EPISODIC DYNAMICS OF TAR SPOT OF ANNUAL LESPEDEZA WITHIN AND AMONG YEARS

The temporal and spatial dynamics of six tar spot epidemics were exami ned at two sites in mid-Missouri during 1990 to 1992. The sites, 48 km apart, differed in topography (flat versus slope), soil texture (silt loam versus silty-clay loam), and cropping history (orchard versus ro w crops). Temporal dynamics of disease incidence differed more among y ears than between sites. During 1990, temporal dynamics of the two epi demics followed a classic monotonic increase, whereas disease increase was episodic for the four epidemics observed during 1991 and 1992. Di sease increase was associated with shading of the plant canopy or with the presence of dodder. Disease increase was related inversely to int ercepted photosynthetically active radiation. Taken together, these ob servations suggest that factors that increase shading in the lespedeza canopy also increase tar spot incidence. For only two of the six epid emics was there a significant relationship between disease incidence ( proportion of leaflets diseased) and disease severity (proportion of l eaflet area diseased), with disease severity rarely exceeding 10%. The se observations suggest that relatively low tar spot incidence may res ult in significant leaf losses, which would reduce lespedeza hay quali ty and yield. Further, this study demonstrates the importance of exper imental repetition in both space and time to fully appreciate the rang e of disease dynamics for a single pathosystem.