Sep 20, 2012

Fall foliar nitrogen applications should be applied now in cherries

There have been many challenges in maintaining cherry tree health this season, even without a fruit crop. Sweet cherry orchards have endured severe bacterial canker infections and many tart cherry orchards had outbreaks of cherry leaf spot. Both of these diseases have caused substantial defoliation throughout the season. Additionally, orchards without irrigation are under drought stress at this time, resulting in very little photosynthesis or nutrient uptake. Therefore, as we head into winter, if trees still have the majority of their leaves, we are recommending fall foliar nitrogen applications in orchards that have had any leaf loss or drought stress this year. 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 lbs 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 Research Center by Theoharis Ouzounis and Greg Lang in 2011, optimum application timing was early September followed by a second application one to two weeks later. However, good results also were found at Clarksville Research Center and at Traverse City, Mich., for split applications in early October.

Since the growing season began very early in 2012, we recommend that growers make their first application as soon as possible and follow that 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. MSUE

By Nikki Rothwell and Greg Lang, Michigan State University Extension.