Ground Cover North : Ground Cover 049 April-May 2004 - North
By ALEC NICOL The worst outbreak of stripe rust in 20 years has “carved a hole right through breeders’ germplasm”, according to researchers reporting to growers in the latest round of GRDC research updates. They made it clear they believe the popular variety H45A, now ranked as ‘susceptible’ to the new WA pathotype, should be abandoned – although they also warned that it ought not be the sole focus. The core message remains the need to quit all susceptible varieties, eliminate the green bridge, and develop a fungicide strategy aimed at delaying any onset of the disease. New chemicals on the horizon are, like the present generation, expected to be better at protecting than curing. The level of interest in stripe rust was refected in the number of papers presented at updates around the country. At Wagga Wagga, NSW, rust expert Dr Colin Wellings, plant pathologist Dr Gordon Murray and visiting New Zealand key-note speaker Dr Nick Poole all delivered papers on the subject and exchanged experiences with agronomists in a lengthy colloquium. What has become known as the WA pathotype is a new strain, unrelated to previously identifed stripe rust strains in Australia and as a consequence was not anticipated by breeders. That has had an impact on breeding lines and many varieties moderately resistant to the older H45A pathotype are now rated as moderately susceptible and H45A itself is now rated as a susceptible variety with no resistance. Dr Wellings says the original stripe rust pathotype, frst identifed in 1979, also still persisted and was found last season in areas as diverse as Cootamundra and Narrabri. These established pathotypes have been joined by another capable of attacking varieties such as Bowie, CammA and the specialty soft wheat Qual 2000, which depended on the resistance gene Yr 17. Another pathotype has been isolated in Tasmania and poses a threat to the long season variety BrennanA. Dr Murray says stripe rust could be a very explosive disease. “It will remain undiagnosed in small hot spots in the crop. If conditions are unfavourable it won’t progress, but given the right conditions it explodes.” He says “step one” in control is to break the green bridge because the rust needed living plants for development. “H45A made up 40 percent of the wheat sown in NSW last year. That means 40 percent of the volunteers out there will be highly susceptible,” he says. With resistance threatening many popular varieties, attention is now turning to developing a fungicide strategy for the moderately resistant/moderately susceptible and susceptible varieties. Drawing on the experience of the 1983 outbreak, Dr Murray says that the earlier the disease got going the more damage it caused, even to resistant varieties. “An epidemic when the crop is at the frst node stage can reduce yields by 85 percent in susceptible varieties and even 25 percent in resistant varieties. An epidemic delayed until awn peep means a 50 percent loss in susceptible varieties but virtually no loss in resistant varieties. That’s why a fungicide treatment designed for early protection is so important. But an early application of some fungicides will compromise grazing strategies.” Dr Murray recommends seed dressing with the higher rates of Triadimenol for protection to the six to eight leaf stage, or Fluquinconazole or Flutriafol for protection at least until fag leaf emergence, and even throughout the season. Visiting NZ expert Dr Nick Poole reports that while these treatments delayed the onset of the disease in the susceptible variety H45A last season, they were still not as effective in maintaining yield as a foliar application at full fag leaf emergence (GS39). “In trials at Harden last season we saw that timing was more important in control than the product used. A single spray at GS39 was more effective than one applied 20 to 30 days earlier at the second node stage, even though this application pre- dated the appearance of disease in the crop. “What it means is that the most important thing is to protect the leaves that produce the yield, and that’s the top three leaves.” Page 21: Airborne attack GRDC PROGRAM 3 For more information: Dr Colin Wellings, 02 9351 8826, email@example.com Dr Gordon Murray, 02 6938 1999, firstname.lastname@example.org Frank McRae, 02 6391 3198, email@example.com Recent trials in northern NSW have shown that brassicas – canola and mustard – can be effective ‘break crops’ for reducing crown rot in following wheat crops. The effect of previous crop species (oilseed, legume and cereal) on the incidence and severity of crown rot and wheat yields was investigated in two three-year no-till feld experiments at Tamworth. The experiments compared the effectiveness of the Brassica break crops, canola and mustard, with chickpea. All break crops (brassicas and chickpeas) signifcantly reduced the severity of crown rot in both a highly susceptible and partially resistant wheat crop, compared to a wheat on wheat or wheat on barley rotation. Brassica crops were generally more effective than chickpea in reducing the severity of crown rot in the highly susceptible durum wheat Yallaroi (by 23 to 26 percent) but the advantage was less evident with Sunco which has partial resistance to crown rot (see chart). Researchers say the effectiveness of brassicas in reducing crown rot appears related to the level of microbial activity under the canopy and its effect on increasing the rate of residue breakdown. Break crops with a denser canopy, such as brassicas and faba beans, are likely to provide conditions more conducive to the breakdown of cereal residues than crops such as chickpeas. Chickpeas generally have thinner canopies and do not close over until later in the season. Brassicas also appear to infuence the soil and residue biology with increased levels of Trichoderma spp., a benefcial fungus. Crown rot is caused by the fungus Fusarium pseudograminearum (Fp) and is a major constraint to winter cereal production across Australia. Although more common in the northern cropping belt, it can occur throughout all mainland cereal growing areas and costs the Australian grains industry an estimated $56 million every year. The fungus survives in cereal and grass weed residues and has three distinct stages. The frst is ‘survival’ of the crown rot fungus in residue. The second is ‘infection’, though while present may not lead to major yield loss. The main infuence growers can have at this stage is the selection of partially resistant wheat varieties. Finally comes the formation of whiteheads when moisture stress occurs around fowering and beyond. This is the stage that leads to signifcant yield loss. Although this is largely at the mercy of the season, there are still management practices growers can follow to reduce moisture stress around fowering. These include effective fallow management, grass weed control, adequate zinc nutrition and matching nitrogen inputs to available water. In the third stage, the fungus is believed to prevent water movement from the soil to the heads – leading to whitehead formation in infected tillers. Without water stress the tiller can still be infected, yet fll normally without yield loss. GRDC RESEARCH CODES DAN485, DAN444, CSP274, Program 3 For more information: Dr Steven Simpfendorfer, 02 6763 1261 firstname.lastname@example.org RESEARCH UPDATE/FORUM 8 APRIL 2004 GROWER FORUM THE CORE MESSAGE n Quit all susceptible varieties n Eliminate the green bridge n Develop a fungicide strategy to delay onset of disease By BRENT ALEXANDER The push for higher yields is, for the most progressive growers, starting to resemble the high jump at a top-level competition. Big improvements across the whole farming system in areas such as rotation and disease control have raised the bar for how high yields can go. Now we are starting to look more closely at where else we can fnd those extra tonnes per hectare for more consistent productivity. And in aiming high, we are looking low – at our soil. While we do have more work to do in disease control in crops such as canola, and subsoil constraints are part of the package, I believe that a better understanding of the nutrient balance in soils is one of the big challenges remaining for growers and researchers. This is one of the main areas I will be researching this year on my GRDC-sponsored Nuffeld scholarship to Europe, the USA and Canada. Fifteen years ago in the Riverina, when we were growing two and three tonne crops in those good years, if you were applying four nutrients it did not matter if you limited one of them because your crop was not yielding up to its potential. But as we move closer and closer to that potential, if you have one nutrient out of balance, it brings the whole system unstuck. Comparisons with “normal” seasons shows that there is room for achieving our most signifcant yield gains – improvements of about 30 percent – in our wetter years. Lack of suffciently accurate, medium-term weather forecasting tools is also holding us back when it comes to nitrogen management. This uncertainty about weather conditions a month or two ahead makes it more proftable for growers to aim for 85 to 90 percent of maximum yields than 100 percent of potential, as research by farmer group Farm Management 500 confrms. Precision agriculture is another area I will be studying while overseas because more work needs to be done into how to make it proftable for the growers who have made this substantial investment. I feel that the only way to make money from precision agriculture is to use yield maps and electro- magnetic surveys and head back to the soil and ask why? What in the soil is causing parts of a paddock to yield half of another part? We need to know more about the causes of the nutrient imbalances that produce these differences. For more information: Brent Alexander, email@example.com MUSTARD CANOLA M CANOLA H CANOLA L CHICKPEA WHEAT T WHEAT S BARLEY YALLAROI SUNCO % CROWN ROT SEVERITY 0102030405060708090 Canola and mustard break stops the rot Nutrient balance in soil the next big challenge CROWN ROT STRIPE RUST Researchers warn growers of disease ‘explosion’ Worst in 20 years: severe head infection.
Ground Cover 050 June-July 2004 - North
Ground Cover 048 February-March 2004 - North