Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Plant Pathology


Alfalfa seedling rot in South Dakota, as in other parts of the United States and the world, is a destructive alfalfa disease. In many fields its abundance prevents the establishment of adequate alfalfa stands, requiring the use of excessive amounts of seed or chemical seed treatments to establish such stands. The absence or near absence of seedling rot in other fields sown with normal amounts of seed often results in stands that are too dense for best growth of plants under semi-arid conditions. Soil-inhabiting fungi of the genus Pythium generally have been shown to be responsible for this disease. Among these, Pythium debaryanum, P. irregular, P. splendens, and P. ultimum hold first place, followed by P. paraecandrum and P. vexans. To account for the differences among soils in the incidence of seedling rot, several possible explanations may apply; namely, (1) that the soil and weather factors were sufficiently different in the various soils to affect disease development, (2) that antagonistic microorganisms to infection were more suppressive in one soil than another, (3) that the quantity of Pythium inoculum was more abundant in one soil than another. All of these unquestionably contribute one way or another to the incidence of disease, but the concept involving quantity of Pythium inoculum in the soil seems most pertinent and appears to have been neglected in most studies concerned with alfalfa seedling rot. The objective of the present study was to determine whether the different amounts of alfalfa seedling rot produced by various South Dakota soils could be attributed to different quantities of Pythium inoculum in such soils.

Library of Congress Subject Headings

Alfalfa -- Diseases and pests
Soils -- South Dakota


Includes bibliographical references



Number of Pages



South Dakota State University


No Copyright - Non-Commercial Use Only