Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Water

Excess moisture may affect wheat and barley in several ways. Small grains grown in soils that are poorly drained or have depressed areas can be prone to drowning from standing water remaining on the soil surface for several days after heavy rains. The result is poor soil aeration that reduces soil oxygen levels needed for root respiration and proper root function. In addition to soil oxygen deprivation to plant roots, nutrient uptake by roots also becomes limited. Excessive moisture keeps soil temperature low and elevates the relative humidity in the crop canopy, favoring the onset of plant diseases. Low soil temperatures slow crop growth and development and can predispose plant roots to soil-borne pathogens. Typical symptoms associated with excessively wet soils are dead or dying plants in low lying areas, uneven emergence, poor growth, and chlorotic plants. See photo 25.

Photo 25 Cold, wet soils cause poor soil aeration that in turn reduces oxygen levels needed for normal root growth.

Under drought conditions, the evaporative cooling process within plants, called transpiration, decreases. Water is primarily utilized in plant transpiration. When transpiration is reduced, heat loss by wheat and barley plants slows down and leaf temperature increases. Drought stress is increased by low humidity, high temperatures, strong winds, and high light intensity. Small grain plants under drought stress will exhibit erect leaves rolled toward the midrib. The inward rolling of the leaf is due to specialized cells that have collapsed. The curled leaf helps to reduce leaf area exposed to the sun, thereby reducing leaf heating. Although this helps to protect the leaves from heat damage, it also reduces photosynthesis. Plants are usually able to take up water at night, but as soil moisture becomes limiting the leaf curling occurs earlier in the day; hence, plant productivity declines.

In severe drought cases, small grain leaves become ashen to bronze in color, hard, dry, and florets can be aborted. Drought stress after heading causes plants to wilt slightly, lose color, and ripen heads prematurely. Below ground, the root systems of drought- stressed small grain plants are limited, and these plants may appear dark blue-green in color. Barley is more tolerant to drought stress than spring wheat. Small grain crop injury from drought stress can appear similar to injury symptoms caused by herbicides from the amino acid, bipyridylium, dinitroaniline, phenoxy-carboxylic acid (2,4-D, MCPA), and thiocarbamate families and difenzoquat (Avenge). See photo 26.

Photo 26 Under drought conditions, small grain plants suffer from cell dehydration. This results in cessation of cell division, reduced biochemical activity, and slowed photosynthesis. Plants suffering under these conditions will appear stunted and yellow or bronze in color.

Temperature

Wheat is especially sensitive to temperatures that exceed 90°F for any significant period. Plants can be injured at seedling emergence, reproductive development, stem elongation, heading, and flowering by high temperatures. When wheat leaves are exposed to high temperatures, photosynthesis slows down and plant respiration increases. During these periods of heat stress and high respiration rates, calories are burned faster than new food is produced. The result is a loss of plant growth. Heat stress results in smaller leaves and fewer spikelets. See photo 27.

Photo 27 Hot weather that occurs during the conversion from vegetative to reproductive stages can cause small grain plants to fall behind in their ability to carry out respiration and supply materials for new growth of leaves, florets, and spikelets. High temperatures during flowering can result in head sterility.

Frost injury to wheat and barley involves the physical disruption of cell protoplasts by ice crystals that enlarge inside and between plant cells. Damage patterns in the field reflect the topographic pattern of cold air flow and where it settles. Plant parts that are the nearest to the crop canopy suffer the most damage. Leaf tips appear water-soaked and dark green, then dry out and turn brown within a few days. Shriveling and water- soaking damage can also appear at the base of the peduncle. Freezing temperature is most dangerous to wheat and barley when it coincides with tiller elongation, heading, or flowering. Frosts that occur during tiller elongation cause the collapse and death of meristematic (growing) tissues immediately above and below nodes. Such damage may lead to early tiller maturity, white heads, and new shoot formation at the crown. Flowering grain heads injured from frost may have sterile spikelets that turn white prematurely. Crop injury from frost is similar to injury produced by herbicides from the amino acid and bipyridylium families and by difenzoquat (Avenge). See photo 28.

Photo 28 Leaves injured by spring frost appear at first water-soaked, then dark green. These injured leaves dry out and quickly turn brown. Frost injured plants quickly recover if the growing point was not exposed to freezing temperatures.

Hail
Spring wheat and barley plants sustain random physical injury when injured by hail. Injury symptoms from hail appear as shredded leaves, lodged plants, broken heads, and kinked culms. It is common to see bleached spots on plants appearing where immature plant tissue was injured from hail stones. Hail injury can mimic injury symptoms from herbicides belonging to the phenoxy-carboxylic and benzoic acid families. See photo 29.

Photo 29 Hail injury occurring on small grains in the preboot stage can cause the grain head to curl up inside the sheath, resulting in twisted heads that are partially exposed and hanging down. The effects of hail injury can also be similar to injury from dicamba applied too late.

Color Banding
Color banding occurs on young grain seedlings because of temperature changes. It is commonly associated with deep planted grains. Color banding occurs when temperatures at the soil surface fluctuate widely. Newly emerged grain plants will exhibit alternate color bands of yellow and green leaf tissue that correlates with high and low temperatures. Color banding can sometimes occur when water laden or frosted leaves are quickly dried by surface heat generated by solar radiation. Emerging leaf tissues of small grains are sensitive to temperature extremes, but quickly lose their sensitivity with age. See photo 30.

Photo 30 Coleoptiles of small grain seedlings exposed to fluctuating temperatures at the soil line exhibit yellow and green color bands. Injury is temporary.

Sandblasting
Sandblasting injury is caused by winds impacting soil particles against plant leaves. Symptoms of sandblasting appear as small abrasions on leaves caused by blowing sand. Lesions from this injury often appear coppery in color. Severe winds can shred plants making them prone to desiccation. Sandblasting injury appears similar to injury caused by herbicides from the benzonitrile and bipyridylium families or propanil (Stampede). See photo 31.

Photo 31 Blowing sand from strong winds can injure small grain leaves resulting in leaf abrasions.

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