Grapes are a warm weather plant and grow best at temperatures above 50F. Juice grapes such as Concord and Niagara were almost in bloom on May 21. Cool, cloudy weather with average temperatures near 50F followed and warm temperatures did not return until June 8. This cool weather virtually stopped the growth of the vines. On May 21, I thought the grapes were ready to bloom. I thought with the cool weather forecast for the week that they would bloom at the end of the week. They looked the same on May 28, a week later; I looked for and saw no bloom. This cool wet weather affected all the crops grown in our area. Corn was stunted in the fields and insect pests were virtually nonexistent as the weather was too cold for insects to be active. Apple growers were unable to apply apple-thinning sprays because it was too cold for these sprays to work.

There was a brief warm period with temperatures in the 60s on May 29-31 when grape bloom began. Bloom was delayed by about 10 days. The delay in bloom by cool conditions probably resulted in some losses but the cool conditions during bloom also resulted in poor pollination. Cool temperatures returned on June 1 with average temperatures at or below 50F. These temperatures again shut down grapevine growth.

The results on the flowers were even more devastating. At temperatures below 60F few flowers open. Some flowers open at the base but the cap does not fall off, so the flower cannot be pollinated. In order to set fruit the flower must be pollinated by several pollen grains. Each pollen grain germinates and forms a pollen tube. This pollen tube then grows down an ovary at the base of the flower and fertilizes one of the ovules inside. There are 2 ovaries in the flower and each ovary has 2 ovules so grapes can have 4 seeds. The fertilized ovule now continues growth as the seed in the fruit. Grape berries with 2 or more seeds are likely to say on the plant and grow. Berries with one or no seeds usually fall off. Grape pollen has an optimum temperature of 65F for grape pollen tube growth, below 60F pollen growth is reduced. The pollen grain has enough energy reserves to live about 24 hours after germination. It is doubtful that the pollen fertilized many berries pollinated before June 8 before it died. On June 8, warm weather returned and bloom was almost finished on June 11. Most of clusters had finished blooming, only a few clusters in the vineyards were still in bloom. I was surprised to see as many fruit as I saw. On June 18, it was obvious that we had lost a lot of berries. The unfertilized berries fell off the clusters as warm weather returned and growth of the fertilized fruit began.

Most of the berries fell off the vines in the week after bloom. I would estimate losses at 60% or more across the entire region. Some of the vineyards have lost almost all their berries, most show reduction to 10 berries per cluster (less than third of normal), and a few vineyards show little damage. Often these conditions can be seen in different areas of the same vineyard. An awful lot of this variation is probably due to the timing of the bloom and the local conditions in the vineyard where some vineyards had warmer temperatures in bloom than other vineyards or were delayed in bloom until the warmer weather came.

Grape growers are now faced with hard questions. Are my grapes worth spraying or can I reduce the amount of sprays to save money? Are they even worth harvesting? I would not recommend any more fertilizer. The vines will be too vegetative already with the light fruit set. Insects that feed on the leaves will not be as important as when we have a full crop, so we can tolerate more damage by Japanese beetle and grape leafhopper. Weed control should probably be reduced since we will want to use the competition from the weeds to slow down the growth of the vines. Grape berry moth control will probably be very important, as there will be fewer berries out there for all the female berry moths to lay their eggs on. So some sprays can be reduced or eliminated and others become more important.

Combing the grape vines will probably be very important this year.  Combing means pulling the shoots of the vine so that they grow perpendicular to the trellis rather than along the top of the trellis.  Combing will allow better sunlight penetration into the vines. One immediate result will be the increase in carbohydrates into the shoot nodes and buds close to the vines, at the base of the shoot.  These buds are the one that we will leave after pruning this winter and will bear next years crop.

Fruit set is extremely variable. On some plants there are only 2 or 3 berries per VINE, while another area in the same vineyard shows a good crop.

Good fruit set is visible in many vineyards