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Gradients of Tan Spot of Winter Wheat from a Small-Area Source of Pyrenophora tritici-repentis . J. SONE, Graduate Student, Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502. W. W. BOCKUS, Professor, Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502; and M. M. CLAASSEN, Associate Professor, Department of Agronomy, Kansas State University, R.R. 1, Box 146, Hesston 67062. Plant Dis. 78:622-627. Accepted for publication 7 March 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0622.

Wheat plots (15.2 15.2 m) were established near Manhattan and Hesston, Kansas, to measure changes in area under the disease progress curve (AUDPC) with distance from a small-area (0.6 0.6 m) source of Pyrenophora tritici-repentis. At both locations during 2 yr, relatively steep gradients of tan spot occurred away from the primary inoculum. Exponential decay equations significantly (P < 0.0001) fit data and estimated that a 90% reduction in AUDPC occurred at 3.6-5.4 m from the source. These results indicate that disease spread by primary and secondary inoculum is limited in Kansas, and that fields where P. tritici-repentis does not occur will not be affected greatly by neighboring, diseased fields. According lo data obtained in vitro and in the greenhouse, high temperatures (32.5 and 40C) that can occur in the spring in Kansas do not have a large effect on conidial germinability; however, temperatures of 40C for 8 hr per day for 2 days immediately after inoculation significantly reduced disease severity. Thus, high temperature may be partially responsible for the relatively limited disease spread in Kansas but is probably not a major factor.