Cotton growers race to treat leaf disease after mysterious arrival

Cotton industry scrambles for ramularia leaf spot treatment after detections in Queensland Tue 28 Jul 2026 at 6:20am The race is on to deal with a little-known cotton disease that has mysteriously arrived in northern Australia and is wreaking havoc on crops. Queensland Department of Primary Industries principal plant pathologist Linda Smith said there were few options to treat ramularia leaf spot, which was known by a number of names. "It's also called areolate mildew, grey mildew, or false mildew, because it looks like a mildew, but it's not a mildew," she said. "It's a fungal disease that loves hot, humid, wet tropical conditions, and when you get these conditions you get disease outbreak." Dr Smith said the disease was more commonly seen in the south-eastern United States and Brazil and appeared briefly in the Northern Territory decades ago. "Overseas, it has been causing some significant yield loss … it can reduce both the yield and the fibre quality," she said. "With the detection now in Queensland and some evidence of defoliation of the crops, we're really concerned about it." Treatment limited Ramularia leaf spot has been detected in crops in north and central Queensland in the last two seasons. Simon Struss, an agronomist based in the cotton, beef and grains producing Callide-Dawson region, said it "really was right throughout the whole area". "Every farm, every field showed symptoms of the disease," he said. "I'll put my hand up and say that I was confusing it with [another fungus] alternaria, until it became apparent that no, there was more to it than that. "That's when the samples were sent down to Brisbane and [there was] a positive identification for the ramularia." Mr Struss said there was no chemical registered for use against the fungus in Australia, which left growers with few treatment options. "Really the only lever they can pull is to try to get their crop in early [before December]," he said. Growers are also being urged to prevent the spores from spreading by burying infected crops after harvest to kill volunteer cotton which grows unintentionally, and not to plant back-to-back. Resistance a common enemy Dr Smith said the race was on for Australia to access a chemical treatment and that industry groups were working together on a control strategy in the meantime. "Which includes what permits do we need for which fungicides so that we can develop a strategy that has multiple modes of action to reduce fungal resistance and also to provide choices for growers," she said. Ramularia is a new threat for Australian growers, but in Brazil the disease is so common that it has developed chemical resistance. Cotton Australia policy officer Doug McCollum said the industry was trying to learn from past mistakes. "We've got a resistance management program in place to make sure that that we've got something that's sustainable in the long-term," he said. "We don't want to have one or two products and then we find after a few years that those don't work anymore. "It's really important that we do this right from the start, and we do have that opportunity." Mr McCollum said where the disease came from remained a mystery. "It probably doesn't really matter where it's come from, the fact that it's here," he said. "I suspect it's probably endemic, it's probably always been in Australia … you'd probably never be able to trace where it where it came from." Mr McCollum said the industry was also focused on figuring out what else it could be unaware of. "There's potential that it could be a serious disease for us here, but we don't actually know that for sure yet," he said. "We're trying to do the research, understand more about the disease and the specific impacts that it might have on our industry." Dr Smith said early detection was key to preventing significant losses. "We do have a very fast diagnostic tool that we developed. It's a molecular tool called LAMP (loop-mediated isothermal amplification)," she said. "We can, from a leaf sample, have a diagnosis of ramularia within three hours. "As long as we get those samples, we can let you know very quickly if that's what you've got."