Brown spot needle blight
MN Dept. of Natural Resources Archive. Bugwood.org
Young pine infected with brown spot needle blight.
Brown spot needle blight causes needle spots, browning and early needle drop. Infected trees have slow growth and often bare lower branches. Disease can eventually kill young trees that have been infected year after year.
Pathogen and susceptible plants
Brown spot needle blight is caused by the fungus Mycosphaerella dearnessii (syn. Scirrhia acicola). This disease severely affects ponderosa (Pinus ponderosa) and Scots pine (P. sylvestris) in the Great Lakes regions. All pine species in Minnesota are susceptible to some degree, including jack pine (P. banksiana), mugo pine (P. mugo), Austrian pine (P. nigra), eastern white pine (P. strobus), and red pine (P. resinosa) but the disease is not commonly found on these species.
- Small yellow to brown, resin-soaked spots form on needles throughout the summer. Spots may occur on needles of all ages.
- Needle tip dies and turns brown, base of needle remains green with random yellow to brown spots in the center of the needle.
- Tiny black pimple-like fungal spore producing structures may be visible pushing through the surface of the needle within the spots and bands.
- Diseased needles turn reddish brown and fall off in autumn leaving bare areas on the limb.
- Infections are most common on lower branches and young trees.
- Disease is most severe during prolonged warm, wet summer weather.
- Lab analysis is often necessary to distinguish Brown spot from Dothistroma needle blight. Submit a sample to the Plant Disease Clinic for testing: http://pdc.umn.edu.
The brown spot needle blight fungus overwinters in diseased needles on the tree or needles that have fallen from the infected tree. In spring, spores are released and spread to new needles by way of rain splash, wind or on contaminated pruning tools. The majority of needle infections occur in spring and symptoms appear several months later. New, succulent growth is most susceptible and is the quickest to show symptoms but all ages of needles can become infected. The needles must be wet in order to be infected by these fungal spores. Once the disease is in an area, annual infections occur as long as weather conditions are favorable.
- Since the disease thrives under warm, wet conditions, do not overcrowd plants – use size at maturity as a spacing guide when planting.
- Remove bottom most branches from trunk to help increase circulation and 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.
- Sterilize pruning equipment between pruning cuts made during wet weather or leave pruning and shearing for dry months.
- If trees have a history of infection with brown spot needle blight and forecasts predict persistent warm wet weather, trees can be protected with an application of a fungicide with copper sulfate or chlorothalonil as the active ingredient when needles are half-grown and again in 3 to 4 weeks later.
Always completely read and follow all instructions on the fungicide label.