By Kathryn Carter and Dr. Wendy McFadden-Smith, OMAFRA
With some growers experiencing cold injury and possibly frost injury in orchards this year, there are some practices than can be used to try to help cold-damaged trees. The impact of frost on the crop load will vary depending on stage of development when the frost even occurred, minimum temperature and crop.
Evaluate the extent of the damage
Some orchards have small amounts of cold injury and will still have enough fruit buds for a full crop this year. The number of fruit buds needed for a crop varies depending on the commodity: a full crop of cherries requires over 50% bud survival, while apples, pears, and peaches may only need 10-15% bud survival in order to have a crop.
Even though cold injury has occurred in some sites, there may be ample blossoms remaining for a full crop. In sites with significant cold injury, lack of crop may result in the need to amend production practices. Make sure to evaluate the extent of the damage in the orchards prior to making decisions about production practices. Here are some things to take into consideration in sites with high levels of cold injury and low yields.
Continue to prune cold-injured peach and nectarines from bud swell through bloom. Research from Michigan has shown that delaying pruning (June or July) of winter injured peach trees produced weaker, less desirable growth compared to pruning soon after budbreak in May (Great Lakes Fruit Grower, 1995). In addition, peach trees pruned in June, have been shown to have increased incidence of Leucostoma cankers (Miller and Byers, 1994). The more rapidly that a wound heals, the less risk there is for infection. Wound healing is temperature-dependent, so pruning should be delayed until forecasts call for warm, dry weather.
In sites with no crop, it might be beneficial to lower the height of peach trees, since lowering the fruiting zone will assist with harvest next year and may help minimize shading.
Use a balanced approach to pruning and avoid hard pruning and the use of big cuts on the lower part of the scaffolds. Avoid excessing pruning which could overstimulate trees resulting in increased growth, and shoots that are less cold tolerant. Large pruning cuts on winter-injured trees result in a tree that is more susceptible to gum production (Miller and Byers,2000).
A reduced crop will also result in reduced nutrient needs. Consider your nitrogen program carefully—trees with little or no crop will require less nitrogen so reduce fertilizer applications as necessary. Excess nitrogen can result in excess growth resulting in increased cold sensitivity and excess shading. However, some fertilizer use may be necessary to maintain tree health. Consider splitting applications to tailor the fertilizer program according to rainfall and crop needs. Monitor tree growth; if the average annual growth is less than 1.5 feet, the tree may be prone to canker and winter damage. If growth is greater than 2.5 feet, the tree may be prone to fall cold injury.
Minimize water stress during the growing season using irrigation if soils become dry. Since flower bud initiation for next years crop starts this year, it is important to minimize stress to promote cropping for next year.
Monitor trees for disease symptoms during the growing seasons. Cold/frost-damaged trees may be more vulnerable to cankers and some pests (borers). Peach, nectarine, apricot, plum and cherry are often susceptible to Leucostoma canker (Cytospora canker) after cold injury. Look for amber-coloured ooze associated with cankers and prune out as needed.