One of the world's leading tropical plant experts has stressed the importance of ongoing research into Panama disease, at the biennial Australian Banana Industry Congress in Sydney.
Professor Randy Ploetz initially worked on race 1 involving Gros Michel bananas, and first discovered the tropical race 4 (TR4) in the Cavendish variety in 1989 in South-East Asia. He noticed it quickly spread to other parts of the world and says it is one of the most serious diseases he has worked on.
"It's clear that TR4 has already killed more bananas, and had a larger economic than race 1 did on Gros Michel and that is saying a lot," Professor Ploetz said. "The Gros Michel outbreak was devastating for many economies in Central America, but through China and the Philippines now, this problem is really serious. More recently and scary to me is the popping up of TR4 around the world. Within the last five years TR4 has been found in several other locations at a great distance from South East Asia."
TR4 was discovered on a property in Tully, in North Queensland in early 2015. Professor Ploetz has praised the way the industry as a whole in Australia responded in isolating it and containing the potential spreading, but admits there are still a number of issues to be answered.
"The question for me as an epidemiologist is how did TR4 get to those areas and we still have an incomplete understanding of that," he said. "How widely spread is this, pathogen now? Certainly the first property that was quarantined and eliminated, it was widespread in it. But how widespread outside that plantation might it be? We don't know. Detecting this pathogen requires the development of symptomatic plants, and when that happens you probably have much wider distribution of the pathogen. We haven't developed a technique to effectively and efficiently detect the pathogen in non-symptomatic situations."
He says this situation gives Australian and international researchers an ideal chance to study the epidemiology of the disease and how the pathogens move around. One reason could be it survives in weeds, and it is currently a vital area of study underway, but the professor says controlling water runoff and infected suckers is just as important in preventing spread.
"Infected suckers are the most efficient way the pathogens move around - certainly it is how race 1 moved widely," Professor Ploetz said. "It's a no-brainer, if you establish new plantings in an area and you've had this disease, you don't want to use traditional suckers. Tissue Culture plantlets are free of this pathogen. Also, early researchers knew clearly that using surface waters from lakes and rivers to irrigate crops once it is infested with this pathogen is a bad idea. Every time you use water from the river (or other surface source), you are inoculating the plantation with this pathogen. If you have the potential to use bore hole water, I highly recommend that."
Frequent visual inspections are also good idea, according to Professor Ploetz, whether that be aerial surveillance or even canopy reflectance data, which allows detection of not only sick plants, but plants that are infected with that disease. He says other detection methods still need some work, as specificity is a problem, by not singling out the exact pathogen.
He also believes that long term management, once TR4 is detected is difficult.
"Whenever anyone considers managing this, they need to consider economic management," Professor Ploetz said. "Is production going to be able to continue with what is being offered - whether it is biological control, cultural control? Can you continue growing banana as you would before you had the problem. In most cases, the answer is no."
There are no fungicides that get inside the plant and eliminate the pathogen. While he adds methods such as soil fumigants, flood-fallowing and rice hull burning are temporary fixes, and the problem often comes back worse than before. Professor Ploetz believes while healthy soil is a good method for managing some diseases such as nematodes, but "the pathogen is too virulent and the host too susceptible" in the case of TR4. He has also raised doubt about the existing work on biological controls, saying most studies were done in greenhouses, and those in the field were not effective, as the disease returned.
The most effective method, the professor believes, is resistance - and that is where future opportunities lie in the way of genetics.
"History has shown clearly the only sustainable way to manage the problem is with resistant cultivars," he said. "But at the moment we are moving from Cavendish to what? I don't know that we have the perfect solution. We are going to hear about some clones. They are improving every day, but (at the moment) that is a dead end."
Before speaking at the conference, Professor Ploetz visited North Queensland plantations, and the area affected by TR4 and says Queensland authorities are doing the right thing.
"People are on red alert," he said. "They are aware of how difficult this is to manage and they are initiating that 'you are not coming on my property without clean boots, we are going spray you off'. That's good news. This pathogen is easily moved. So being aware of this is really important."