When the landscape is white, we are especially appreciative of our beautiful green pine trees. Remember, pines are the ones with the long needles. However, like every other tree, pines are susceptible to diseases. Dothistroma needle blight, or red band disease for those of us who don’t speak Latin, is common in Minnesota. It is relatively easy to diagnose as the needles turn to brown. As with many other tree diseases, it is caused by a fungus. It is primarily a problem on Austrian pine, ponderosa, red and mugo pines; Scots pine are relatively resistant. It isn’t difficult to recognize this disease. Usually the lower branches are either completely dead and brown or half brown at the tip and green at the base with a red band dividing the two colors. Usually the needles closest to the trunk are more severely affected than the younger needles at the ends of the branches. If you look closely, you should be able to see red spots and bands on the green sections of the needles. Fruiting structures, tiny black pimples, might be visible pushing through the surface of the needle in the red areas. These symptoms usually appear in September, however if a tree has been infected for several years, dead needles and red spots may be seen at any time of the year.
This fungus produces spores from May through October. They can start new infections whenever we have cool wet weather for several days in a row. The infection may remain symptomless until the fall. In September, infected needles develop reddish-brown spots that grow into the red band that completely girdles the pine needle. The needle beyond the red band dies and turns brown. Eventually the needle dies and falls off. Infected needles are most common in the lowest 6 feet of the tree as this is where humidity is the highest.
This is a slow-moving disease that takes over a full year to complete its life cycle and several years to build up into a serious problem. If you notice this on your tree, spray with a copper based fungicide just before the buds open in the spring, usually in mid May, than again once the needles have grown to their full length. Knock off and rake up infected needles. This helps to reduce the number of fungi that are capable of surviving from one growing season to another.
There are other things one can do that can protect your pines from needle blight. Because the fungus needs moisture on the needles to start a new infection, anything you can do to keep them dry will reduce the problems with the disease. Space your trees far enough apart so they have good air circulation. This may mean you will need to remove every other tree but isn’t that better than losing all of them? They shouldn’t be cuddled up to any building especially your house either as that prevents the air from circulating. Keep weeds from under or near the tree. You may need to cut a few of the lowest branches off so air can get under the tree. A bark mulch will keep the weeds down and help keep moisture on the roots.
If you have a low-lying area that is usually cool and moist, plant Scots pines instead of the more susceptible species.
Bev Johnson is a Master Gardener with the University of Minnesota Extension. Her column appears in the Weekend Edition.