Dothistroma needle blight
A.S. Munson USDA Forest Service, Bugwood.org
Young tree infected with Dothistroma needle blight has browning of older needles as well as bare branches from needle drop.
Dothistroma needle blight is a fungal disease that turns needles brown and results in early needle drop. Needle loss slows tree growth and severe infection several years in a row can result in tree death.
Pathogen and susceptible plants
Dothistroma needle blight is caused by the fungus Dothistroma septosporum (syn. Mycosphaerella pini). In Minnesota, plants most severely damaged by this disease include Austrian pine (P. nigra) and ponderosa pine (P. ponderosa) and to a lesser extent, mugo pine (P. mugo). Red pine (P. resinosa) and Scots pine (P. sylvestris) are generally resistant to this disease.
M. Grabowski, UMN Extension
Needle spots, bands and dead needle tips from Dothistroma Needle blight.
- Reddish brown spots scattered on green needles. Spots grow into a band encircling the needle.
- Tip of the needle is brown and the base of the needle is green. These two sections are separated by a reddish brown band. The transition from green to dead areas is abrupt.
- Eventually needles turn completely brown and fall off.
- Older needles found close to the trunk are more severely affected compared to the younger needles that are found towards the ends of the branches.
- Tiny black pimple-like fungal spore producing structures may be visible pushing through the surface of the needle within the spots and bands.
- Needles on lower six feet of a tree are most noticeably affected whereas needles about 20 feet high are rarely affected.
- New infections often appear in late summer to fall whereas dead needles and spots may be seen at any time during the year on a plant with reoccurring disease.
- Lab analysis is often necessary to distinguish Dothistroma needle blight from Brown spot. Submit a sample to the Plant Disease Clinic for testing.
M. Grabowski, UMN Extension
Needle spots on current year needles and needle tip death on previous year's needles.
Dothistroma needle blight is a slow-moving disease that takes over a full year to complete its life cycle and several years to develop into a serious problem within the tree canopy. Spore producing structures known as stromata are produced within infected needles on trees. Spores are produced throughout the growing season whenever cool, wet weather occurs. These spores are windblown or rain-splashed to mature needles. Second year or older needles are susceptible to infection anytime during the growing season but new needles are resistant until they reach maturity in early to mid summer. At this point they become susceptible to infection. Several consecutive days of cool (41-77°F), wet weather are needed for successful establishment of new infections.
In Minnesota, symptoms do not typically appear until the fall when needles develop reddish brown spots. These spots expand into a reddish brown band that entirely girdles the pine needle. The needle beyond the band then dies and turns brown leaving the bottom portion of the needle green. Infected needles may remain attached to the tree for one or two years depending on the age of the needle at the time of the infection. Eventually infected needles turn completely brown and fall off prematurely. Tiny black fungal fruiting bodies appear in the bands or in dead areas of the needles. These fruiting bodies will release spores the following year to begin a new disease cycle.
- Do not overcrowd plants – use size at maturity as a spacing guide when planting.
- Remove bottom most branches from trunk to help increase circulation around the tree canopy.
- Control weeds under the trees with wood mulch.
- Maintain a 3-4 inch deep layer of mulch around your tree. Do not mound the mulch around the trunk of the tree but lay a flat layer with at least a 2 inch space between the mulch and stem to allow for air movement. Annually reapply mulch and inspect to ensure levels are maintained.
- If the tree is in a landscape with a sprinkler irrigation system make sure that water is not spraying the needles.
- In low lying or other areas with cool, moist air, plant resistant Scots pine or Red pine trees instead of more susceptible pine species.
- If the disease does occur, a copper fungicide can be applied once just before buds open in the spring (typically in mid-May) to protect needles from previous years and once after new needles have grown to their full length (in early to mid summer).