Winter Injury In Okanagan Fruit Trees (2010)

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By William McPhee

"Trees which have been frost-bitten, when they are not completely destroyed,
soon shoot again, so that they immediately bear fruit."

Theophrastus (373-287BC)

Clearly there is a potential for trees to recover from winter damage. However, winter injury has been misdiagnosed and mismanaged for years in the Okanagan and resulted in significant losses.

The Okanagan, although relatively mild compared to some other parts of Canada, does experience Canadian winters and temperatures can drop significantly below zero. Although damage can be to roots, bark, buds etc., with today's technology recovery can be greatly assisted. Trees can actually withstand temperatures significantly lower than those experienced this year provided they "winter-off" prior to the arrival of the low temperatures

The extent of winter injury is greatly influenced by environmental conditions during the winter. Snow cover, temperature pattern, tree condition etc. all influence the extent of the damage that can occur.

Damage is more severe when there is no snow cover and when soils are dry. To help protect roots from freezing temperature make sure the orchard goes into the winter with soil water level up. Snow cover is not controllable but a good irrigation prior to water shut off is.

Winter root damage is easy to identify and different from pathogen damage. Freeze injury on roots results in complete rupturing of the cell integrity so that the root tissue disintegrates (Figure 1). Under dry conditions winter injured root tissues crumble like sawdust. When wetted, as when winter injured nursery stock is soaked prior to planting, the root tissues become slimy and slough off.

Trees with root damage from winter injury tend to push normally in the spring utilizing reserves within the tree. However, when there is a demand for water, usually coinciding with the first hot spell, these trees exhibit a general loss of vigour and show wilting

For maximum recovery during the season the winter injury should be diagnosed early. This is accomplished using a shovel and a little bit of no-how.

In spring new root development begins when soil temperature reach about 8 - 10oC but winter injury can and should be assessed as soon as the soil is thawed the roots can be observed and the level of response determined.

Table 1: Guide to managing trees that have suffered from winter damage.

Damage Level Within the Root System

Response

Feeder Roots These normally are regenerated each year and not associated with significant winter injury. There should be no growth retardation Standard spring phosphorus treatment *
Secondary & Tertiary Root Damage

This indicates more severe winter damage. The extent of the damage depends on the soil temperature, soil type etc. and is important to the normal regeneration of the flush of spring feeder roots that are associated with the tips of these roots. Damage at this level may delay recovery of the roots in the spring.

A standard spring phosphorus program followed by foliar sprays in the spring as necessary.
Primary Root Damage When winter injury is severe enough there can be significant damage to the main root system and even to the crown area. Damage at this level can result in tree death or significant decline during the spring and summer. This level of damage requires an agressive recovery program. This level of damage requires an aggressive program and may require several phosphorus treatments to the soil as well as an aggressive foliar feeding program.

*Always combine a phosphorous program with a follow-up inspection to determine if soil pathogens are also of concern.

Assessing Root Damage

In high density plantings start about 60 cm (2ft) from the trunk and dig soil away gradually while moving toward the trunk area. By digging down and in this way the roots are exposed allowing one to examine them closely. It also avoids damaging the root system as much as possible. The 3 root zones to assess, as per Table 1, are shown in Figure 2.

Moderate root damage in the Okanagan is the rule rather than the exception. However, good soil and root management can have a significant impact on tree health and orchard production under all conditions. It is appropriate for growers to assess their root systems and manage them just as they manage other orchard disorders.

Contact you horticulturist or BC ministry specialist for help in assessing root condition.

Figure 1: Shown is typical major root damage due to winter injury. Tissue completely disintegrates when scraped.

Figure 2 A, B and C illustrates the levels of root damage.

A (green arrow) indicated the primary roots which are directly linked to the shank (trunk).

B (white arrows) show the second level considered here as secondary and tertiary roots.

C (red arrow) shows the feeder root level. This is the most important segment of the root system during high water demand in hot weather.

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