Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: This problem, also called scald, is most serious in the hot desert valleys of the southwestern United States, subtropical regions in eastern Australia, and western Asia and northern Africa (Middle East) where fields are established and irrigated under high temperatures. The disorder also occurs to a limited extent in temperate regions where soil is saturated for long periods after irrigation or rainfall. The disorder is related to the combination of high soil and water temperatures and saturation of the soil with water. The critical factor is the length of time soil is saturated at high soil temperatures; that is, the higher the temperature, the shorter the time necessary for flooding to cause damage. Symptoms: When the air temperature approaches 39 degrees C, alfalfa becomes extremely susceptible to flooding injury. Foliage of all affected plants in the flooded area fades to light green and wilts, even though the soil is wet. Roots of some plants completely rot and have a putrid odor when removed from the soil. The xylem tissue of affected roots that are not rotted becomes brown and necrotic. Affected plants die within 3-4 days after irrigation. Recently mowed plants or those run over by a truck or tractor are much more susceptible to flooding injury than are noninjured plants. Scald may be confused with Phytophthora root rot because both occur in saturated soil conditions, but scald occurs only at high soil temperatures. Management: Flooding for short periods (about 4 h) when temperatures are high reduces the likelihood of injury; however, some soils remain saturated long after irrigation because of the soil type, the slope of the land, and the length of the irrigation run. Irrigation should be avoided when the air temperature exceeds 40 degrees C. Newly mowed plants should not be irrigated until enough regrowth occurs to prevent submersion of entire plants.