Sunday, April 19, 2009

Landscape and Nursery - Needlecast Diseases

The following is information on two common needlecast diseases of evergreens in Delaware that are now showing up.

Two major needlecast diseases have been observed this past week. Rhabdocline needlecast infects Douglas fir especially in Christmas tree plantations and nurseries where trees are planted close together. In spring needle spots are yellowish to reddish brown. Sometimes spots coalesce to discolor the entire needle. Discolored needles are cast in spring after spore discharge. Infections of newly emerging needles persist until the following spring when infected needles drop. It is usually not a serious disease in the landscape but Christmas tree growers and nurserymen should be checking trees for symptoms. If found, prepare to spray with chlorothalonil (Bravo, Daconil) when the first buds break and again 2-3 more times at 2-3 week intervals depending on the weather.

Rhizosphaeria needlecast, like Rhadbocline, infects spruce needles in spring and infected needles turn brown in fall to early spring. The first sign of infection occurs in late fall or in spring one year after infection. At that time, the fruiting bodies (pycnidia) of the fungus emerge from the stomata or "breathing" pores of infected needles--visible with a hand lens. Fruiting bodies resemble tiny black dots in neat, even rows. The second summer after infection, symptoms appear as yellow needles, which later turn purplishbrown and drop from the tree. A few of these infected needles may persist on the tree over the winter and drop off the following spring. Because of the long delay between infection in spring and needle drop the following summer, the ends of infected branches appear green and healthy. Branches appear to lose their needles from the trunk outward. Branches that repeatedly lose needles for three or four years may die. Norway spruce is more resistant than Colorado spruce, but both are affected. If control is needed, spray with chlorothalonil when the needles are half grown and repeat 2-3 times.

Information from Bob Mulrooney, Extension Plant Pathologist, UD.

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