Flexstar Carryover Symptoms on Corn

May 26, 2000
We have received several plant samples and inquiries related to corn plants demonstrating injury symptoms associated with carryover of the active ingredient (fomesafen) of Flexstar. The occurrence of cases does not appear to be isolated in any specific geographic area of Illinois, and other Midwest states have recently reported similar instances (see the Iowa State University weed science Web page at http://www.weeds.iastate.edu/mgmt/qtr00-1/Flexstar.htm. While there have been previous cases in Illinois of fomesafen-carryover injury, the frequency of occurrences is greater this season than in past seasons. Why has the frequency of carryover cases increased this year? Several factors have probably played a role.

The use of Flexstar for weed control in soybeans has increased since its introduction. In general, the greater the number of acres treated, the greater the probability that carryover may occur. While the active ingredient of Reflex and Flexstar is identical, the Flexstar formulation has a "built-in" additive system. Flexstar has been a popular choice for postemergence waterhemp control and is frequently the diphenyl ether herbicide chosen for late-season (late-July to early August) applications. Late-season applications can often make satisfying the corn rotational interval difficult, especially when the following spring is dry enough to allow for early corn planting.

Fomesafen has the longest soil-residual activity of the three post-emergence diphenyl ether herbicides (Blazer and Cobra being the others). Soil half-life values (the time required for half of the applied herbicide to degrade) for fomesafen have been reported to be from100 days to 6 to 12 months. The range is dependent on several factors, including soil type and soil moisture. For example, the soil half-life of fomesafen under anaerobic conditions (flooded soil) is only 3 weeks, but persistence is extended as soil moisture becomes more limited. Because of the soil persistence of fomesafen and sensitivity of corn to fomesafen residues, the Flexstar label has a 10-month rotational interval for corn. Other crops are also sensitive to fomesafen soil residues (rotational interval for sorghum, alfalfa, and sunflowers is 18 months).

Dry soil conditions following post-emergence applications in 1999 were conducive for persistence of fome-safen. Again, degradation of fome-safen is most rapid under wet soil conditions and slows considerably as soils become drier. Dry conditions during the spring of 2000 have allowed for early corn planting, and in many instances corn planting was completed before the 10-month rotational interval was satisfied.

The most common corn injury symptom caused by fomesafen carryover is veinal chlorosis or necrosis, which results in a striping effect on the corn leaves. In some instances, the leaf veins almost appear clear or transparent. The root system of effected plants usually shows no symptoms. While these foliar symptoms may cause concern, corn generally recovers fairly quickly from this injury. Zeneca data suggest corn can survive fomesafen residues from up to a 3X rate applied the season before corn planting. Several calls have indicated that a small percent of the corn plants showing the carryover symptoms have died, but it is not completely clear if other stress factors (insect feeding, plant diseases, etc.) may be contributing also.

We have also received several calls about corn plants' expressing injury symptoms similar to those caused by fomesafen carryover but growing in fields that were not treated with Flexstar in 1999. It's difficult to determine the causal factor(s) in these situations, but possible causes might include the seedling blight stage of Stewart's wilt or sunscald. Corn plants with Stewart's wilt will also include symptoms such as cavities in the stalk pith tissue near the soil line and the presence of bacterial ooze.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague