Geostatistical analysis of Phytophthora epidemic development in commercial bell pepper fields

Spatial patterns of disease, soil population levels of Phytophthora capsici, and soil water content were characterized during epidemics of Phytophthora root and crown rot using geostatistical techniques in three commercial bell pepper fields in two consecutive years. Disease incidence and severity increased with time in all fields, ranging from a maximum linear rate of 1.01% per day and 0.060 severity units per day (0-6 scale) in the lower portion of field three in 1992 to a minimum rate of 0.33% per day and 0.011 severity units per day in the upper portion of field three in 1993. Semivariograms constructed over four directions of orientation (0 degrees, 45 degrees, 90 degrees, and 135 degrees) were used to determine the spatial dependence of mean disease severity by quadrat (1 m2) in each field over time. In most fields, disease severity displayed strong spatial dependence and a high degree of anisotropy over time, indicating strongly aggregated patterns of disease with distinct directional orientation. The within-row (0 degrees) orientation generally was the direction of maximum spatial correlation and the predominant direction of disease spread. Diagonal directions also demonstrated considerable spatial dependence in most fields, whereas spatial dependence in the across-row direction varied with soil water conditions and was lower than other directions in three of four fields. The range of spatial dependence for final disease severity averaged 15 m or greater within rows and 3-15 m across rows for fields one and two in 1992 and the lower portion of field three in 1992 and 1993. Disease severity displayed little spatial dependence in field one in 1993 and the upper portion of field three in both years. Two-dimensional distance class analysis indicated nonrandom spatial patterns of disease incidence in all fields, as well as an increasing level of aggregation of disease over time and greater spread of disease within rows than across rows in most fields. Patterns of soil water content also were spatially dependent and correlated with patterns of disease severity in fields that had distinct moisture gradients. Early season population levels of P. capsici, as estimated by either a leaf disk bioassay or soil dilution plating, did not consistently demonstrate strong spatial dependence and were not closely associated with the occurrence or severity of disease throughout the season. These results emphasized the importance of soil water in the development and spread of disease caused by P. capsici in bell pepper.