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 40°C) that can occur in the spring in Kansas do not have a large effect on conidial germinability; however, temperatures of 40°C 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.