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.