Fall foliar nitrogen applications to cherries should be applied now

Sweet and tart cherry trees that have had cherry leaf spot infections or drought stress would benefit from fall foliar nitrogen applications.

Maintaining cherry tree health is important as we head into winter. In southwest Michigan, many tart cherry orchards had outbreaks of cherry leaf spot, and this disease has caused substantial defoliation throughout the season. Additionally, orchards without irrigation are under severe drought stress at this time, particularly in northwest Michigan, and drought stress results in very little photosynthesis or nutrient uptake. Therefore, as we move into the winter months, if trees still have the majority of their leaves, Michigan State University Extension is recommending fall foliar nitrogen applications in orchards that have had cherry leaf spot infections or drought stress. Fall foliar nitrogen applications have been shown to increase winter hardiness as well as improve tree growth and fruiting in apples and cherries the following season.

Nitrogen (N) and carbohydrates are stored in tree tissues in fall and are vital for fruit tree growth and development in spring. Fruit trees accumulate carbohydrate and N reserves prior to leaf drop, which are stored through the winter until they are remobilized to growing points (flower buds, new shoots and expanding spur leaves) the following spring. Reserves provide trees with the necessary energy for new growth when leaves are not yet present for photosynthesis and roots have not yet begun taking up adequate amounts of N from the soil.

In 2004, Marlene Ayala and Greg Lang investigated carbohydrate reserves in sweet cherry and found that stored carbohydrates are used for the development of fruiting and non-fruiting spur leaves during the first few weeks after bud break, whereas new shoot leaves develop using carbohydrates from the current season’s photosynthetic activities later in the spring and summer. Further work shows that spur leaf size and the total leaf area per spur increased with foliar urea applied the previous fall.

Spur leaves play an important role in sizing fruit, since larger leaf area close to the fruiting clusters equates to larger fruit. Spur leaves also play a role in development of Montmorency tart cherries. The Montmorency study also showed that tree winter hardiness actually improved with fall foliar nitrogen applications. Therefore, if trees are heading into winter under substantial stress, fall foliar applications are likely to improve winter hardiness as well as promote strong early season growth in 2013.

The recommended rate for foliar sprays is a total of 40 pounds of urea split into two applications; growers should apply this spray to the leaves (not ground applied) and be sure the product is formulated for foliar applications (i.e., a low biuret urea). In research trials at Clarksville, Mich., by Theoharis Ouzounis and Greg Lang in 2011, optimum application timing was early September and followed by a second application one to two weeks later. However, good results were also found at Clarksville, Mich., and at Traverse City, Mich., for split applications in early October; timing of applications will depend on the numbers of leaves remaining on the tree. For instance, if trees are quickly defoliating at this time, applications should be made as soon as possible. However, if leaves are still green and the majority of the canopy remains, applications can be made at the end of September or the first of October.

Whether growers apply the foliar applications sooner or later, the initial applications should be followed with a second spray within two weeks if adequate leaves are still on the trees. Trees with substantial leaf loss will not benefit from these applications because the leaves need to absorb the material and translocate it down to the storage tissues in the buds, bark and roots.

Dr. Lang’s work is funded in part by MSU’s AgBioResearch.

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