Rhizosphaera Needlecast                                         Bulletin #5104

Rhizosphaera kalkhoffii is a fungus that can cause extensive defoliation of spruce and fir, especially where these trees are grown out of their natural range. Colorado blue spruce is especially susceptible to infection, but most spruces and true firs can sustain some damage. Damage in Christmas tree plantations can be extensive. Losses are due mainly to lower tree market values due to needle casting, but tree vigor is also reduced and successive years of defoliation can lead to death of the tree.

Rhizosphaera spores require free water to germinate. Water must be present for two days for the spores to germinate and infect the needles at the optimum temperature of about 77oF. Such conditions occur during extended rainy weather and also under poor drying conditions. Shaded trees, trees with dense foliage, and trees in areas of poor air circulation are the most susceptible. Tender new needles and older needles of trees stressed by various environmental and cultural factors are vulnerable to attack.

Needles infected in the spring will be without symptoms until late in the summer when they may start to turn brown or purple. Browning continues through the winter, but needles are not generally cast until a year or more after infection. This leads to an early season pattern of apparently healthy current-year needles at the branch tips followed by browning of the previous season’s growth. In general, the disease tends to begin in the lower portion of the tree and work its way up. In the spring, needles that were infected the previous year will show rows of tiny black dots on the underside. These are the spore-bearing structures (pycnidia) emerging from the stomata and may be capped with a whitish waxy plug.

The fungus overwinters in needles on the tree and in needles which have fallen. In the spring, spores are splashed or otherwise spread from these needles to the new susceptible growth where infection occurs. In plantations, the spores may also be spread mechanically by workers as they walk down the rows or by pruning shears and other equipment.

1) Keep trees in a healthy vigorous condition. 2) Do not prune during wet weather. 3) Prune healthy trees first. 4) Sterilize shears to prevent disease spread. 5) Remove unhealthy unwanted trees or stumps that may be a reservoir for the disease. 6) Apply a preventative fungicide to the emerging shoots when they are half grown and again when growth is complete. Appropriate fungicides include Bordeaux mix, chlorothalonil (Bravo, Daconil), and manganese/zinc (Cleary's Protect T/O).

When Using Pesticides  ALWAYS FOLLOW  LABEL DIRECTIONS! Bruce A. Watt, Extension Plant Pathologist 2004 Where trade names are used, no discrimination  is intended and no endorsement by Cooperative  Extension is implied.

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