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Cabernet Sauvignon with (left) and without nitrogen deficiency during bloom.

Cabernet Sauvignon with (left) and without nitrogen deficiency during bloom.

Markus Keller

To a casual observer, a cluster thick with berries might forecast a bumper crop. But with fruit set, what you see is not necessarily what you get, cautions ­Washington State University researcher Dr. Markus Keller. If you are looking for a predictor, count the flowers on your clusters.

Better yet, he joked, hire an intern to do it.

Growers can control the number of vines per acre at planting; they can control bud formation, as well as the number of shoots per bud, through pruning; and they can control the number of clusters per shoot, berries per cluster, and berry weight through ­irrigation and nutrition.

But they can’t control the number of flowers per cluster, and that one step reveals more about the future crop than anything else, Keller said. To better manage yield, he said, pay close attention to the vines in the weeks surrounding bloom.

Only 20 to 50 percent of flowers will produce berries, Keller said. That’s largely influenced by temperature and day length. Warm weather will accelerate fruit set, while cold weather will delay it. It can also give a glimpse of future berry size, information that can be valuable in preventing diseases. Large flowers mean larger berries, while smaller blooms indicate smaller, more tightly clustered berries with less air flow surrounding them, conditions favorable to fungal diseases, such as powdery mildew and bunch rots.

The number of flowers per cluster can help predict yield, but the vines’ development during bloom can give other clues, too. Keller described several key indicators of poor fruit set:

  • Flower size. Small flowers indicate poor fruit set, while large flowers usually mean more fruit set and larger berries. “It’s almost like they got a head start.”
  • Poor leaf area. In order to develop, blooms need a good canopy to supply the ­necessary nutrients.
  • Climatic stress. Low light, sudden light exposure, and temperatures below 55 degrees Fahrenheit and above 95 degrees can all cause poor fruit set.
  • Nutrient and water stress. Without good nutrition (nitrogen, phosphorus, and boron), and the moisture to carry them through the plant, fruit set will suffer.