Wet Season Fruit Problems

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

It has been a wet week and excess moisture can cause problems in fruit crops. It has been so wet that during blackberry harvest we are seeing slugs on ripe fruit (we even saw some slugs on blueberries!). While slugs are not a normal problem in fruit, excess moisture is a problem in some years. Currently, brambles (raspberries and blackberries), blueberries, peaches, plums, and apricots are being harvested. Cherry season is past but we did have wet weather during cherry harvest.

Of the fruit crops, sweet cherries are the most sensitive to excess moisture. Cracking due to excess moisture is a major reason that we have problems growing sweet cherries on Delmarva. It is not the uptake of water through the plant root system that causes the cracking; it is the absorption of water through the cherry fruit cuticle that causes the fruit splitting. The theory is that as a cherry nears ripening with the accumulation of sugars, cherry fruit exposed to extended periods of wetness from rain, dew, or high humidity conditions will absorb water through the fruit skin and swell until the fruit cracks. Some cherry varieties are more susceptible to cracking than others.

Growers of sweet cherries in the east often lose large portions of their crop due to fruit cracking. In the past, the tools that growers have used are to physically remove water from cherry fruit surfaces using helicopters or blower sprayers. Use of Calcium Chloride sprays prior to rain events acts to reduce the osmotic potential of rain water. Chemical barriers have also been tried to prevent water movement into the fruit with varying success.

There has been great interest in the use of high tunnels with dwarf sweet cherries to control cracking by eliminating wetness on fruit surfaces with these plastic covers. There is a new planting of dwarf sweet cherries in a high tunnel at T.S. Smith and Sons in Bridgeville, DE as a part of a Specialty Crop Block Grant from the Delaware Department of Agriculture. We will be holding workshops on this production technique over the next 3 years.

Similar skin cracking can occur in nectarines, plums, and peaches. When the skin of these fruits stays wet for an extended period near ripening, the fruit can absorb so much moisture that it also cracks. This is a problem for growers that do “tree ripe” fruit. Earlier harvest and ripening off the tree can help control this problem.

Some blueberry varieties are also susceptible to fruit cracking at ripening. Research has shown that both fruit absorption of water and internal water accumulation from root uptake cause this cracking.