Spotting stalk rot in grain sorghum
Sorghum harvest is getting near. One of the few remaining potential stumbling blocks to a successful crop could be lodging -- often caused by stalk rot. Stalk rot can be an even bigger problem in grain sorghum than in corn due to a generally thinner stalk in sorghum.
Annual losses are difficult to determine, because unless lodging occurs, the disease goes mostly unnoticed. The best estimates are at least 5 percent of the sorghum crop is lost each year to stalk rot -- severe cases could create yield losses of 50 percent. The most obvious losses occur when plants lodge. More important could be the yield losses that go unnoticed.
In sorghum, these losses are caused by reduced head size, poor filling of grain and early head lodging as plants mature early.
In grain sorghum, the two most common types of stalk rot are charcoal rot and Fusarium stalk rot. Although caused by many different organisms, the symptoms of the various stalk rots are somewhat similar. Symptoms generally appear several weeks after pollination, when the plant appears to prematurely ripen. The leaves become dry, taking on a grayish-green appearance similar to frost injury. The stalk usually dies a few weeks later. Diseased stalks can be crushed easily when squeezed between the thumb and finger and are more susceptible to lodging during wind or rainstorms. The most characteristic symptom of stalk rot is the shredding of the internal tissue in the lowest internodes of the stalk, which can be observed when the stalk is split. This shredded tissue might be tan, red or salmon colored (Fusarium stalk rots); or grayish-black (charcoal rot).
Hot, droughty weather with soil temperatures in the range of 90 degrees or more are ideal for the development of charcoal rot. Drought does not cause the problem, but it weakens the plants' defenses to the disease. Charcoal rot is usually less severe if drought stress is reduced. A good rule of thumb to discern charcoal rot from drought stress is plants will die prematurely about two earlier.
Fusarium stalk rot is favored by dry conditions early in the season, which decreases nutrient solubility, making the nutrients unavailable to the plant. Later in the season, following pollination, warm (82 to 86 degrees), wet weather can leach remaining nutrients from the soil resulting in late-season nitrogen stress and an increase in stalk rot.
Stalk rot is a stress-related disease. Any stress on a crop can increase both the incidence and severity of stalk rot. Research has indicated when the carbohydrates used to fill the grain become unavailable due to nutrient shortage, drought stress, leaf loss from insects, hail, disease or reduced sunlight, the plant uses nitrogen and carbohydrate reserves stored in the stalk to complete grain fill. This loss of nitrogen and carbohydrate reserves weakens stalk tissues and results in increased stalk rot susceptibility. Early maturing hybrids are generally more susceptible than full-season hybrids.
Other than irrigation or rain, there is little that can be done to prevent stalk rot by late summer. No hybrid has complete immunity to the stalk rotting pathogens. When choosing a hybrid, a grower should select a hybrid that is not only a high yielder, but one that has good standability and "stay-green" characteristics.
This will help assure that if stalk rot does occur, losses due to lodging will be minimal. A balanced nutrition program based on soil tests should be used.
Overall fertility levels should be adjusted to fit the hybrid, plant population, soil type, environmental conditions and management program. An excess or shortage of nitrogen can lead to increased stalk rot problems.
Producers can check their sorghum for stalk rots by squeezing the lower stem with their thumb and fingers. If the stalks crush easily, they are probably infected with one of the stalk rot organisms and could lodge at any time. Check 100 plants across the field to determine the percent of affected plants. If the percentage of stalk-rot-infected plants is high, sorghum should be harvested as soon as possible, even if it hasn't dried down adequately in the field. If the stalks are firm, the plants probably will be able to stand just fine in the field for several more weeks.
Rotation with non-susceptible crops, such as small grains and alfalfa, will reduce the severity of stalk rot but will not eliminate it. A good insect control program is a must in limiting losses to stalk rot. Pathogens can enter stalks or roots through wounds created by insects. Hail damage generally will increase the amount of stalk rot damage.
For more information, see "Stalk Rots of Corn and Sorghum," K-State publication L-741, at www.ksre.ksu.edu/bookstore/pubs/L741.pdf.
Stacy Campbell is Ellis County agricultural agent with Kansas State Research and Extension.