Grape disorder shrivels profits
The cause of the disorder is still a mystery to researchers.
Berry shrivel, a problem in vineyards around the world, results in sour-tasting grapes that are unwanted by wineries.
Berry shrivel, a grape disorder causing problems in vineyards around the world, is easily confused with other shrivel maladies that can affect grape quality. Being able to identify the different types of shrivel can help growers know when to take action to improve overall grape quality.
Berry shrivel was one of four types of shrivel impacting grape quality found during the 2006 harvest in Washington vineyards by Washington State University scientists Drs. Markus Keller and Bhaskar Bondada. Sunburn, dehydration, and bunch-stem necrosis, also known as waterberry, were the other types they identified.
Each type of shrivel has a different cause and different impact on grape quality, said Keller, noting that a recent WSU survey of Washington growers showed confusion in the industry about shrivel disorders.
"Growers need to learn to distinguish between end-of-the-season dehydration, sunburn, bunch-stem necrosis, and true berry shrivel," said Keller. While management practices can take care of or lessen the problems from the other types of shrivel, the cause and cure for berry shrivel has eluded viticulture researchers so far.
"Berry shrivel is an increasing problem in Europe," he said, adding that researchers there are calling it the "disease of the twenty-first century." They attribute 10 percent of crop losses in certain regions to it in 2005. "In a report I read, the symptoms they are describing are exactly like ours."
In Australia, according to a search on the Internet, berry shrivel is a particular problem in the Shiraz (Syrah) variety and, depending on the location, can result in crop losses of up to 40 percent due to reduced crop quality.
True berry shrivel becomes apparent between veraison and harvest and is characterized by the sudden shriveling of berries accompanied by low sugar, low color and flavor, and high astringency. Berries in affected clusters do not ripen normally--the juice tastes sour and there may be other off flavors. In samples collected by WSU, sugar levels in the shriveled berries have been as low as 7° Brix, though they are usually around 11° or 12° Brix, Keller said, whereas healthy berries on the same cluster are around 25° Brix. The disorder seems to be prevalent in red varieties like Cabernet Sauvignon, but also in whites like Sauvignon Blanc. It can result in significant yield and quality losses and high costs from thinning out the shriveled berries. Symptoms are often found throughout the cluster or only on the cluster tip. Some growers remove affected fruit before harvest because it is unacceptable to wineries.
Bunch-stem necrosis has similar symptoms to berry shrivel, but the rachis is not green and healthy as with berry shrivel. Necrotic patches develop on the rachis or peduncle, he explained. Sunburn, which results in shriveled or raisin-like berries, can be minimized by canopy management and timing of leaf removal. Shrivel from dehydration--dimpled berries--does not negatively affect fruit quality. The concentration of sugars resulting from dehydration is often desired by winemakers.
There are remedies for sunburn, dehydration, and bunch-stem necrosis, Keller said. "But with true berry shrivel, all you can do is cut the problem berries out or hope that the occasional berry shrivel cluster doesn't wind up in the winery sample."
Using hand labor to thin or cut out the shriveled berries is expensive. But he noted that thinning can be even more expensive if crews don't know what they are looking at and confuse the disorder with other symptoms like dehydration. Keller's research crews taste the berries to help them tell the difference between shrivel types.
"Tasting in the vineyard is an important teaching tool," he added.
Looking for clues
Scientists worldwide are trying to solve the mysteries of berry shrivel. European researchers are taking a pathogen/virus approach in their search and have undertaken huge acreage surveys to look for patterns, while others are focusing on mesoclimate and vine physiology, Keller noted.
"We're all fishing in the dark," he said. "Researchers around the world have no clue. We think that it's something physiological, but we can't even exclude pathogens or viruses. We have to consider every possibility."
One of the difficulties of berry shrivel is its variability. Some vineyards show symptoms every year but not always. The disorder usually likes a particular block, but within the block, it moves around. "In one vineyard [where] we sampled and found shrivel, we had zero overlap of shrivel in the next year of sampling the same vineyard."
Keller and Bondada are collaborating on research to find the causes of shrivel. Bondada's anatomical research uses a chemical dye put in water to follow the plant's uptake of the chemical in its clusters. They have also collected cuttings and clusters from grapevines known to have shrivel and are propagating plants to help them learn if the disorder is genetic.
"Our goal is to be able to induce berry shrivel, so we can then expose the vine to different things like changing temperatures, and such, to develop potential remedies," Keller added.