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Back during the dry summer of 1999, Bob Pool and Alan Lakso wrote an excellent article on drought stress in vineyards. It appeared in the September issue of the Lake Erie Vineyard Notes and in the August issue of the Finger Lakes Newsletter (click here or go to http://www.cce.cornell.edu/programs/finger-lakes-grape/No8%20082499.pdf to see the article - this is in pdf format and will take a while to download so be patient). I would urge you to re-read the article because it is very applicable to the 2001 season and the measurements that we are taking in the field.
As promised, we are continuing to monitor vine water status and leaf photosynthesis throughout this unusually dry season. Concord vines grafted onto 3309 rootstock on the deep gravel at the Fredonia lab continue to have high leaf photosynthesis. However, there are some indications from pressure bomb measurements that they are getting closer to the mild stress point. In this case, rooting depth is helping the vines tap into a larger soil water reservoir. Own-rooted, irrigated, Concord vines are also maintaining high leaf photosynthesis. For own-rooted, non-irrigated vines, leaf photosynthesis depends on the soil water holding capacity and depth. On the deep gravel soils at the Fredonia lab, mid-day photosynthesis is down about 50% from maximum potential photosynthesis. Many of you may have seen pictures of the root excavations that we have done at the lab showing how many Concord roots are relatively shallow and spread throughout the vineyard floor while a smaller proportion of roots go deep into the soil. This year, large proportions of those spreading roots are in soil that is too dry to supply the vine with water and the vine is relying on the deep roots for water. Therefore, the water supply is decreased and the vines are responding accordingly. Wednesday, I stopped by the mechanical pruning experiment at the Betts' vineyard and took some photosynthesis measurements. On the deep heavy soil in that vineyard, own-rooted vines had very high photosynthesis and did not show any signs of water stress (good news for the vineyards on that soil type). Heavy soils have higher water holding capacity than light soils and it appears that the vines on the deep heavy soils in the lake plane are withstanding the drought. In contrast, calls are starting to come in describing symptoms of leaf yellowing, leaf burn, and basal leaf abscission. These are typically from young vineyards and/or mature vineyards on shallow soils. The shallow soils are close to running out of water and the young vines have shallow rooting depth. Well-watered vines do not wilt. Mild water stress causes leaves to relax in the afternoon but they recover at night. Vines with more severe water stress will wilt in the afternoon and do not recover over night. Some vineyards with shallow rooting are getting close to the permanent wilting point, will not recover at night, and may start to loose basal leaves.
We are very close to veraison and many people are asking about how the drought will affect the crop. Again the Pool-Lakso article is excellent. At this time, the crop and the wood is ripening at the same time but at different rates depending on the situation. If mild water stress continues, the crop should develop normally but it will come at the expense of the vine. The fact that there is a low crop does help the situation. Rick Dunst has observed early periderm formation in dry vineyards, which is a symptom of drought stress, but the periderm formation may be incomplete and vine capacity may be lowered.
The low crop in 2001, the potential high bud fruitfulness for 2002, and the potential low vine capacity at the end of 2001 may set us up for some interesting pruning decisions this winter. We are all going to want to leave up enough buds to get a maximum crop next year; however, we also have to be careful not to set up an overcrop with high fruitfulness and low vine capacity. We will talk more about pruning decisions after harvest when we have a clearer picture on the 2001 season.