Fungicide resistance in Australian vineyards is not at crisis levels but, as Barbara Hall puts it, there are ‘red warning flags all over the place’.
Mrs Hall, a pathologist and Senior Research Scientist with the South Australian Research and Development Institute (SARDI), is leading a collaborative national project designed to determine the incidence and severity of resistant fungal populations of the big three diseases (powdery mildew, downy mildew and botrytis), better understand how resistance develops, and work out the best response.
The researchers have largely completed the first bit and are making progress with the others but have quickly discovered just how complex a task it is, and why very few researchers have ventured into the area.
First things first. ‘What we can say at this point is that fungicide resistance is an issue, but growers are still getting control if they’ve got a good spray program’, Mrs Hall said. ‘But if we don’t start looking after the fungicides we’ve got, there could be serious issues.
‘By looking after fungicides I mean understanding them, knowing and following resistance management strategies, mixing and matching, and minimising their use.’
The trap with fungicides is that a grower can be technically doing the right thing in terms of how often they spray, but it might be the wrong thing for their vineyard at a given point in time.
The ultimate goal for Mrs Hall and her colleagues is to be able to understand each of the 15 or so individual fungus / fungicide combinations to the extent that they can develop a testing service so growers can know where they sit and how to respond. Or, in the first instance, discover whether it is a resistance issue, or just an inappropriate spray program and/or poor spray technique.
‘With any fungus / fungicide combination, if you can keep the resistance below that level you are going to keep achieving efficacy. Once resistance goes above that level you’re going to start seeing loss of field control.’
While this research work continues, the best approach is to stay vigilant and to understand the basics. The rule of thumb is that the more often you use an individual fungicide, the more likely it is to build resistance.
‘Look what happened with Cabrio,’ Mrs Hall said. ‘Everyone embraced it as the most fantastic fungicide because it controlled powdery and downy and had an effect on botrytis. The industry used it widely and resistance grew. Scientists found in America that resistance can start with just 20 applications, so if you spray 3 times a year that can happen in less than 8 years.’
Resistance usually – but not always – develops when mutant genes that already exist at a low level in a fungal population are able to survive and thrive after exposure to a fungicide. Whether or not that happens depends on how many mutant genes there are, how strong they are relative to the other genes, and whether the conditions are right.
What makes the science so complicated is that mutant genes can be present at even quite high levels without a vineyard actually having a resistance problem. There are relatively simple molecular tests to assess whether mutants are present (in Europe they are part of a compulsory annual test for all chemical companies) but they don’t tell you whether this is leading to failure in the field.
The path Mrs Hall’s team plans to take will use more complicated phenological and genetic testing in vineyards and in the laboratory. They started with the DMI and QoI fungicide groups, which the sector told them were the priorities, and will now start to look at the newer SDHI group.
The project, funded by Wine Australia, is a collaboration between SARDI – a division of Primary Industries and Regions SA (PIRSA), the Australian Wine Research Institute, Curtin University, the University of Adelaide, the National Wine and Grape Industry Centre and the WA Department of Agriculture and Food.