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Why Is Your Grain Sorghum Falling Down?
Lodging in grain sorghum is showing up in the Grain Sorghum Belt, just as farmers are getting ready to harvest the crop. The lodging is due to a couple of stalk rot diseases, and there's not much you can do at this point.
Those areas that are already lodged? Those are wildlife feed at this point. So protect the yield you have left in the field, says Ignacio Ciampitti, cropping systems specialist at Kansas State University. "Don’t wait to harvest these fields. You cannot assume the rest of the field will stay standing," he emphasizes.
Check for stalk rot by squeezing the lower stem of the plant. If the stalks crush easily, they are probably infected with one of the stalk rot organisms and may 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 will probably be able to stand just fine in the field for several more weeks if necessary.
Where sugarcane aphid pressure was heavy this year, that will likely increase the incidence of stalk rot, and producers should be prepared to harvest as soon as the grain is ready.
How we got here
Stalk rots - which include charcoal rot and Fusarium rot - are a fairly common occurrence in grain sorghum; they reduce grain yields about 5% each year. This year, however, the yield loss will be much more. One reason is the wet spring, during which farmers planted grain sorghum in less-than-ideal conditions. In many cases, soils were wet at planting, and impaired root development. Since then, much of the Grain Sorghum Belt has been dry, and coupled with compaction in these fields, roots never fully developed.
Sorghum plants typically have a thinner stalk than corn and are more prone to lodging.
Although charcoal rot and Fusarium rot are two different diseases, there are some similarities, says Doug Jardine, plant pathologist at KSU. For both diseases, 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 easily crushed 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 may be tan colored (Fusarium stalk rots); red or salmon, (Fusarium and Gibberrella stalk rots); or grayish-black (charcoal rot).
Stress makes it worse
Any stress on a crop can increase both the incidence and severity of stalk rot. When the carbohydrates used to fill the grain become unavailable due to nutrient shortage, drought stress, leaf damage from insects, hail, disease, or reduced sunlight, the plant uses nitrogen and carbohydrate reserves stored in the stalk to complete grain fill.
The loss of nitrogen and carbohydrate reserves resulting from leaf damage weakens stalk tissues and results in increased stalk rot susceptibility. Early-maturing hybrids are generally more susceptible than full-season hybrids.
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 also 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.
Jardine suggests growers rotate with nonsusceptible crops, such as small grains and alfalfa, to reduce the severity of stalk rot.
A good insect control program is a must in limiting losses to stalk rot. In addition to the effect of leaf damage on stalk integrity, pathogens may enter stalks or roots through wounds created by insects.
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