Weekly Fruit Update - 9/10/2020

Author: Annie Klodd, Extension Educator - Fruit and Vegetable Production

In this week's update: 

1. Wait until dormancy to prune grapes, fruit trees, shrubs
2. Fall soil testing and applying amendments
3. Critical freeze temperatures for grapes
4. Splitting on apples and grapes due to recent rainfall

Note: Because harvest for apples and grapes are in full swing, you will notice that this week's update is heavy on grape and apple content. However, bullets 1 and 2 also apply to brambles and shrub fruits. 

Wait until dormancy to prune grapes, fruit trees, shrubs

It is still too early to do any pruning on grapevines, fruit trees, brambles, or fruiting shrubs (blueberries, aronia, etc.). While it can be tempting to get a head start on pruning before cold winter temperatures set in, it is critical to wait to prune anything at least until the plants go dormant (leaves turn brown and fall off). Better yet, wait until the winter or early spring to prune.

Here is why: From now until dormancy (late November and onward), the plants are performing an essential task that allows them to survive from year to year. They are moving sugars and carbohydrates from the leaves and canes down to the roots for winter storage. In the spring, the plants will use those stored sugars (carbohydrates) and nutrients to being growing again. If canes or shoots are removed now, you are preventing the plants from being able to develop winter storage of these resources. Without those stored carbohydrates, they have a very hard time growing again in the spring. As a result, the plants may be severely stunted next year. 

Please wait until dormancy, or until the winter, to prune fruit trees, shrubs, or vines.

For more information about how grapevines "go to sleep" for the winter:

Cornell University Grapes 101: Sources and Sinks: Allocation of Photosynthates during the Growing Season  

Cornell University Grapes 101: How Defoliation, Defruiting, and Extreme Shoot Reduction Affected Clusters, Fruit Composition, and Bud Hardiness 

Fall soil testing and applying nutrients

While September is a poor time to prune, it is an excellent time to take soil samples and applying fall fertilizers! Soil sampling now means that the results will come back in time to apply fall soil amendments if needed. 

Fruit growers should take a soil sample in each field every few years to help make sure that pH and soil nutrient levels are still in adequate ranges for each fruit crop. Soil samples are also important if the plants are exhibiting issues that have not been diagnosed, or that may be due to soil health factors. If a specific part of the field is experiencing problems, separate samples can be taken for the problem part and a healthy part of the field to help diagnose the issue. Information on taking soil samples in fruit crops can be found in this video.  

Soil samples can be submitted to the UMN Soil Testing Laboratory for analysis.

Phosphorus and potassium can be applied in the fall. However, fruit growers should avoid applying nitrogen in the fall, especially before dormancy. Fall nitrogen may cause the plants to put on new growth, which can delay hardening off for winter. Additionally, much of the fall-applied N may leach out before spring. 

This article discusses fall fertilization practices for vineyards. Many of the points made in the article can be adapted to other fruit crops as well.

Critical freeze temperatures for grape berries and leaves

Thank you to the Great Lakes Fruit Workers group, particularly Tim Martinson, Renee Moran, Imed Dami, and Bruce Bordelon, for contributing the information for this post. 

As more nights of cold temperatures roll in, grape growers should know what temperatures to expect freeze damage to occur. However, even if a freeze does occur, that does not necessarily mean grape growers need to rush out an harvest before the freeze. Grapes can still be harvested shortly after a freeze, but they do become more susceptible to diseases as the cell walls break down.

Early defoliation on a grapevine at the UMN Horticulture Research Center, due to an early fall freeze. Photo: Annie Klodd.

28 degrees F is the temperature at which grape fruit and foliage begin to die from cold. Some varieties can be damaged at 29 degrees F. The duration of the cold temperature matters; 3 hours below 29 degrees puts the vines at higher risk than a 1 hour freeze right before dawn. The site of the vineyard also affects risk of freeze injury. North facing slopes are less favorable than south facing slopes. Frost pockets that occur in valleys and low lying areas also increase the risk of frost damage.

Consequences of fall freeze are worse for the leaves than the berries. Freeze damage below 29 degrees accelerates leaf senescence and may cause leaves to drop before the canes have been able to properly harden off for winter. If berries freeze, they will stop ripening and start deteriorating, but this does not necessitate an emergency harvest. 

From Tim Martinson, Cornell University Senior Extension Associate in Viticulture: "Modest frosts may have minimal effects. Mature leaves are tougher than tender spring shoots. Mainly, it might hasten leaf senescence, and should motivate growers to harvest if the fruit composition is at or close to ballpark targets.  If leaves are frosted before harvest, this can reduce storage of starch and possibly nitrogen mobilized from leaves, with carryover effects to winter hardiness and early development next spring." (See #1 above)

Splitting on Grapes and Apples Due to Recent Rainfall

Rain-induced cracking on a Kerr crabapple in September, 2019. Photo: Annie Klodd.

As I have discussed individually with several growers, rains in September often cause splitting on nearly ripe grapes and apples. 

Apples/crabapples: Apples that have split due to rain are unmarketable for the fresh market and will deteriorate quickly. Crabapples are particularly susceptible to cracking. If significant cracking is observed in an apple variety following a rain, and if the fruit is ripe enough to be marketable, that variety should be harvested before further splitting occurs. If the variety is not ripe, it may be best to remove the split fruit and let the rest ripen before harvesting.

Grapes: Grape growers should go out to the vineyard in the days following a rain to scout for berry splitting. The difficult question is whether the grapes should be harvested ahead of schedule if splitting occurs. The answer is not perfect, and depends on multiple factors like current ripeness, winery standards, and the extent of the splitting. It is ultimately the decision of the grape grower, and not something that I can prescribe here in this article. 

If splitting is observed and the brix, pH, and TA indicate that the berries are ready, or close-to-ready for harvest, consider harvesting soon to reduce the continuation of splitting. If the berries are far from being ready for harvest, consider leaving them on the vines and implementing measures to address the split berries. Split berries are vulnerable to sour rot infection as fruit flies are attracted to the juicy berries. Oxidate is one of the products that can be sprayed to desiccate split berries. Insecticides can also be sprayed for fruit flies to reduce sour rot infection. While recently split berries can be included in the harvest, sour rot-infected split berries should not be included in the harvest bins because they contribute harmful flavors and aromas that make the juice un-usable. 

See this article for more information on mitigating sour rot.

For more information on the mechanisms that cause splitting, see this article.