitae may occasionally become infected. Pines that have five needles per bundle are highly resistant to the disease.
In the United States, Diplodia tip blight has been reported in planting sites across thirty Eastern and Central states, as well as sections of Hawaii and California. The fungus infects more than twenty pine species. It is commonly found on Austrian pine, but is also frequently reported on Scots, Jack, red, ponderosa, and Mugo pines. Infections occur on Monterey pine in California.
During summer and fall, tiny, black fungal fruiting bodies called pycnidia will appear at the base of needles under the fascicle sheaths. The pycnidia may also develop on the scales of second year seed cones, and on infected bark. Pycnidia can be found on the shorter needles of shoots infected the previous year, as well as on needles that have turned ashen-grey. If rainfall is abundant in late summer, high numbers of pycnidia may develop on emerging needles, and second-year cones.
Moist conditions are required for infection; infection levels tend to be low when there is very little rain. During rainy periods, pycnidia will expel large numbers of spores, which are dispersed amongst pines. Spores are released from March to October. They are transparent at first, but later turn a brown hue. When temperatures become consistently warm, spores begin to germinate, before growing and penetrating needles and shoots. Once the fungus penetrates the needles, healthy tissues are destroyed, resulting in distorted shoots and needles. New shoots of Austrian, red, ponderosa, Mugo, Scots, and Jack pine are most susceptible when buds begin to open. Second-year seed cones are typically infected in late May. This period of vulnerability persists until mid June.
Brown, stunted, or curled new shoots, with short, brown needles may signify the presence of Diplodia tip blight. Symptoms are characterized by resin droplets, which indicate that a new shoot has been infected. Entire new shoots are killed rapidly by the fungus. Needles on infected shoots often turn tan or brown while still encased in their fascicle sheaths. Infection varies among major branches. Damage is first evident in the lower crown, before progressing upwards. Diplodia tip blight can form perennial cankers on mature trees and saplings, inducing sudden branch death. After two or three successive years of infection, the tree’s crown may incur significant damage.
Infection is most common in mature pines. Damage from Diplodia tip blight is generally confined to new shoots; however, the fungus may infect older tissues through damage resulting from wind and hail, or perforations in the wood created by insects. Tissues wounded during pruning or shearing operations may also become infected. Stressed or injured trees remain vulnerable to infection for several days, especially during warm, dry periods. Symptoms on new shoots can be detected in late May. The extent of the infection can be effectively determined in late June or July.
Prevention & Management:
- Infection of new shoots can be reduced by applying a fungicide to pines. Perform the first application as buds begin to swell, followed by a second application two weeks later. Fungicides containing potassium bicarbonate or benomyl as the active ingredient, or the 4-4-50 Bordeaux mixture are most effective. Fungicides applied during late April and early May will not prevent infection of seed cones.
- Prune and dispose of blighted needles, twigs, and cones. Use rubbing alcohol, or a solution comprised of one part bleach and nine parts water to disinfect pruning tools before each cut. This will help prevent the fungus from being spread to other trees or branches.
- Forego pruning or shearing of pines during periods that are favorable for infection.
- When planting, avoid selecting vulnerable conifer species in areas where infection is common. Native species are more susceptible when planted on poor soil sites, or locations prone to environmental stressors.
- Ensure trees are sufficiently watered, especially during periods of extreme heat.
- Fertilizing trees in spring or summer will improve tree vigor; avoid using fertilizers containing high quantities of nitrogen.
- Maintain a layer of mulch around the base of the tree to improve soil quality, and conserve soil moisture.
- Avoid mechanical wounding of pines.
Photo courtesy of Jacinta Iluch Valero.