Powdery and downy mildew can destroy entire wine harvests. More than 150 years ago, breeders attempted to increase the plants’ natural resistance to mildew by crossing them with more mildew-resistant grape varieties as an alternative to external abatements using fungicides. The majority of breeders soon abandoned the effort; only a handful of breeders, including biologists from the State Viticulture Institute in Freiburg, still believed that this could be achieved. Today, the Department of Resistance and Clone Breeding at the State Institute of Viticulture and Enology (WBI) in Freiburg under the leadership of Dr. Volker Jörger is a European leader in terms of fungus-resistant vines. In addition, the department is seeking collaborators amongst molecular biologists and within industry.
The beginnings of globalisation were a disaster for wine connoisseurs. Powdery (Uncinula necator) and downy (Plasmopara viticola) mildew have parasitised American vine varieties for millions of years, and forcing them to develop resistance to mildew. European emigrants, who liked to grow grapes, established trade relations with their home countries at the beginning of the 19th century and exported American vines to Europe – along with the fungi as hidden cargo. The European vines, selected and bred over several thousands of years, were vulnerable to the American diseases since they had never before been in contact with the intruders. Thousands of hectares of vine areas were soon littered with dead vines. “Downy mildew needs no more than three years and powdery mildew less than five years to completely kill a vine plant that is susceptible to fungi,” said Dr. Volker Jörger, head of the Department of Resistance and Clone Breeding at the State Viticulture Institute (WBI) in Freiburg. “Measures had to be taken immediately, otherwise European vines would have ceased to exist.”
Powdery mildew tends to infect vines during dry periods. In order for the fungus to penetrate the green parts of vine plants, a period with relatively high temperatures and low air humidity is required. The fungus spores attach to the tissue from the outside and use specific organs to penetrate the cell walls. Powdery mildew can penetrate green plant parts at any spot, and it then goes on to develop strongly branched threads inside the plant. Mildew-affected grapes burst, infected leaves turn yellow and drop off. The plant starves. One of the external characteristics of mildew infestation is a grey spore film on either side of the leaves. Downy mildew generates different effects and infection usually occurs during wet periods. Its spores require an air humidity greater than 95 per cent or dripping water on the leaves and grapes. The former director of the WBI, Prof. Dr. Karl Müller, carried out a detailed investigation of the conditions that favour infection, incubation and re-break-out during many years of research. His findings were published in 1930 in “Müller’s Incubation Calendar”. The downy mildew spores, which resemble tadpoles, cannot penetrate the cell walls, and can therefore only enter the leaves through the stomata, which the plant needs for the exchange of gas, located on the lower side of the leaves. When the fungus has reached the interior of the leaves, it also develops a branched network of threads and the leaves eventually die.
"Between around 1830 and 1850, the first winegrowers started to use chemicals such as arsenic or nicotine as fungicides," said Jörger. "However, a number of other viticulture specialists and winegrowers had the idea of initiating defence mechanisms from inside the plant." The cells in the tissues of the more fungus-resistant American vines are able to harden their cell walls relatively quickly by embedding cellulose, hemicellulose or lignin. That is how they wall in the intruding fungal appendages and prevent the spread of the fungus. The fungus-sensitive vines of European origin try to use the same defence strategy but usually lose their battle against the attacking parasites. Early ideas involved the crossing of the gene pool of the resistant American vines with that of the European vines (Vitis vinifera). However, the problem was that the American vines did not have tens of thousands of years of cultural history in which their taste had been optimised by the specific selection by experts, as was the case for their noble European relatives. Although the winegrowers managed to come up with varieties that were relatively resistant to mildew, the wine had a mediocre taste only.
Despite the protests of many winegrowers and the putting in place of repressive laws, the former head of the vine breeding department of the Freiburg-based WBI, Dr. Johannes Zimmermann, continued his research into the breeding of French hybrids during his spare time. He crossed them with tasty European varieties. However, the offspring of a specific cross normally only included between one and three per cent of individuals that were resistant. Therefore, it is necessary to run the offspring through intensive tests. In greenhouses, the breeders expose the offspring of the crosses to fungi under optimal infection conditions. This is where another problem arises. The information on properties such as yield, sugar content, grape colour, taste or resistance to dry periods is stored at different locations on the chromosomes. Therefore, a cross might well be resistant to fungal attacks, but on the other hand may have lost other important properties. Assiduous, sustainable vine breeding therefore requires the researchers to plant the laboratory-grown vines on the field and monitor them over several vegetation periods. It takes about 20 years before the results of the quality of a new cross can be regarded as sufficiently secure.
As early as 1960, Zimmermann's work led to the Merzling grapevine variety, a quality vine variety that is resistant to fungi. In 1968, this work resulted in the Johanniter grapevine variety. In the meantime, the WBI has more than 14 fungus-resistant varieties used to produce wine and 5 fungus-resistant varieties used for consumption. The wines taste quite good. These grapevine varieties only need to be treated against fungal infestation during extremely favourable infection conditions, which not only saves time and money, but also protects the complicated ecological structures within and around a vineyard. "The advantage of these vines is that they have a true, genetically anchored resistance to fungi that has developed over millions of years," said Jörger. "This is different from a range of apple varieties that seem to be more resistant to disease as a result of breeding, but which lose this resistance after three to four vegetation periods."
The Freiburg team of breeders led by Jörger is continuing the work of Zimmermann and his successor Dr. Norbert Becker. But is it not possible to speed up the breeding process? Other breeding research groups are looking for genetic information that confers fungal resistance to grapevines and other culture plants and are working to locate it in the entire genome. Molecular biologists envisage that this will provide them with more targeted breeding steps and a quicker progress in the breeding of plant resistances in general. However, such investigations are very difficult and still in their infancy. It is estimated that 40 switches in the grapevine genome need to be switched on in order to make the plants sufficiently resistant to specific fungal infestations. It is difficult to predict the functions that are simultaneously altered when the plants are artificially manipulated. Jörger hopes to develop cooperations with industry or molecular biologists. However, the wine industry will also in the future have to focus on classical breeding research.
Dr. Volker JörgerStaatliches Weinbauinstitut Freiburg/State Institute of Viticulture and EnologyAbteilung Weinbau/Department of ViticultureMerzhauser Straße 119D - 79100 FreiburgTel.: +49 (0) 761 / 40165 - 60Fax: +49 (0) 761 / 40165 - 64Mobile: +49 (0) 174 32 79 413E-mail: volker.joerger(at)wbi.bwl.de