Crop Injury - More Than Just Herbicides?

Tim Trower* and Chris Boerboom

This is the time of the year when growers finally get the opportunity to walk their fields and gauge the health of the crops and the effectiveness of the weed control program. This is also the time that consultants, dealers, chemical reps, and university personnel receive calls concerning crop injury. The extreme weather patterns observed the last month (frost, hail, heavy rains, and extended cold temperatures, etc.) have presented field diagnosticians the difficult task of separating the environmental effects from the potential chemical effects. Many times the crop injury may result from a combination of environmental and chemical factors. For example, environmental stress may cause the crop to be more susceptible to herbicide injury by increasing herbicide uptake or slowing herbicide metabolism. The following is a discussion of non-chemical factors that may cause crop injury and may mimic herbicide injury.

Diseases

Corn: The cool, wet soils favor the development of many seedling and root rotting pathogens. Pythium root rot (Pythium spp.) has been identified as a factor in reducing the corn stand in at least three fields. High soil moisture levels and low temperatures favor pythium development. A quick diagnostic tool to identify pythium is to gently pull the root between the thumb and forefinger. The cortex (outer portion of the root) will readily pull off, leaving the white steele (inner portion of the root) in a pythium-infected root. Corn plants infected with pythium may be stunted and yellow in color.

Soybeans: Water-saturated soils favor the development of Phytophthora (Phytophthora megasperma f. sp. glycinea) and Pythium (Pythium spp.) seedling blight. Both pathogens are favored by wet and cool soils with phytophthora more prevalent in soils with high clay content. Affected plants may rot prior to emergence or emerge and die at a later time.

Plant Genetics: Not all corn hybrids or soybean varieties are created equally and weather extremes can cause differences to become evident. Fortunately, most of the commercial corn hybrids and soybean varieties have fair to good disease tolerance. Matching disease tolerance and hybrid selection to field conditions (i.e. a low field with poor drainage) is an important consideration.

Insects

Corn: Seedcorn maggot has been identified as one component of a crop injury complaint. Seedcorn maggots are yellowish-to-white in color, cylindrical in shape, about 0.25 inches long, and legless. The seedcorn maggot can be found in the seed feeding on the germ which usually results in a non-viable seed. Seedlings that emerge may be weakened and may die. Seedcorn maggots are usually a problem in fields that have been spread with manure and are favored by cool and wet soils that delay germination.

Although usually not a serious pest in Wisconsin, cutworm feeding can cause a stand reduction ranging from mild to serious. The key to identifying cutworm is to look for corn plants cut at or below the soil surface. A close inspection of the surrounding soil (up to 3 inches below the soil surface) or crop debris (if the soil is saturated) should reveal the larvae.

Soybeans: Soybeans usually are not affected by most common soil-borne insects. However, a severe stand reduction was caused by a heavy infestation of seedcorn maggot feeding on the soybean cotyledons at the Arlington Research Station. Cutworms have also been known to feed on soybeans.

Environment

Frost: Frost damage has been widespread throughout the state. Corn leaves that have been frosted may become water soaked, turn dark green and then brown. Frost damaged corn may mimic the leaf burn of contact herbicide injury.

Crusted or hard soil surface: Corn emerging through crusted or hard soil surfaces, as well as heavy trash, can express symptoms very similar to acetanilide injury (Harness, Surpass, Dual II, Lasso, Frontier, and their premixes). Seedlings may leaf out prior to emergence and exhibit laddering or "shepherd’s crook" after emergence.

Soil compaction: Corn or soybeans growing on compacted soil may become stunted and have restricted root growth. The root stunting can partially mimic the root pruning caused by ALS inhibitors (eg. Hornet, Pursuit), dinitroanilines (eg. Prowl), or preemergence growth regulators (eg. dicamba).

Saturated soils: Saturated soils can weaken or destroy a crop standing by drowning the root system. Corn is somewhat more tolerant to flooded soils compared to soybeans, but the risks of root diseases increases the longer the soil remains saturated.

Herbicides: Although there is more non-chemical stress on our crops this year, it does not exclude herbicides as the reason for crop injury. It just makes it more difficult to confirm the role of the herbicide when more factors are interacting. When diagnosing injury, take an adequate amount of time to determine if: 1) if the injury symptoms are consistent with the herbicide’s mode of action; 2) if the appearance of symptoms is consistent with when the herbicide was applied or taken up by the crop; and 3) if the injury patterns in the field are consistent with the application pattern of the herbicide (eg. no injury in herbicide skips or increased injury in overlaps).

*  Senior Outreach Specialist