By: Sherri Roberts AAg Regional Crops Specialist – Weyburn
Have you noticed your magnificent shelterbelt spruce trees looking poor?
Do they have bare branches and significant needle loss?
Are some of the needles showing a purplish tint to them?
These are all classic symptoms of the fungal disease rhizosphaera needlecast. While this fungal disease prefers spruce trees it will also infect pines, cedars and fir trees.
Examining the needles under a hand lens will reveal what looks like black specks of pepper on the needle surface that don’t brush off. These are pycnidia; the fungal fruiting bodies that have infected the stomatal openings on the needle surface. On spruce, the pycnidia are present on all sides of the needle, whereas on fir, they occur only on the underside of the needle.
The disease begins in the lower portion of the tree with infected needles being shed, causing branches to look sparse. The fungus overwinters in needles that have fallen to the ground or are still attached to branches. Spore dispersal from infected needles occurs during wet weather, spread by rain from the needles infected the previous season to newly emerging needles. As it progresses, the infection moves its way up the tree, eventually killing enough needles so that the tree dies. Optimal conditions for Rhizosphaera spp. are during times of excess moisture and humidity.
Cultural methods can be adopted to reduce the spread of rhizosphaera needlecast:
- Avoid pruning or shearing trees during wet weather and sterilize pruning tools frequently by dipping in 70 per cent alcohol for three minutes;
- Remove severely infected branches;
- Rake fallen needles from the base of trees, where practical;
- Promote good air circulation and encourage rapid drying of foliage by mowing the weeds and grass from around trees.
Chemical controls registered for rhizosphaera needlecast include fungicides containing chlorothalonil or copper sulfate. Apply as per label instructions beginning in the spring when new shoot growth is one to five centimetres in length, and at three- to four-week intervals until conditions no longer favour disease development.