Cherry trees are very active in the spring. Shoot, root and fruit growth all take place at once. This uses up the stored reserves of the tree very quickly. Shoot growth ends at pit hardening and fruit growth ends at harvest. Root growth is very active in the spring and then again in the fall. The tree uses the time after harvest to accumulate reserves for next spring's growth spurt.

Cherry leaf spot causes premature leaf drop in tart cherries. This was a wet growing season and it is easy to see differences between orchards as some growers lost the battle to control this disease. Other growers saved money after harvest by reducing the number of sprays to control leaf spot disease and lost all or most of the orchards leaves before fall. What are the consequences of losing leaves early in the season?

In the 1970s research was done at MSU to determine if defoliation during the growing season had any influence on winter hardiness. It was found that trees that were defoliated in June and July were slow to begin acclimating to winter and began growth earlier in the spring. These trees also did not harden off to the same extent as trees that maintained their leaves longer into the fall. These are serious consequences for cherry growers. Most fruit losses in cherries are due to spring frosts that strike the trees as the buds begin growth in the spring. Cold temperatures that leave dormant trees unaffected will damage and kill the buds on trees that have already begun growth. The resultant crop loss reduces profits. Defoliated trees are also more susceptible to early winter freezes because they are slow to begin hardening off for winter. Most cherry tree losses are due to early winter cold snaps and freezes or absolute cold during the winter. Defoliated trees are very susceptible to both these events.

This fact was brought home to me forcefully my first year in extension when in the winter of 1993-94 we had temperatures in the -25 F range in SW Michigan. This good damaged peach and cherry trees. One grower had been unable to control leaf spot in his orchard and gambled that 1993 would not be a bad leaf spot year. He lost that gamble and most of his trees were defoliated by harvest. These orchards died. About 20% of the trees leafed out but most of the trees showed no sign of life. Several thousand acres of tart cheery trees were lost in the prime of the bearing years because the grower gambled on disease control. Other orchards bore a marketable crop that year and the main difference was the lack of winter hardiness due to defoliation of the trees. The effect lingers for several years as the trees try to catch up. Applications of Potash might induce increased winter hardiness but since the trees were defoliated before fall root growth began root growth and nutrient absorption will be reduced. Your best investment in cherries is to maintain a healthy leaf canopy as long as you can to improve winter hardiness.