If a waterhemp population that has resistance to a PPO inhibitor applied POST, which one is true?
1. has resistance to PPO inhibitors applied PRE, yet aPPO inhibitor applied PRE will still provide somecontrol
2. does not have resistance to PPO inhibitors appliedPRE
3. will not be controlled at all by a PPO inhibitorapplied PRE
There is not a clear-cut answer to this question. The resistant and susceptible waterhemp populations will respond differently when treated with PPO-herbicides (i.e. sulfentrazone or flumioxazin). However, these responses to preemergence applications are at rates lower than commercial use. When treated preemergence at typical use rates, differences between resistance and susceptible plants are not observe.
This means under field conditions, PPO-resistant waterhemp survives a postemergence application but is often controlled when treated preemergence. However, under controlled conditions differences can be detected between resistant- and susceptible-populations when treated preemergence with a range of below labeled rates.
Most people think of an herbicide-resistant weed as either surviving or controlled with a herbicide application. But resistance can be manifested in others ways. Research from Purdue shows that after a preemergence application of a PPO-herbicide (flumioxazin or sulfentrazone) the first waterhemp to emerge (2 or 3 weeks after preemergence application) is usually a PPO-resistant biotype. Therefore, length of control from preemergence application is shorter for the resistant-biotypes than the susceptible biotypes.