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ISSUE 3   May 17, 2001

 

SPRUCE TREE DISEASE-RHIZOSPHAERA NEEDLECAST VS. CYTOSPORA CANKER

There are two commonly occurring diseases on spruce trees in ND, Rhizosphaera needlecast and Cytospora canker. Both are named after the fungus that causes the disease, and both result in needle browning that often begins in the lower part of the tree. It is important to distinguish between the two, however, since management of each disease is quite different.

Cytospora canker is, as the name states, a canker disease. These types of diseases are caused by establishment of infection in a branch of the tree. The canker will then cut off flow of water to any parts beyond the canker resulting in dead or brown needles. Cytospora canker symptoms will show on the ends or tips of branches and move back toward the inside of the tree, resulting in entire branches being killed. These cankers may also ooze large droplets or masses of gray-ish or silvery-white sap. Small black pycnidia, the fruiting bodies of the fungus, Cytospora kunzii, can be seen just under the bark in the cankered area with some magnification. Managing Cytospora canker requires judicious pruning out the dead branches to eliminate the cankered areas from the trees. There are no fungicides to treat this disease. For more information on how to prune, contact the lab or see your county extension agent

Rhizosphaera needlecast, on the other hand, is a disease of the foliage or the needles of spruce. Spores of the fungus, Rhizosphaera kalkhoffii, land on and infect needles of the tree. Like Cytospora, R. kalkhoffiii also produces fruiting bodies called pycnidia. Rhizosphaera pycnidia appear in the stomates or breathing pores of the needles. Unlike Cytospora canker, symptoms of Rhizosphaera needlecast appear first on the inside of the tree. Needles nearest the trunk, usually in the lower branches, will turn brown and fall to the ground. New growth at the ends of the branches remains green. If left untreated, the entire tree will eventually become infected and die. Pruning will not manage this disease since symptoms of the disease take a year or more to appear after infection. Both chlorothalonil and Bordeaux mix have been listed as appropriate fungicide treatments, however chlorothalonil has been better researched and is more widely used. Several Bravo formulations are available: Bravo Ultrex (at 5 lbs/100 gal), Bravo 500 (at 8 pints/100 gal), and Bravo Weatherstik (at 5.5 pints/100 gal). Ortho Multipurpose fungicide Daconil Weatherstik (at 1.5 tsp/gal) is one of the most commonly stocked products for the urban homeowner. timing of the fungicide application is critical for disease control. The first application should be made when new growth is about 1/2 to 1 inch, probably any time now in most parts of the state. A second application must be made in 10-14 days. The product needs to cover all symptomatic branches, those next to or near symptomatic branches, and 2-3 branches above symptomatic branches. Two consecutive years of judicious treatment will generally control the disease in the infected tree, however spores from this fungus may be wind blown from another tree, resulting in subsequent infections.

If there is any doubt about disease identification in the case of these two spruce diseases, a lab identification can be important. It is evident from the fungicide formulas listed above that each product formulation is different, and it is very important to read the label for the specific instructions.

Cheryl Biller
Plant Diagnostician
diaglab@ndsuext.nodak.edu


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