January 28, 2010 at 8:54 a.m.
Rhizosphaera Needle Cast is the most common needle disease in Minnesota. Drought like we have experienced the last two summers, weakens the trees and invites disease. Poor planting practices like planting new Colorado spruce next to old and planting in an area where the air circulation is poor, are causes of needle cast. You can identify needle cast by seeing yellow needles by mid to late summer. Infected needles turn brown or purplish brown by late winter or early spring. Needles closest to the trunk are often discolored while the needles at the tips of the branches remain green. Diseased needles at the tips of the branches remain green. Diseased needles usually fall off in the summer, about a year after initial infection. Damage usually starts in the lower branches and move up the tree. Infected branches may begin to die after three to four years of severe infection. Chlorothalonil can be sprayed twice in the spring to protect new needles. The first spray should be applied when needles are half the length of the mature needles. A second spray should be applied three to four weeks later as is prescribed on the fungicide label. Read and follow all the instructions on the label when applying a fungicide. Before spraying fungicides, confirm that Rhizosphaera is the fungal pathogen causing damage by sending a lab sample to the U of M Plant Disease Diagnostic Clinic. Several other fungi results in symptoms very similar to Rhizosphaera.
Lirula Needle Blight can affect most spruce species. In late summer second year needles become discolored, yellow to brown. In the third year needles form a raised black line along the midrib of the infected needles, mostly on the lower surfaces, which are affected first. In some cases, horizontal black bands form across infected needles. Infected needles turn grayish brown after spores are released but remain attached to the tree for several years. Management includes avoid planting spruce where Lirula needle blight already occurs. Apply Chlorothalonil or a Bordeaux mixture when new needles are half the length of mature needles and a second time one month later. Since the disease cycle is three years long, applications must be applied for three consecutive years. As hard as it may be, remove heavily infected trees.
Spruce Needle Rust can affect all spruce except the Norway spruce, which is only occasionally infected. The needles at the tip of the current year needles turn yellow. Pale orange to white tube-like projections appear on infected needles in July or August and release powdery orange spores. Severely infected trees may have a tan to pinkish cast. Infected needles fall off in September and some species of rust can cause witches' brooms, which is a clump of small weak branches arising from one point on a large branch. In most cases spruce needle rust is a cosmetic problem and no management is needed. Reduce moisture on needles by redirecting lawn sprinklers away from the trees and spacing them to allow good air circulation around the trees. If witches' brooms are present they can be pruned out and destroyed.
By the end of January information about our Bonanza plant orders and our spring class schedule will be sent to those on our mailing list. Our Bonanza a will be Saturday, March 12, at Chisago Lake Lutheran Church in Center City. If you are not on our mailing list and want more information, please call our Extension Office at 213-8904.