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In the Orchard


Minimizing the impact of summer stress involves several key management factors.


T


his past summer, through cherry harvest, peach picking and leading up to apple


harvest, we experienced a prolonged period of heat. Although we certainly have had extended hot seasons in the past, this one could be a harbinger for the future. So as we head into the late fall and


the preparation for next year what can be done to ensure that the trees are as prepared as possible for the next and the coming years? We have a number of


tools at our disposal to reduce the impact of summer heat. Certain sunscreen foliar spray materials, some calcium products and orchard cooling techniques are aimed at fruit protection. In the case of orchard cooling, it involves moderating leaf canopy temperatures to encourage leaf stomates to stay open so gaseous exchange continues and carbohydrates continue to be assimilated in order for fruit to keep sizing. They can also protect against direct sunlight that literally cooks the exposed fruit areas. In addition, the affected side of the fruit can reach maturity sooner and affect fruit quality and storagability. Generally, these measures are


aimed at immediate mitigation of sudden changes in weather, such as intense hot sunny periods. There are broader more long-term production practices that can affect the tree’s ability to withstand heat and water stress. More efficient irrigation systems


such as drip and mini-sprinklers reduce evaporative losses and maintain soil moisture status. With


22


By Peter Waterman Heat, tree performance and fruit quality


any irrigation system, mulching greatly reduces moisture loss by creating a loose barrier where humidity can be maintained more effectively close to the soil surface. On a trip to Israel a


number of years ago we saw a drip system that was installed just below the soil surface, with relatively loose soil acting as a mulch that was highly efficient. This was coupled with automatic soil moisture measuring systems that triggered water application when required. Mulching with various materials


such as bark mulch, grass mulch and plastic mulch, etc., (note that Dr. Eugene Hogue of PARC showed that soil microflora and fauna are negatively affected under plastic mulch) is very effective in reducing evaporative losses from the soil surface. Beyond this measure, pruning and


fruit thinning, soil (especially monitoring soil pH in the tree row) and general tree nutrition and water management techniques keep trees from entering water stress and subsequent impacts on fruit sizing and fruit quality. Management practices that encourage a high leaf to fruit ratio, heavy shoot growth and biennial bearing negatively impact fruit calcium levels fruit quality and fruit storagability. On the flip side of the coin, trees


with excessive fruit numbers, coupled with poor shoot and leaf growth, growing in a light or gravelly soil, are a perfect storm for heat and water stress resulting in weak small fruit


British Columbia FRUIT GROWER • Fall 2013


that will suffer sunburn. Fruit does not have to have obvious sunburn to be impacted to some degree. Determining fruit maturity can be very difficult due to variability from one part of a tree to another as well as tree to tree and area variability throughout the orchard. I mentioned mulching earlier; it


does have the effect of reducing variability throughout a block of fruit. Dual drip lines in very sandy areas help to guarantee a more uniform moisture supply for the root zone. It is interesting how quickly, in conditions like those experienced this summer, moisture levels can be reduced to levels that put trees in stress, even in heavier soils. If you have not been checking soil moisture levels this past season, plan to get into a regular program next year.


Tree condition tells the story of


water stress, but unfortunately, by the time you see visual effects, damage to crop and shoot growth will have occurred already. On that trip to Israel we saw systems that demonstrated how quickly various parts of a fruit tree show reduction in turgor pressure of plant cells. Researchers were able to monitor leaf thickness, shoot thickness and branch and trunk diameter differences. Within a few minutes leaf cells showed reduction in turgor pressure that was expressed as a reduction in leaf thickness. Within 2-2.5 hours instruments measuring trunk diameters showed measurable differences in response to water need. These instruments were part of a system that then triggered the irrigation system to start up. Thus they were able to demonstrate how quickly water stress can affect a tree. By not monitoring at all you can


easily put on too much water that can create too much shoot growth and affect fruit quality, or, conversely, put on too little or not respond quickly enough to water stress and put tree growth and fruit quality at risk. To stay in the business of producing good volumes of high- quality fruit you must manage heat stress. A lot of management factors need to come together in a effective program.


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