MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

In the last few weeks, we have experienced below normal amounts of precipitation and certainly below normal relative humidity. Combined with high winds, canopies of trees are beginning to express stress, especially as we approach harvest. This situation is becoming extremely critical, especially in Northern Michigan where rainfall and humidity has certainly been below normal.

Additionally, the majority of sites in Northern Michigan are on coarse, sandy soils which accentuate the problem. In many cases growers are either not providing supplemental moisture through irrigation or perhaps have an irrigation system installed, but not running it or they are running it at a level that does not adequately supply plant needs. Unfortunately this comes at a time when fruit trees and wine grapes are in their highest demand for water to satisfy fruit growth (cell expansion) and mobilize nutrients to the canopy through the water stream. The result can be reduced fruit size and a canopy vulnerable to cold temperature injury this coming winter. Some growers have tried to get by, using mulch to conserve soil moisture. While a study in the past on sour cherry (Kesner) demonstrated the benefits of mulch (alone) being equivalent, it still is inadequate in especially dry conditions and where fruit size is critical, sweet cherry, apple, peach, and plum. This year, many apple growers experienced a significant reduction in fruit set. There is plenty of research data that has demonstrated drought stress incurred during the growing season has deleterious effects on flower and fruit bud development and subsequent flowering a fruit set for the next growing season. Additionally, many apples are now growing in high density systems on dwarfing rootstocks. Trees closely planted must compete with one another for soil moisture and nutrients as compared to trees planted at a greater distance. The result is that closer spaced trees will be smaller. Research has shown also that dwarfing rootstocks such as M.9, B.9 and even M.26 have concentrated root systems, rather than extensive, which only explore a small area of the soil profile for soil moisture. Therefore, fruit grown in the contemporary high density systems today must be irrigated to ensure good productivity, return bloom, fruit set and large crops of large quality fruit.

The same argument can be made for sweet cherry on dwarfing rootstocks (Gisela). According to Dr. Greg Lang, fruit size is reduced on these dwarfing rootstocks and survival on non irrigated trees is also placed into question. More and more, fruit growers are opting to grow and produce fruit for fresh market to take advantage of premium prices for their fruit. The cooler climate of Michigan often is deceptive when it comes to plant water needs, and often growers under estimate the need and under-irrigate. Evapo-transpiration rates can still be as high in Michigan as even the arid west due to wind which can drive the plant system. If nothing else, irrigation is a must in young developing orchards, vineyards and blueberry plantings to insure rapid and healthy canopy development.

There is still some dogma out there regarding wine grapes, which supports the strategy that a stressed vine produces high quality fruit and subsequent quality wine. In Europe, irrigation is prohibited in most Appellation Control regions leading to the establishment of dry-land farming for wine grape production. Unfortunately, there is plenty of research to demonstrate that this strategy is deleterious under our conditions in North America. One problem that can occur is seeing reduced monoterpene chemicals, a key component, in the berries of varieties such as ‘Riesling’, ‘Gewurztraminer’ and ‘Muscats’ in drought stressed vines. Secondly, ‘Riesling’ grapes, a popular variety planted in Michigan, are vulnerable to producing wines with Atypical Aging (ATA) and illicit wines with a characteristic “diesel fuel smell” (inadequate nitrogen levels available during fermentation and aging).

We still have an opportunity with the remaining season to irrigate using portable systems up through mid September. Drought stress and its critical impact is accentuated for stone fruit and apple high density orchards. Apple trees on dwarfing stocks such as Mark, M.26 and M.9 have concentrated (non‑extensive) root systems that don't fully explore the soil profile for water. Secondly, we have found that these root systems are inefficient in absorbing soil moisture. Young fruit tree orchards and grape vineyards are especially vulnerable at this time due to the fact that their root systems are small and shallow where the soil profile dries out first. Unfortunately if stressed, they go into the fall at a disadvantage with not only a small canopy but with a canopy vulnerable to winter temperature injury. This is especially dangerous to peaches, sweet cherries, high density apples with tender varieties and V. Vinifera wine grapes which are especially vulnerable to winter injury.

Apple on semi-dwarfing stock  non-irrigated.

Young Vinifera grape
vine stressed on sandy
soil.

Dr. Perry's work is funded in part by MSU's AgBioResearch.