This winter is going down as one of the snowiest on record and has far surpassed the record for consecutive days with snow cover on the ground. These accumulations of snow are good in moderating soil temperatures and protecting plants under the snow from exposure to extremely low air temperatures. However, the presence of snow cover that has accumulated and the spring thaw that will inevitably occur can present several problems to the various fruit crops grown in Iowa. These problems include diseases, wildlife, and the physical weight of the snow accumulation.
Over-wintering diseases. With the snow cover, diseases that over-winter on leaves, such as leaf spot, leaf scorch, and powdery mildew on strawberries, downy mildew on grapes, and apple scab will be more prevalent in the spring. This is because fewer leaves have blown away and temperatures under the snow are more favorable for their survival. Leaf scorch in strawberries can continue to develop beneath snow cover at temperatures between 37 and 25 degrees F. With greater carryover of inoculums, early season disease control will be important to keep diseases under control.
Soil borne root rots. With a prolonged snow melt, there is a good chance that soils will be saturated for extended periods of time and create conditions conducive for the development of soil borne root rots caused by Phytophora, Pythium, or Verticillium wilt, particularly when they have been planted on soils that are not well drained. Symptoms of these diseases will first appear as reddening of the young leaves followed by stunted grown and plant decline. Fungicide drenches are available for Phytophora, but not for Pythium or Verticillium wilt. Proper site selection with a well drained soil and planting resistant cultivars (when they exist) are the best solutions for controlling soil borne root rots.
Rabbits. With a limited food supply, rabbits will feed on raspberry gooseberry, currant, blueberry, Aronia berry and grape canes; fruit tree buds and shoots. Rabbits feed above the snow so the extent of damage will be determined by how soon they began feeding on the fruit crops and the amount of snow that has accumulated to allow them to feed further up on the plants. On shoots, rabbit feeding can be identified by clean cuts through the tissue and de-barking up to a foot or more above the snow. Rabbits can be controlled by eliminating rabbit habitat, improving the habitat for predators, exclusion by fencing (provided the snow does not drift over the fence), and hunting.
Voles (Mice). Meadow, prairie, and pine voles all feed under the snow cover. They form tunnel-like runs in the sod and on bare soil when snow cover is present. They will feed on fruit tree trunks, gooseberry, currant, blueberry, and Aronia berry canes; and occasionally on raspberry canes and grape trunks. Typically the feeding is within three inches of the soil surface, although I have seen where they have climbed up on a trunk guard and fed. On trunks, the feeding can be distinguished by very narrow tooth marks. If canes are girdled or chewed off, fruit production will be reduced, but the plants will recover. If trunks are girdled, the above ground portions will die. For grapes on their own root system, suckers originating below the injury can be trained up to re-develop the vine. For fruit trees, completely girdled trees must be either replaced because any suckers that develop will be from the rootstock, or attempts can be made to insert grafts over the injury to replace the lost bark (phloem tissue) and eventually the wood (xylem tissue). This can be done by bridge grafting or inarching (grafting in the top of a sucker above the injured portion or planting a rootstock next to the tree and grafting it in). Information of grafting can be found on-line at University of Minnesota, North Carolina State University and Cornell University.
Information on controlling voles is available in PM 1282 2010 Midwest Tree Fruit Spray Guide that is available as a printed copy through the ISU Extension On-line Store or it can be downloaded as a pdf file.
Deer. Deer cancause serious injury during the winter, particularly when their food sources are limited by snow cover. They will feed on gooseberry, currant, blueberry, and Aronia berry canes; and fruit trees, particularly on apples. On occasion they will feed on raspberries and grapes. Injury from deer can be distinguished by tattered cuts through canes and shoots. With heavy snow packs, deer are able to reach much further up into trees to feed. Exclusion, either by permanent fencing or electrified fencing, is the best solution for controlling deer. If electrified fencing is used, a ground wire must be run beside the charged wire during the winter months. Additional information on managing deer is available in PM 1302G Managing Iowa Wildlife: White-tailed Deer.
Breakage. With the heavy snow pack that has accumulated, especially in areas with high drifts, there is a risk of limb breakage in fruit trees; and pulling of canes from trellises, staples giving out, and wires breaking in grapes. As snow melts, it becomes harder and heavier near the surface. This puts greater strain on tree limbs covered by snow and pulls any trapped grape canes downward as it settles. Where limbs and canes are trapped in the snow, breaking the surface crust above the limbs and around the canes will help to lessen the load, but can be time consuming. Consider doing it on the deepest drifts.
Frost heaving. Frost heaving should be a minor concern for strawberries this spring. However, a special set of circumstances may cause frost heaving to occur. For it to occur, the snow has to have melted, the soil remains saturated, and temperatures drop low enough to re-freeze the top few inches of soil. Evidence that frost heaving has occurred would be strawberry crowns laying on the soil surface with many of their roots exposed. This is more likely to happen in areas of a field that are poorly mulched and lack good internal drainage. The best protection against frost heaving is planting strawberries on well drained sites.