Stressed Out Trees

One of the most recurrent words that I have heard this summer is stress. This refers mostly to stressed out trees but does not diminish the stress factor on growers looking for more predictable outcomes after hundreds of thousands of dollars in expense. 

No doubt that this is a result of what I would call an asymmetrical weather pattern for the 2007 growing season. If I was asked what normal or seasonal is, I'm not sure how I would respond. 

What we seem to be getting on average is earlier seasons and more and more volume of heat during our traditional growing season. There are elements in this "warming scenario" that could be argued as favourable but overall the unfavourable elements of this type of growing season can create havoc in the ways that we manage our orchards. Our harvest dates for indicator varieties seem to be on average getting earlier. 

Rootstock selection can play a critical role under this aforementioned warming condition. What is not well understood by many is the degrees of tolerance that various rootstocks have for soil temperatures during the summer months. Way back in the 70's researchers Nelson and Tukey in the U.S.A. reported that the EM 9 was the most intolerant of high soil temperatures in the summer when compared to M26, EM 7 and MM 106. In fact the EM 9 performed best at 55 degrees F, poorly at 66 degrees F and poorest at 77 degrees F. This does help explain why we have better results from certain rootstocks during a hot summer like we are experiencing. Different strains of 9s no doubt vary in their tolerance to various soil temperatures. 

One of the more outstanding ways of combating heat stress in the orchard that we have found is with the use of particle film "Surround Crop Protectant". In some cases, I would have to say that it mimics the effects of irrigation in terms of tree performance. In controlled studies, it allowed trees to produce fruit in size classes that one would normally expect from an irrigated orchard. Fruit finish is an added bonus when using 'Surround® WP Crop Protectant'. Growers that are doing a lot of summer pruning to enhance fruit quality will also find that it largely prevents high cullage rates due to sun burning of shoulder areas of newly exposed maturing apples. 

Figure 1 - Aurora Golden Gala trees treated with a season-long 'Surround Crop Protectant' program are being compared to untreated controls. 

No doubt at some point there is not a replacement for water delivered to tree in the form of irrigation. Many growers are reporting that micro irrigation systems had trouble keeping up with tree demand this year. This is understandable since evapo-transpiration rates were enormous in general. These temperature extremes started during the fruit set period of crop development and contributed in my opinion to excessive June drop in some cases. Other physiological results from a high heat growing season can include bitter pit and water core. Stressed out trees do not absorb and utilize growth regulators the same way as trees that are not suffering from the effects of excessive heat or poor canopy development as in the case with ReTain. ReTain works best on trees that are considered to be functioning close to peak performance. 

There seems to be more interest in Ontario in various forms of mulching. Mulching can be a costly option but if one was to do a cost benefit analysis for a high value cultivar I think the results would be positive.

If you do plan on introducing a mulching system based on non-synthetic materials (straw, wood chips) you will likely have to adjust nitrogen requirements since the amount of carbon present in the mulch will demand free nitrogen to build up bacterial populations. Mulches of course do keep soils relatively cool compared to bare ground. Under a heavy layer of straw, bait stations for mice are recommended. The longevity of mulches relates to the source and thickness. While a layer of woodchips may take several years to break down, wheat straw may last only 2-3 years. Both types of mulch systems can require dressing up on an annual basis, as soils vary in their relative degree of biological activity.