Researchers at the renowned Agricultural Research Service of the US department of agriculture are close to finding a solution to powdery mildew.

The fungal pest uncinula necator makes fruit unmarketable and appears as white powder first as spots on leaves before affecting the fruit itself.

But now a team lead by David Ramming at the ARS research station in California have found a way to exploit the natural resistance to the disease found in some wild grapes.

Although any new variety releases are still years away, the research against the table grape world’s most widespread and devastating disease is making keen progress.

The fight against powdery mildew costs producers dearly every year in terms of fungicide applications - sometimes up to eight treatments are required and the canny disease has been able to develop resistance to a number of the products developed to control it.

Work to develop varieties resistant to the mildew has been ongoing for decades, but so far, fruit quality on many of the types bred has been too poor to be of commercial value. The California scientists have been working with colleagues in New York state and found a number of species with natural resistance. Using this resistance, they have developed molecular markers to help them make subsequent selections without the need for years of nursery and field studies used in conventional cross-breeding.

“Our ultimate goal is to develop both table grape and raisin varieties that are resistant to powdery mildew,” said Dr Ramming, adding that it seems that the resistance trait they have discovered actually prevents the fungus from penetrating the vine itself. “This should be a very durable source of resistance,” he said.

Dr Ramming’s team already has a large number of selections planted in plots at the Parlier USDA station, and continues to work with additional resistant species on backcrossing with selections made earlier.

The team’s work so far shows a high proportion of the desired characteristics such as large berries, flavour, and seedlessness, along with powdery mildew resistance are likely to be able to be preserved in a number of the crosses.

Although Ramming admitted that so far, despite this increased fruit quality while keeping resistance, no individual cross yet has all the commercial table grape qualities necessary to compete with current industry standards.

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