2 Troubleshooting Soybeans 

Looking for Patterns in the Field

A logical starting point for diagnosing soybean problems is to look for patterns of occurrence within the field - areas where the problem occurs and those where it is absent. If the condition appears uniformly throughout the field, the cause of the problem must also have been distributed uniformly over the field. Problems that correspond to the topography of the field or to soil type (which is also often related to the topography) are more likely to be soil related than caused by pests or field operations. Problems that are worse on one side or edge of the field are likely to be related to spray drift or to the movement of insects into the field from one side. Some problems, such as root and stem rots and bacterial chlorosis, may occur on isolated plants throughout the field.

Problem areas with sharply defined boundaries are frequently related to some field operation - something that was done differently on one pass than on an adjacent pass (Figure 2). Nematodes, however, are immobile enough that the edge of a nematode-infested spot may also be very distinct (Figure 3). Problems that are concentrated in one row but do not appear in an adjacent row are usually equipment related. The distance between affected rows will provide some insight into how wide a piece of equipment was involved.
 

Plant Symptoms

The abnormal symptoms of individual plants are also useful in diagnosing problems. Since some symptoms are more useful than others, relying solely on plant symptoms is often unsatisfactory. If plants fail to emerge, the cause may be low seed quality, root rots, use of certain chemicals, planting too deep, or severe crusting. If seedlings emerge but do not survive, root rots and chemicals that inhibit photosynthesis (for example, metribuzin, linuron, or atrazine) are very possible causes. Misshapen leaves (wrinkled, cupped, or elongated) are most likely related to chemical use but may be a result of virus infection.

Some leaf discolorations are fairly distinctive. Potassium deficiency causes yellowing of the leaves, starting at the outer edges. Manganese deficiency also causes yellowing at the leaf edges, but the veins stay green noticeably longer than the interveinal areas of the leaf. Viruses often cause a bronze coloration of the leaves. Nitrogen and sulfur deficiencies are typically indicated by lighter-colored to yellowish plants, but nitrogen deficiency can be created by several different problems. An uncommon but fairly distinctive leaf symptom is caused by bacterial chlorosis. The top leaves on isolated plants look like the plant has a combination of manganese and potassium deficiencies, but the discoloration develops almost immediately on normal-sized plants, and nodulation is usually poor.

Holes in leaves suggest damage by foliage-feeding insects or hail, although hail damage looks more like a tear than a spot where the foliage was removed. Spots or lesions on the leaves (or perhaps stems) suggest the presence of diseases or tiny insects such as thrips. Twisted plants are most commonly a result of chemical damage (frequently by chemicals in the phenoxy family). Dead leaves or other plant parts help pinpoint where to look for causes of the problem - somewhere on or below the dead plant part.

Generally poor growth suggests poor growing conditions. Problems of this type often involve limited root activity or an insufficient supply of one or more of the inputs that roots supply to the plant. Abnormally shortened plants with shortened internodes (stunted plants) are often caused by viruses or chemicals. Absence of normal pod or seed development late in the season is often an indication that growing conditions were poor earlier in the year.

Roots are not nearly as easy to see as the upper parts of the plant, but they can provide very useful information. Stunted roots are often the result of chemicals or nematodes. Rotting of the roots - whether it occurs throughout the root, on the surface, or only on the interior - is usually caused by the root and stem rot diseases. Small cysts on smaller roots are typically caused by soybean cyst nematodes, while galls on the roots are usually a sign of one of the root-knot nematodes. Limited or unhealthy nodules may be caused by lack of adequate rhizobia, low pH, molybdenum deficiency, or insect injury. The condition may also be caused by factors such as nematodes or inadequate fertility that limit root activity or by something that limits the overall supply of photosynthate available to the roots.
 

Ability of Soybeans to Compensate for Problems

The later a problem is noticed after it first develops, the more difficult it is to diagnose the cause, partly because soybeans have a relatively great ability to compensate for problems. If plants are missing from a "normal" stand, adjacent soybean plants tend to produce extra branches, and they produce about as much extra yield as would have been produced by the missing plant. If the growing point of a plant is killed or removed (for example, by hail, deer, or rabbits), one or more new stems tend to emerge from a secondary bud at a lower leaf axil.

The better the growing conditions and general plant vigor at flowering, the more pods are set. If no or few pods are set early in the flowering period, more pods tend to be set later. If no or few pods are set anytime during the normal flowering period, the flowering period tends to be extended, thus allowing for later-than-normal pod set. If pods are set markedly later than normal, they tend to develop and mature seeds in fewer than the normal 55 to 60 days (as much as 10 to 15 fewer days).

The number of pods per foot of row is much more constant than the number of pods per plant. The number of seeds per pod tends to reflect growing conditions during the seed set period as well as the number of pods set. (Setting more pods leads to fewer seeds per pod, and vice versa.) Thus the number of seeds per foot of row is more constant than the number of seeds per pod. Similarly, seed size tends to reflect growing conditions during the seed-filling period and the number of seeds set per foot of row.