Recognizing and Managing Common Crop Disease in South Dakota

Diseases of plants may be expressed differently under varying environmental conditions or in various host species. Disease is the outward expression of an infection, yet in some cases there may be no outward evidence of disease. Outward expression may be some deviation from normal growth, change in appearance, or alteration of the quality or value of the crop. Plant diseases may be caused by biotic agents, infectious microorganisms, or abiotic causes such as chemical deposition, nutrient imbalances, or environmental factors. This publication concentrates on biotic or infectious diseases and provides information on common abiotic problems. Infectious plant diseases may be caused by fungi, bacteria, viruses, nematodes, or phytoplasmas. Each of these microorganisms has characteristics that can be keyed on in the control or management process. The most common cause of plant diseases are fungi. Outward indications may be present that indicate there is something wrong with a plant. Most often you observe symptoms of a disease—the reaction of the plant to a pathogen. Symptoms are defined as any perceptible change in host structure or function that indicates disease or disorder. Slimy, dark, roots; lodging; holes in the leaves; and blasted heads are all examples of symptoms. The pathogen that is causing the disease may or may not be visible. Sometimes the pathogen can also be seen. When the pathogen or a structure of the pathogen is present it is referred to as a sign. Signs may appear as an ooze, a cottony mass, or sometimes a black or pink mass on the plant. Some types of bacteria, fungi, nematodes, and viruses cause plant diseases. Weather and unfavorable growing conditions may interact with the pathogen to increase or decrease severity of the problem. The diseases listed here are historically found in South Dakota. Depending on the crop varieties planted or currently prevalent forms of the pathogens, diseases may become more or less important over the years. Occurrence of the diseases listed is reported as widespread, common, fairly common, or rare in South Dakota. The symptoms of each disease are described and the period when the disease is most common is provided for each crop. Be sure to monitor fields between the dates given for the presence of specific diseases