Rhizosphaera Needlecast of Spruce: One Possible Explanation for Browning Spruce Needles

A common problem encountered on spruce trees is browning of the needles. There are a number of possible causes, but one that is frequently a problem in Manitoba is Rhizosphaera needlecast (Rhizosphaera kalkhoffii) Rhizoneedle.mp3

When the needles are examined closely tiny black fruiting bodies, (pycnidia) are arranged in lines along the length of the needle (See Figure 1). It is primarily a disease of blue spruces (Picea pungens), occasionally white spruce (Picea glauca) but has also been reported on Douglas Fir (Pseudotsuga menziesii). This disease can be a significant problem in Christmas tree plantations, where it can greatly reduce the attractiveness and the value of the tree, but can be a concern in urban landscapes as well.

With a disease cycle that lasts longer than one season, disease management is a challenge. This disease tends to be a problem in years with greater than normal spring rainfall. Infection takes place in the spring, and symptoms are not visible until a full year to 15 months later. Thus an outbreak of symptoms this year, is actually the result of suitable conditions in the spring of the previous year. Failure to manage this problem in year one (e.g. 2004), will result in needle yellowing and browning in the second year (eg. 2005). This disease is most severe on the lower branches and under high disease pressure, loss of needles on entire branches and multiple branches, is possible. Over consecutive seasons the disease can progress upward on the trees, after several successive seasons of leaf fall, the tree may die.

To reduce the risk of Rhizosphaera needlecast outbreaks, fungicides are often necessary to protect new uninfected growth. Two fungicides are registered for use in Canada, chlorothalonil and copper oxychloride (See Table 1). Generally, late May to early June (about 1-5 cm of new growth) is the time to begin applying these products, so if this problem has been observed in previous years, it may be worth considering chemical applications soon.

Table 1: List of Products registered in Canada for use against Rhizosphaera needlecast or needlecasts in general.

In conjunction with the use of fungicides, mowing of grass around the trees to keep humidity levels low, removal of all fallen needles away from the base of tree (to reduce levels of the pathogen), and avoiding pruning or shearing of trees during wet weather ( to avoid spreading spores of the fungus), will also reduce the impact of Rhizospheara needlecast. When pruning or shearing, at any time, tools should be sterilized between cuts by dipping in 70% alcohol for 3 minutes.

Rhizosphaera needlecast is one explanation for the browning of needles in spruce but there can be other factors to consider, be sure to have an accurate diagnosis prior to starting a control.

For further information on Rhizosphaera needlecast see the references below:
 

References:

Hansen, E.M and Lewis, K.J. 1997.  Compendium of Conifer Diseases.  St. Paul: APS Press.

Skilling, D.D. and Walla, J.A. 1986. Rhizosphaera Needle Cast of Spruce. in: Riffle, J.W. and Peterson, G.W.  Diseases of Trees in the Great Plains. Gen Tech Report. Fort Collins, Co: USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, pp. 124-125 .

For further information, contact your GO representative.