Tree Fruit Leader, Vol. 1(1) Feb. 1992

Apple Shading to Reduce Heat Damage

H. Arndt, P.Ag. Horticultural Specialist BCMAFF, Kelowna

Apple quality in the Okanagan has suffered from heat damage (sun scald) in certain years. Individual apples become blemished on the exposed side due to cell breakdown from extreme heat. This injury may occur any time during the months of July, August or September, when air temperatures reach 28^oC to 32^oC or higher. Damaged fruit is downgraded or culled, depending on the severity of the damage, resulting in major economic losses to the producer.

Most apple varieties are subject to some degree of sunburn during hot summer years. Jonagold, however, appears to be more sensitive to this condition than other varieties. This is especially so on younger trees. The damage usually occurs to exposed apples hanging on the outside of the canopy and on the southwest side of the fruit. It is usually the early to mid-afternoon sun that causes this type of damage. On Jonagold, losses in the Kelowna area have been as high as 50 percent of the total crop.

Various methods have been tried to reduce or prevent sunburn to apples but with limited success.

In 1991 a trial was carried out in the Kelowna area using 30 percent shade cloth. This was hung over a six-foot-high, four-year-old slender spindle planting on a post and wire structure. The variety was Jonagold. Tree rows were planted north and south.

Results showed that this technique reduced total sunburn by up to 15 percent over the non-covered treatment.

Sunburn occurred under the shaded area to fruit on the lower limbs which were exposed to direct sun. This probably occurred during the late afternoon where the shade cloth did not protect the fruit from the angle of the sun. Structural changes will be made to address this problem for the 1992 trial. Slight damage showed on some fruit on the top of the trees under the shaded area.

Damage occurred in the week of September 22 to 28 when temperatures shot up to 27 and 29^oC. No fruit burn had been observed earlier. In 1991 sunburn occurred more on the southeast to south side of the fruit as opposed to the southwest side in other years.

No noticeable differences in fruit colour were observed under the shaded plots. A side benefit of the shade cloth is that it can serve as a hail net. This became apparent on July 31 when a freak hail storm occurred. No hail damage was found under the shade cloth.

The cost of erecting the shade cloth for this experiment was approximately 22 cents per square foot. This included the shade cloth and structural materials. At this price, if one were to cover 2/3 of an acre (actual canopy of planted area) it would cost $6,325. The shade cloth is required to be in place for only three months of the year and thus could be reused for quite a number of years. It should not be left up over the winter, as it would suffer snow damage and weathering.

At this time the cost per acre seems too high to be economical for most growers. Until a cheaper source of shade cloth is found and a better structure is built to reduce sunburn damage greater than 15 percent, growers should view this practise with caution.

 

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