Fig. 30

Potassium Deficiency

Soybeans require large amounts of potassium, especially during periods of rapid vegetative growth. Potassium influences plant growth, and nutritional balance of the plant. Deficiencies can be induced by heavy applications of other nutrients such as calcium.

Symptoms of potassium deficiency include chlorosis of the leaf margins which may eventually encompass half the leaf, leaving only the base green (Fig. 30). The yellow areas coalesce and become necrotic followed by downward cupping of the leaf margins. The dead tissue may shred and give the leaf a ragged appearance. Severely deficient plants produce wrinkled or misshapen seeds. Some cultivars show leaf hopper injury that mimics potassium deficiency.

Potassium deficiency can be corrected by applying fertilizers that contain potassium. A soil test should be used to determine the correct amount of potassium to apply.

Fig. 31

Manganese Toxicity

Manganese is important in respiration and nitrogen metabolism. Manganese toxicity is common on light soils in the southeastern United States, and usually occurs when the soil pH drops below 5.0.

Affected soybean plants are stunted and leaves exhibit crinkling, hence the common name "crinkle leaf" (Fig. 31). In severe cases a brownish orange necrosis may develop along the leaf veins. Youngest leaves are infected first. The symptoms at this stage resemble mottling caused by a virus.

Manganese toxicity can be corrected by adding lime to the soil to increase the pH to 6 or slightly higher.

Ozone

Ozone is the most widespread air pollutant that damages plants, including soybeans. Ozone is generated from the reaction of sunlight with nitrous oxides and oxygen in the presence of hydrocarbons. Such a mixture is commonly referred to as smog and arises primarily from automobile exhausts in urban areas. Most damage occurs when contaminated air stagnates over soybean fields.

Prolonged exposure may result in reduced growth and yields depending on the cultivar and the stage of plant growth. Damage is most severe when plants are growing under optimal conditions. Drought-stressed plants are resistant. Soybeans exposed over a long period (several days or more) to low ozone levels suffer chronic injury. Acute injury occurs when plants receive a high dose over a short period (a few hours). Soybeans are generally sensitive to ozone levels of 0.06 ppm or greater, although damage and symptom development vary depending on the cultivar.

Leaves are most sensitive during the later stages of leaf expansion and symptoms appear 48 hours or more after exposure. For this reason, symptoms are first observed on older leaves. Symptoms often referred to as leaf stippling or bronzing, include small cream to bronze colored lesions on upper leaf surfaces between the veins (Fig. 32). Injured leaves may ultimately exhibit chlorosis and die prematurely.

Examination of local air-quality data in addition to symptom appearance will help confirm ozone injury.

Fig. 33

Sulfur Dioxide

Sulfur dioxide is produced primarily from the buming of fossil fuels and is most damaging near coal-fired power plants and ore-smelting operations. Relatively high levels of sulfur dioxide (25 ppm or greater) must occur for symptom development, thus damage is generally confined to localized areas downwind from a point source. Recent installation of emission control devices has reduced the occurrence of sulfur dioxide injury.

The youngest fully-expanded leaves are most sensitive. Plants growing under adequate soil moisture are more sensitive than drought-stressed plants. Yield may be reduced if exposure is prolonged or occurs during early reproductive stages of growth. Sensitivity varies among soybean cultivars.

Symptom development may be chronic from exposure to low levels over a long period, or acute following a brief exposure to high levels of the pollutant. Chronic symptoms consist of leaf chlorosis occurring between veins. Acute symptoms include watersoaked areas between leaf veins that later become necrotic and appear bleached (Fig. 33). Veins of severely affected leaves also become necrotic and leaves often drop prematurely. Symptoms often appear on both leaf surfaces.

The proximity of affected fields to a point source of the pollutant, and air quality data are useful for diagnosis.

Dinitroaniline Herbicides

Dinitroaniline herbicides include many common preemergence herbicides such as Treflan, and Prowl. Injury usually occurs when rates that exceed the label are used. Dinitroaniline injury is often temporary and is not economically important, unless stands are reduced.

Symptoms include delayed emergence and some stunting under cool, wet soil conditions. Hypocotyls are often enlarged and secondary roots are short and stubby (Fig. 34).

Fig. 35

Triazine Herbicides

This includes herbicides such as atrazine and metribuzin (Lexone and Sencor). Atrazine which is not labeled for use in soybeans, kills soybeans often as a result of residual herbicide from previous crops.

Although labeled for soybean use, metribuzin injury occurs when application is made at rates exceeding the label or when applied to sensitive cultivars. Injury may also occur from herbicide being concentrated in low areas following heavy rains. Symptoms includes chlorosis, marginal necrosis, leaf abscission, and death of the plant (Fig. 35). P>

Fig. 36

Fig. 37

Phenoxoy or benzoic acid herbicides

Injury from phenoxy herbicides (2,4-D and 2,4-DB) is probably the most common herbicide problem encountered on soybeans. Injury results from improper application within the field, but may also result from herbicide drift from surrounding fields. Soybeans are more sensitive to phenoxy injury when they are in a rapidly growing vegetative state. Symptoms of injury include cupping of leaves (Fig. 36), strapping of leaf veins, and epinasty of stems. Severe injury can result in cracking of stems (Fig. 37) and adventitious root formation.

Injury from benzoic acid herbicides (e.g. Banval) is usually the result of drift from other crops. Damage resembles that of phenoxy herbicides. Leaflets are cupped upward, crinkled, and may exhibit a pronounced tip at the apex. Other Injury

Fig. 38

Lightning Damage

Lightning kills or injures soybean plants in a circular pattern within the field. The spots are typically about 5 meters in diameter, but may be as large as 15 meters with clearly defined margins (Fig. 38). The affected area does not expand over time. Few plants or weeds within the circle survive, but plants near the edges may show only slight injury. Plants on the edges may exhibit splitting of the epidermis, dieback of terminals, and separations in the pith. Stems may be blackened with dead leaves remaining attached.

Fig. 39

Rhizobium Induced Chlorosis

The cause of this condition has been attributed to an inefficient strain of nodule forming bacteria. It may also be associated with acidic soil conditions. Although this yellowing is common in some years, yield is not significantly reduced. Rhizobium chlorosis commonly appears in the tops of 6-8 week old soybean plants. The yellowing ranges from light green to nearly white and usually disappears a week or two after it is obser-ved (Fig. 39).