Grains Research and Development

Date: 01.04.2002

Crop protection: balance variety selection and fungicide use by Alex Nicol

Spray or new varieties, or a judicious mix of both?

Crop protection, and how best to accomplish it, was a major theme and the subject of some stimulating debate at the GRDC-supported southern region Grains Research Adviser Updates in recent months. Also prominent were presentations on frost and how to weigh the risk, and precision agriculture - a technology where promise is now becoming firm reality. Also, in this special Ground Cover Update report, you'll find some of the latest research on herbicide resistance.

YOU DON'T use fungicides to control disease; you use fungicides to increase profits. That's the message from Anton Nicholls, keynote speaker at a series of Grains Research Adviser Updates in the southern grains region. Drawing on his New Zealand experience, Mr Nicholls told delegates that yield potential, fungicide rate response and fungicide price were the factors that determined whether the use of fungicides would generate more profit.

In New Zealand, he said, "fungicide protection is profitable in most seasons on most cereal cultivars, and environmental conditions in some parts of Australia along with the incidence of disease suggested that the same could be true here". But Mr Nicholls warned that knowing what you were trying to protect and understanding how the chemicals work were vital.

In the same forum Hugh Wallwork, from the SA Research and Development Institute, insisted that the use of resistant varieties had to remain the ,first line of defence against disease. He warned against adopting the English approach where some very susceptible, high-yielding varieties were released on the assumption that fungicides would control the disease. He also had some words of warning about the role 'sucker' varieties play in breaching disease defences.

"These very susceptible varieties," he said, "produce many more spores, have a faster life cycle and are less fussy about the environmental conditions they require for infection. There's a real danger that this build-up in activity leads to changes in the pathogen population and the collapse of resistant varieties.

"My concern is that, in the current climate of competitive and commercial plant breeding, the temptation will be for breeding companies to release high-yielding varieties, at the expense of disease resistance. This will lead to ever-greater dependence on fungicides in the future.

"Growers have made a considerable investment in developing resistant varieties and we should not be putting that investment at risk," Dr Wallwork said. "We've experienced the recent loss of resistance of the Lr24 rust gene and that's a significant problem. For some time now we've been mining resistant genes from wild grasses, as traditional bread wheat resistance sources have been depleted. That's not an infinite supply and, while we may eventually be able to look to gene technology to manipulate new resistance, I wouldn't want to depend on it.

"That's not to say for a minute that I'm on an anti-fungicide crusade. Far from it," he added. "I think that fungicides should be used strategically, particularly in areas such as barley scald where there are rapid changes in the causal fungus and we don't have good resistant varieties.

Fungicides are an essential tool in managing the leaf rust risk. They also have an insurance role to play where growers are protecting a potentially very high yield."

Planning - key to a fungicide program

Mr Nicholls told his audience that understanding what produces the goods is vital in planning a fungicide program. "When you're talking about winter cereals, it's the leaves produced during late spring that fill the bin," he said. "In wheat, the flag leaf and number two leaf contribute 65-70 per cent of the yield. Leaves three and four, 10--15 per cent. In barley, leaves two, three, four and the emerging awns are the important structures to protect."

He emphasised that probably the most important aspect of planning a fungicide defence was an understanding of how the disease worked. He talked about the "latent" period of a disease, the period of time between the infection taking place and the symptoms becoming apparent. "Different diseases have different latent periods and these are affected to some extent by temperature. Systemic fungicides, those which are absorbed into and move through the plant, have the ability to control an infection if applied a number of days after the infection has occurred."

Mr Nicholls cited the example of the ttiazole group, which could be effective if applied up to 10--14 days after infection. Newer chemicals pushed this protection period out by another couple of days. But "the warmer conditions are, the shorter the fungus life cycle and the narrower the protection window He wamed, "most systemic fungicides move through the plant's xylem. That means if you don 't cover the whole of the leaf, you' ll only get protection from the point of application to the point of the leaf'.

Timing, he suggested, was even more important if you were depending on 'contact or protectant fungicides. These don't penetrate the plant tissue - they work by preventing the germination of a spore on the leaf. ''To get the best results with protectant fungicides, you have to anticipate an infection, applying the fungicide before it occurs," he said.

Mr Nicholls' attendance at the adviser updates was sponsored by the Nufarm group.

Program 3 Contact: Dr Hugh Wallwork 08 8303 9382 Mr Anton Nicholls email nicholll@far.org.nz