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Facts for Fancy Fruits 

The unusually mild winter has many people asking about what effects the mild temperatures are havingon fruit crops. It’s a bit complicated. Most temperate zone plants like apples, grapes, blueberries, etc. go dormant in the fall and will remain dormant even though we may have some mild temperatures during the winter months. They are adapted to this climate by having a “chilling requirement” necessary for them to break dormancy. They must receive a certain amount of “chilling” in order to complete the dormancy process and begin normal grow in the spring. “Chilling” occurs when temperatures are in the range of 34 to 54˚F, with an optimum of about 42˚F. Temperatures colder than 34˚F don’t count towards the chilling requirement and temperatures above 54˚F can actually cause a loss of accumulated hours. There are several models to monitor chilling hour accumulation and the Utah model, developed in peaches, is widely used. The amount of chilling necessary varies by species and varieties, but for most fruit crops is in the range of 500-1,000 hours. Grapes are on the low end of that scale, and vary by variety and species background. The same is true of peaches with some Florida varieties having a chilling requirement of as little as 150 hours while others like Redhaven and Contender can require around 1000 hours. Obviously, in Indiana we would want to plant cultivars with higher chilling requirements to help hold back early spring bud development. Only after the chilling requirement has been fulfilled can trees/vines respond to warm temperatures in the spring.

During normal winters, we accumulate some (300 or so) chilling hours in November, but few in December and January, as the average temperature gets too low to count towards satisfying the chilling requirement. Chilling hours accumulate again in February and March, with most crops achieving the necessary hours to fulfill the requirement during the last month of winter. After that point, plants will be ready to respond to warm conditions and begin growth. This year is unusual in that we received a significantnumber of chilling hours during December and January. In Tippecanoe County, for instance, from Nov.1 until last week we accumulated 1218 hours. During the same period in 2011 we accumulated only 603 hours. So at this number of chilling hours (1218) we have satisfied the chilling requirements for all fruit crops grown in the state. Since the chilling requirement has been met, the only thing preventing crops from budding out and growing is warm temperatures. Of course growers will be hoping for cool temperatures for another few weeks to prevent early budding out and higher risk of frost damage. With temperatures in the 70’s forecast next week, we could be in a vulnerable state. (Bordelon and Hirst)

In addition to being no longer dormant, plants have also likely experienced cold hardiness deacclimation in the past month. My colleagues in Ohio recently posted a nice article on this topic in the Ohio Grape-Wine Electronic Newsletter http://www.oardc.ohiostate.edu/grapeweb/images/OGEN_02142012.pdf

It means that your crops are no longer dormant, probably have reduced cold hardiness, and are ready to grow as soon as we get a few days of 50 degree weather. The risk of winter cold damage from temperatures near zero is diminished as we move through February, but the risk of spring freeze damage would be very high if plants begin growth before mid-March or early April. Recall that the frost free date (50% chance of 28˚F) is early May across central Indiana, we have a long way to go.

Growers can avoid frost damage by delaying pruning as long as possible. Unpruned plants are more cold hardy and their buds develop more slowly compared to pruned plants. Grape growers can also use a technique called “long or double pruning.” This method is especially good for varieties that tend to bud out early. The procedure utilizes the apical dominance of buds on a cane. The first buds to begin growing are those on the tip of a cane, while buds closer to the base begin growth ater. To perform long pruning, select canes to be used for fruiting spurs during the normal pruning practice and remove the rest of the wood. Instead of cutting the selected canes back to 3-4 node spurs, leave them long, with 10-15 more buds than desired. The extra buds will help delay the development of the desired basal buds, which helps avoid frost injury. After the date of the last probable spring freeze has passed, the canes are shortened to the desired length to properly adjust the bud number for the vine. Growth of the basal buds can be delayed as much as two weeks if weather conditions are favorable. While this procedure requires an extra trip through the vineyard, it can mean the difference between a full crop and little or no crop. (Bordelon)

Impact on tree fruits

 With mid 70 degree weather forecast this coming week, we could start to see some buds start to push on all treefruit crops. Hopefully they won’t push too far, too fast because the earlier they develop, the higher the risk of a frost during the early budding out and flowering stages. (Hirst) Early season disease control - start the season right. Everyone wants to begin the season by starting off on the right foot. So begin by reviewing your last season and by walking through your disease management history. Were there problems with scab management? Bitter or brown rot? Fire blight? Regardless of your history, pruning is always a great way to start, and is probably mostly done by those of you in southern Indiana. With pruning, start with apple and pear to remove fire blight, moving on to plum or other Prunus species, to remove black knot. Remember, you want to go in reverse flowering order—late flowering pome fruits (apples and pears) should be pruned first, followed by stone fruits (apricot, cherry, plum, peach)
• Remove infected branches from orchard and destroy.
• For the incredibly motivated: Remove nearby chokecherry, pin cherry, or wild plum that can serve as hosts for black knot. Removing cankers should be a primary goal for growers seeking to manage fire blight. Remove any observed mummies that may have been caused by bitter rot, white rot, black rot and brown rot (on stone fruit). This holds true for all fruit crops—stone fruit, pome fruit, cane fruit, berries and grapes. Ideally, this should be done after harvest, but if that didn’t happen, get them out now,
Source: Facts for Fancy Fruit – March 13, 2012 - This newsletter can be accessed free
at www.hort.purdue.edu/fff/.