Weeds with resistance to multiple herbicide groups are becoming increasingly common. The most alarming cases are where plants are resistant to herbicide groups that they have never been exposed to.
Dr Roberto Busi is interested in understanding the rapid evolution of mechanisms that plants use to survive a herbicide application.
The most alarming herbicide resistance cases are where plants are resistant (right) to herbicide groups that they have never been exposed to.
“Each herbicide group interferes with different physiological mechanisms, such as photosynthesis or protein and lipid synthesis, within the target plant,” he said. “There are two main ways that plants can be resistant to a herbicide group.”
The first is when the plant possesses a mutant version of the target protein. When the herbicide reaches the site of action, it cannot bind to the target protein and so the physiology of the plant is unaffected. This is known as ‘target site resistance’ and has been the most common mechanism in herbicide resistance.
The second way occurs when the herbicide is prevented from reaching the target site. In this scenario the target site remains susceptible but the plant is able to de-activate or de-toxify the herbicide before it has its intended effect. This is known as ‘non-target site resistance’ and is the main mechanism involved when plants are resistant to more than one herbicide mode of action (MOA).
“Cross-resistance is becoming more common and in glasshouse experiments we have proven that it can occur within just a few generations,” said Dr Busi.
AHRI researcher Dr Roberto Busi is interested in understanding the rapid evolution of mechanisms that plants use to survive a herbicide application.
If I am careful to rotate herbicides with different modes of action will I avoid cross-resistance?
Short answer: Not necessarily.
Longer answer: In some crops there are only a few registered herbicides for the control of weeds. Even if these chemicals are rotated they are all still being regularly applied and it is possible for survivors to be resistant to both modes of action. For example, annual ryegrass plants have been found to be resistant to wheat-selective herbicides from Groups A, B, D and, most recently, J and K.
Is it possible for a weed on my farm to be resistant to a herbicide MOA that I have not used?
Short answer: Yes.
Longer answer: If a plant possesses genes that give it ‘non-target site resistance’ to one MOA group, it may also be automatically resistant to another MOA group. This can occur even if these chemicals have not been applied to this plant. Importantly, annual ryegrass plants can stack target-site and non-target-site resistances.
If I have cross-resistant weeds do I still have weed management options?
Short answer: Yes.
Longer answer: Preventing seed set on herbicide resistant plants is absolutely critical. It is necessary to look for survivors after a spray application and to eliminate them. Sending plant samples away for testing against a range of herbicides will tell you if there are herbicide options available to kill the resistant plants. Once the immediate threat has been dealt with it is important to develop a weed management plan than includes several non-herbicide tactics to reduce the frequency of herbicide use.
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