A challenging harvest in 2024

By Kathryn Carter and Wendy McFadden-Smith, OMAFA

The 2024 wine grape harvest is continuing, with harvest dates up to 10 days earlier than average this year because of the warm season, and the light crop for some varieties.  Unfortunately, many growers still have unsold grapes and must make decisions on what to do with fruit that can’t be sold.  

Unsold grapes

The Grape Growers of Ontario provide a platform to connect Ontario grape growers with potential buyers. For more information see Grapes (grapegrowersofontario.com).

Grape growerswith Production Insurance and/or AgriStability coverage who have questions about their coverage, should contact Agricorp to discuss their situation. 

Allowing unsold fruit to rot on the vines is wasteful, but without a market for grapes, it is not economical to harvest the grapes nor to drop the clusters. So, what is the impact of leaving fruit on the vine?

Disease pressure

The warm wet preharvest period has been ideal for late season downy mildew development.  In many vineyards this has manifested as browned leaves in the canopy above the top wire, leaving enough functioning green leaves to produce sugar. However, in vineyards where downy mildew was not well-managed, it resulted in significant defoliation.  If defoliation occurred before the stem of the cluster hardened off, fruit left on the vines could still act as a sink for carbohydrates that are critical to cold hardiness.  Together, these factors could have significant implications on wood and bud hardiness.  If significant defoliation occurred in your vineyard, go out and check to see if the part of the rachis attaching it to the shoot is brown.  If it isn’t, it might be worthwhile to drop the crop to prevent it from competing with trunks, canes and buds for sugars that are important for optimizing cold hardiness.

With respect to overwintering inoculum, the late season downy mildew flareup could affect disease pressure next spring.  However, this very much depends on the spring weather conditions in 2025 as the dormant structures of the downy mildew pathogen can remain dormant for several years.  The most effective strategy in 2025 will be to maintain excellent early protection from fungal diseases and canopy management to increase air flow and spray penetration.

While some crops may not be harvested and therefore fungicide applications stopped, this unprotected fruit can provide inoculum of Botrytis and sour rot, as well as Drosophila vectors for other vineyard blocks.  This applies to both fruit on the vine as well as that dropped to the ground.  Be vigilant in crops that are destined to be harvested and apply appropriate control measures if Botrytis or sour rot start to develop (see https://onfruit.ca/2024/09/03/pre-harvest-pest-problems-in-vineyards-2024/).

The amount of inoculum of downy mildew, powdery mildew or Botrytis on pruning debris is minimal.  However, in vineyards where black rot was a problem, to minimize possible inoculum carryover of the black rot fungus on infected fruit and canes, try to prune vineyards with severe black rot infection as early as possible.  The earlier the rotted berries are on the soil, the more chance soil microbes and worms, will decompose them (see https https://onfruit.ca/2024/07/26/why-is-black-rot-so-bad-in-some-ontario-vineyards-in-2024/).

Cold acclimation

How does delaying or not harvesting grapes impact the cold hardiness of the vine?

Fruit ripening or extended periods of ripening (ie., extra hang time) do not normally interfere with cold acclimation in grapevines provided that the vines are well balanced with fruit and foliage production (Keller et al, 2014). As a result, leaving grape clusters (healthy or rotting) on the vine will not have a negative impact on autumn cold acclimation, midwinter hardiness or spring deacclimation.

However, overcropped vines are more susceptible to cold injury, and may not have enough reserves to harden off buds and shoots in preparation for winter. As a result, dropping grapes on overcropped vines, may help to increase cold hardiness. 

Summary

Generally, extended periods of ripening do not impact the cold hardiness of the vine.  In situations where vineyards have been completely defoliated or vines have been overcropped, dropping the crop may help increase the cold hardiness of the vines.