Temporal and spatial dynamics of microsclerotia of Macrophomina phaseolina in three fields in North Carolina over four to five years

The spatial and temporal dynamics of microsclerotia of Macrophomina phaseolina were investigated in three fields planted to common agronomic crops in North Carolina over 4 to 5 yr. Two fields, C-1 and C-2, were divided into 180 contiguous quadrats (6.1 X 6.1 m) arranged in an 18 X 10 grid, and one field, W, was divided into 144 contiguous quadrats (4.6 X 4.6 m) arranged in a 12 X 12 grid. During the period of study, inoculum density ranged from 35.3 to 77.5, 20.4 to 59.7, and 6.4 to 18.0 microsclerotia per 10 g of air-dry soil in C-1, C-2, and W, respectively. In C-1 and C-2, no characteristic seasonal fluctuation or general trend in number of propagules was found. In W, an increase in inoculum density in the first half of each year was generally followed by a decrease in the second half of the year. A small but significant decrease in inoculum density was found over the 4 yr of sampling in W. In each field, the propagules remained aggregated over the years of sampling as indicated by values of Lloyd's index of patchiness. The degree of aggregation fluctuated, which indicated the existence of factors with opposing effects on the level of aggregation. The level of aggregation usually decreased when the soil was tilled but was also probably affected by nonrandom deaths of propagules. Mean location (e.g., the centroid of occurrence) of the propagules remained nearly constant in each field over the period of study. Spatial correlation analysis and density maps demonstrated the development of new foci near old ones in C-1. In C-2, no new foci formed. In W, the spatial pattern of the microsclerotia was characterized by a relative high density on one side of the field and a relative low density on the other side during the years of sampling. A sensitivity of spatial correlation analysis to extreme values was observed