Here are are some strategies to avoid or minimize apple scalding.
unscald is one of several fruit physiological disorders and is described by Dr. Beth Mitchum of the UC Davis, California. What is interesting is that one of her references was to work done by Meheriuk and others at our own Ag Canada research station at Summerland.
In the Southern Okanagan and Similkameen Valleys there can be severe problems, with symptoms being white, tan or yellowed patches that can develop into blackened depressions on fruit on the southwest quadrant of the tree. Sunburn can be especially severe when extreme heat follows moderate or cool conditions with water stress. Moderate skin discolouration can become more severe after several weeks in storage. Injured cortex apple flesh is brown and firm but can become sunken and spongy in storage.
Sunscald can result when the surface of the fruit gets to be 18 degrees Celsius above air temperature and eight to nine degrees warmer than the shaded side. The biggest problem occurs when fruit is suddenly exposed to extreme solar radiation, such as a change from cool cloudy days to sudden high temperature. Shifting of branches with excessive fruit load will also expose surfaces that otherwise would not be exposed. Much of the affected fruit is not easily detected at harvest and won’t be obvious until later in storage. So just throwing down damaged fruit excludes the obvious fruit, but more subtle damage causes major internal quality problems. Flesh firmness and soluble solid concentrations increase but titratable acidity (TA) decreases as severity of sunburn increases. During cold storage the TA declines markedly in fruit with more severe sunburn. Therefore, the soluble solids and TA ratio increase dramatically, affecting taste. Anything growers can do to not only eliminate severe damage but also the virtually negligible damage will result in a higher quality product.
Well-constructed trees are the first priority; that is, trees with branches that hold their position firmly as fruit size and weight increase. Branch renewal and spur pruning encourage good bourse
By Peter Waterman When the heat is on...
shoot development, with healthy leaves that assist in shading fruit.
Although sunscald is primarily a problem in the south Okanagan – less obvious damage can happen anywhere in the Okanagan-Similkameen, likely creating more fruit quality problems than
most growers realize.
By the time one reads this article the ability to prune and affect the solid nature of branches will have passed. But several options remain. Branches that bend from excess fruit on branch tips and somewhat back from the branch ends can be removed fairly quickly with touch-up fruit removal.
Another method is to wait until very close to harvest and remove affected fruit before the picking crew does the first main pick. (I regard this as less than satisfactory if another practice could have avoided the damage in the first place).
As a grower of Gala, Fuji, Jonagold and Ambrosia I employed cooling with sprinkler misters. This requires a fairly sophisticated irrigation timing clock that will allow short multiple cycles in addition to regular irrigation schedules. Growers would either pick a start time before noon that coincided with a surge in air temperature above a critical temperature, in the high 20s (note, it is hard to cool fruit that is already too hot) and continue the cooling cycles until late afternoon. Other, even more sophisticated setups, start the cooling using a thermocouple embedded in the fruit. This practice with appropriate nozzles uses very little water and has very little effect on soil moisture. The
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water is quickly evaporated as it cools the orchard and tree environment. Other options include sun protectants to shield the fruit from UV and infrared light. The recommended materials are Raynox, Surround WP and Purshade-O. Applications of these materials are commenced shortly after all spraying efforts have been completed. Raynox timing is when fruit is golf ball size or 42 mm, whereas the other two start a little earlier. The decision to apply these materials must be made prior to hot weather. None of them will reverse damage. There are specific details regarding timings, rates and precautions regarding rates of water per hectare, timings with other spray materials etc. As I stated earlier, yellowing and brownish patches are easy to spot; more subtle damage that affects skin and cortex tissues will impact fruit maturity readings and ethylene production by the fruit, and, more important, fruit quality and taste aspects of the fruit, so it is wise to assess your site and situation carefully to avoid cullage and fruit shrink—especially in pre-grade, pre-size operations.
—Retired orchardist and
horticulturist Peter Waterman can be reached by writing email@example.com
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