Temperature and UV Effects on Apple Trees and Crop Maturity

Last winter's minimum temperatures have played a big role in hurting and indeed killing out blocks and individual trees through winter freeze injury. This is especially true for some of the more tender and less hardy apple cultivars grown in Ontario.

In contrast to the low temperatures this past winter, it's not difficult to convince anyone these days that this is one of the warmest summers on record. Compared to last year, one would think we're in a totally different climatic zone.

Apple trees and crop can suffer from extended periods of heat and high UV in the absence of liberal supplies of water in the soil environment. Rains in mid-late July in many of the districts have helped immensely in relieving stress symptoms in various blocks of trees. In this article I will attempt to summarize some of the temperature effects and possible outcomes.

The cost of maintaining cell function in apple at temperatures approaching 30°C can be very high in terms of tree respiration and net loss of carbohydrate in Ontario. This is one reason why a country like New Zealand can produce such high volumes of fruit on a per acre basis as compared to North Eastern North America.

The 2200 hours per year of sunshine in the Hawke's Bay area of New Zealand are typically coupled with relatively cool temperatures. Because of these moderate temperatures during the growing season, the trees do not have to work as hard in holding and sizing the crop. This is completely unlike what we have experienced in Ontario during the 2005 growing season. We have the sunshine but also extreme heat and UV to go with it.

The effect of high summer temperatures on certain cultivars is very obvious. Excessively high temperatures and solar radiation can cause sunburn. Cultivars like 'Crispin' or 'Mutsu' are especially vulnerable. This may be made even worse with summer pruning and opening up the canopy and removing foliage that would otherwise shade the fruit. The effects of this can be buffered with the use of irrigation water and more reliably products like Surround™ Crop Protectant containing kaolin. The Surround is also known to increase the rate of photosynthesis when compared to trees under heat stress.

The volume of heat we have had in the early part of this growing season will likely influence fruit maturity of cultivars like 'McIntosh'. Growing degree days @ base 10ºC for the first 30 days after petal fall and time of pollination is more influential on maturity than higher temperatures around the onset of maturity. Growers are encouraged to use the starch iodine test to follow maturity 2-3 weeks ahead of anticipated harvest dates for the 2005 growing season. Fruit maturity this year could be accelerated depending on cultivar, spray program and/or other factors influencing ethylene development in the canopy.

We have found through experience that aggressive use of calcium sprays for instance can accelerate maturity of cultivars like 'McIntosh' and 'Northern Spy'. The mechanism by which this occurs is not well understood but it appears to be related to the generation of canopy ethylene far above threshold values. Tests done in the late 90's in Ontario revealed that 'McIntosh' treated aggressively with various calcium salt formulations matured up to 10 days ahead of untreated checks. This represented a net loss in fruit volume, as apples did not have the opportunity to size. There was also the potential for premature abscission of fruit due to the presence of ethylene in the canopy several fold greater than what is considered to be a threshold value of 1 ppm.

Other treatments or conditions that can influence maturity of cultivars like 'McIntosh' include rootstock, varietal strain, presence or absence of infectious diseases and insects (mites), and the use of particle films such as Surround ® crop protectant.

How long into the growing season this heat will stay with us is anyone's guess. Temperatures during the 4-6 weeks immediately preceding harvest can influence the quality of the fruit at harvest and during the storage period.

There is plenty of evidence to relate the incidence of scald and watercore to temperatures being too high in the preharvest period. See the discussion on 'ReTain'^TM, in Publication 360, Fruit Production Recommendations. Please note that watercore (or "honeycore" as it is known in some cultivars) is a desirable characteristic and does not take a cultivar like 'Fuji' out of grade. 'Fuji' is a cultivar that was selected for its ability to produce honeycore. This is a trait that in some markets commands a premium price.

Cooler temperatures in the preharvest period result in less watercore (internal and visible watercore) and a reduced susceptibility to superficial scald. Researchers claim that what is needed is 100-150 hours of temperatures less than 10ºC in the month or so preceding harvest in order to reduce the chances of these disorders showing.

Colour development for some cultivars is related in part to night time temperatures, as all growers know. The pigments responsible for colour development in apples activate more efficiently with temperatures under 21ºC in the preharvest/colouring period, provided there is sufficient light exposure on cultivars like 'McIntosh', 'Fuji', 'Empire' and 'Gala'.

Related Links

• Caution using ReTain on Stressed Trees
• Growth Regulators for Apples, OMAFRA Publication 360