The number of phone calls and plant samples received during the past week concerning corn injury from Balance (isoxaflutole) herbicide increased significantly. Although it is not feasible to estimate the percentage of treated acres affected, injury appears to be generalized across much of Illinois. While the 1999 growing season marks the initial year of Balance commercialization in Illinois, many universities have evaluated this herbicide in research trials for several years, and few have reported corn injury as severe as is currently expressed in many producer fields. So what appears to be different this year as compared to previous years, when little or no injury was reported? What are the symptoms of Balance injury?|
Mode of Action of Balance
Pigments are a group of compounds produced within the plant that function in a variety of processes. The best known plant pigment is chlorophyll. Chlorophyll absorbs several wavelengths of visible light during the process of photosynthesis and passes along the absorbed energy to electron carriers involved in the "light" reactions of photosynthesis. Although chlorophyll can absorb several wavelengths of visible light, green light is not well absorbed; most of the green light is reflected from the leaf, and thus plants appear green in color.
Several other plant pigments are involved in the process of photosynthesis, and with respect to isoxaflutole, carotenoids are an important group of accessory pigments. Carotenoids funnel additional light energy to the chlorophyll molecules and also help dissipate excess energy when the chlorophyll molecules are in the excited state following absorption of light energy. If carotenoids are absent, chlorophyll molecules are unable to dissipate the excess energy, and damage to plant membranes can result. Chlorophyll molecules may also be damaged from this excess energy through a process known as photo-oxidation. If the chlorophyll molecules are damaged extensively enough, injured plants may lose their characteristic green color and appear white.
Balance inhibits a step in the carotenoid synthesis pathway, specifically the enzyme 4 HP dioxygenase (HPPD). Command (clomazone) also inhibits carotenoid synthesis, but acts at a different site of action than Balance. When carotenoid synthesis is inhibited, affected plants appear white, which is the primary injury symptom of Balance. This "bleaching" symptomology generally occurs first along the margins and tips of new leaves. Unlike Command, corn generally has good tolerance to Balance because the corn plant can rapidly metabolize the active ingredient to nonphytotoxic forms.
Why has corn injury from Balance appeared? Although we don't have definitive evidence or data to elucidate the cause, we suspect that cool, wet growing conditions may have contributed to the observed injury. The mechanism of isoxaflutole selectivity to corn is the ability of the corn plant to rapidly metabolize the active ingredient to a nonphytotoxic form. Factors such as cool weather and very wet soils, which slow overall plant growth, can also slow the plant's ability to rapidly metabolize the herbicide. If the amount of active herbicide within the plant exceeds the plant's ability to metabolize it, crop injury symptoms are frequently the result.
Field observations indicate that where Balance injury has occurred, symptoms are often very distinctive along the headland rows where there may have been a double-up of the application. Additionally, soils lower in organic matter and/or with a higher sand content may have more herbicide in solution and thus more available for plant uptake than soils with a higher organic matter content.
Will the corn plants recover from this injury? Again, we do not have sufficient data to offer anything more than recent field observations. Some recently visited fields where injury has been present for a week or longer have begun to show signs that the crop is recovering, whereas portions of other fields have already been replanted due to the severe injury. With additional precipitation, our hope is that Balance will not "recharge" the corn.--Aaron Hager, Marshal McGlamery