If we can better understand plant resistance to pathogens like fungi and oomycetes, we can successfully introduce genes for resistance into crops like grapevines, which will increase productivity and quality, and simultaneously decrease our dependence on chemicals for disease control.
Hunting for resistance to costly pathogens
With an estimated cost to the Australian industry of approximately A$140 million per annum, powdery and downy mildew are the most economically important diseases in viticulture, causing reduced yield and loss of berry and wine quality.
Powdery mildew is a fungal disease which affects a wide range of plants, including apples, peas and roses. In grapevines, the fungus Erisyphe necator causes reduced yield and loss of berry and wine quality.
Downy mildew is an oomycete pathogen and is a particular problem on grapes and cucurbit vegetables such as cucumber and pumpkin. The oomycete pathogen responsible for downy mildew on grapes is Plasmopora viticola and symptoms are normally observed on both leaves and berries. Downy mildew requires a rain event for infection. During dry conditions the control of downy mildew is not a problem. However, under favourable conditions, uncontrolled downy mildew infection can cause defoliation and complete crop loss.
Wine grapes meet mildew pathogen
The cultivated grapevine, Vitis vinifera, has no natural genetic resistance to powdery or downy mildew. This is due to the two organisms evolving on different continents. Vitis vinifera first evolved in Europe while the pathogens came from North America. For this reason, the only method growers have to control these pathogens is by using fungicides.
Grapevines are responsible for the lion's share of fungicide use in agriculture. In 2002, for example, grapevines accounted for approximately 6 per cent of the total crop area in Europe, however, of all fungicides used, around 70 per cent were applied to grapevines. Since then, Europe has been working towards reducing its fungicide use. The Australian industry is determined to do likewise, ensuring a more sustainable wine industry and continued exports to Europe.
Sniffing out resistance in wild relatives
As the pathogens evolved in North America it was likely that a wild American relative of the European grapevine would carry resistance to one or both pathogens.
Investigating this, CSIRO research scientists upon years of breeding research carried out by collaborators at the National Institute for Agronomic Research (INRA), in France. The INRA team had identified genetic resistance expressed in the wild American grape species Muscadinia rotundifolia.
Our team was then able to identify seven individual resistance gene candidates which were tested using gene technology to determine whether they conferred resistance to powdery or downy mildew. Each gene was introduced into several grapevine varieties including Shiraz, Tempranillo, Cariganan and Portan which were then tested for resistance.
The experiment revealed two resistance genes, one for powdery mildew and another conferring resistance to downy mildew. These two genes MrRUN1 (Resistance to Erysiphe necator) and MrRPV1 (Resistance to Plasmopora viticola) provide breeders with an alternative to fungicides in the constant battle against these deadly pathogens.
Breeding resistance into existing wine varieties
There are two methods in which these genes can be utilised. Firstly, genetically-enhanced disease-resistant versions of existing premium varieties like Shiraz could be produced through genetic transformation. While Australian and world acceptance of genetically modified grapevines is most likely to be some time away, this method would provide protection against these important pathogens without the negative impacts on wine quality associated with traditional breeding techniques.
The second option uses the knowledge of these genes to create DNA markers for application in marker-assisted breeding. Rather than directly manipulating the plant's DNA, marker-assisted breeding uses DNA markers to identify specific traits in the plant at the seedling stage. This dramatically reduces the time required to identify varieties which express the desired trait, in this case resistance, in a breeding program.
Marker-assisted breeding provides significant advantages to the industry in the development of new varieties. New varieties pose their own problem when it comes to the current market; however, the past ten years has seen a small shift in the consumer market indicating future interest in new varieties and styles.
Future research direction
While the discovery of these two resistance genes is a massive leap forward in protecting grapevines from mildew attack, there is a great need for continued research to identify new sources of genetic resistance in order to remain one step ahead of these costly pathogens.
To this end we're investigating resistance to powdery (E. necator) and downy (P. viticola) mildew expressed in other wild grapevines such as the Chinese Vitis species V. romanetii and V. amurensis. The old adage, two heads are better than one, applies to breeding durable disease resistance. Combining, or pyramiding, MrRUN1 and MrRPV1 with other resistance genes will overcome potential problems associated with the deployment of race-specific single gene resistance and maximises the durability of disease-resistance vines in the vineyard.
This research was carried out in collaboration with the National Institute for Agronomic Research (INRA) in France and is supported by the Australian Grape and Wine Research Development Corporation (Wine Australia).