Are soil-applied herbicides immune to resistance?
Imazethapyr (the active ingredient in Pursuit and some herbicide premixes) is an ALS-inhibiting herbicide that can be applied to the soil or plant foliage. Worldwide, biotypes of 128 weed species have evolved resistance to ALS-inhibiting herbicides. As mentioned previously, many Illinois waterhemp populations contain plants resistant to ALS-inhibiting herbicides (including imazethapyr). These plants demonstrate a high magnitude of resistance to imazethapyr regardless of whether it is applied to the soil or plant foliage.
What about weeds resistant to herbicides that share a common mechanism of action but that are usually applied to either the soil or foliage, such as herbicides that inhibit the PPO enzyme? Table 1 lists examples of PPO-inhibiting herbicides commonly used in Illinois, and indicates whether they are most commonly applied to the soil or foliage. Waterhemp resistant to PPO-inhibiting herbicides is becoming increasingly common across Illinois, but some incorrectly believe this type of resistance exists only to foliar-applied PPO inhibitors. Biotypes of waterhemp resistant to PPO-inhibiting herbicides are resistant to those herbicides regardless of whether the herbicide is applied to the soil or foliage.
Table 1. Examples of PPO-inhibiting herbicides commonly used in Illinois.
|Trade name||Active ingredient||Most commonly applied to:|
1Flumioxazin, sulfentrazone and saflufenacil possess some foliar activity, but greater weed control is achieved when these herbicides are taken up from the soil.
2Fomesafen and lactofen are labeled for soil application.
click image to zoomFigure 1. Waterhemp plants resistant (back row) or sensitive (front row) to lactofen (Cobra). Numbers indicate the application rate of Cobra (fl oz/acre) applied when plants were 4 inches tall. Suspicions of waterhemp resistance to PPO-inhibiting herbicides generally begin after a foliar-applied PPO inhibitor failed to control plants that were treated within label guidelines. PPO-resistant waterhemp plants treated with a foliar-applied PPO-inhibiting herbicide typically demonstrate injury symptoms, such as leaf necrosis, characteristic of this herbicide family. However, unlike susceptible plants, the leaf necrosis is generally much less and the resistant plants begin to recover within 7 to 10 days after the application (Figure 1). But, when a soil-applied PPO-inhibiting herbicide is applied to this same field, the level of waterhemp control often appears comparable to that of a susceptible population. So, why do soil-applied PPO-inhibiting herbicides seem to control a PPO-resistant waterhemp population that foliar-applied PPO-inhibiting herbicides do not control? The answer is because the dose of the soil-applied herbicide is sufficiently high enough to overcome the mechanism of resistance, at least for a while.