Spatial and temporal spread of soybean stem canker from an inoculum point source

Incidence and severity of stem canker arising from inoculum point sources of Diaporthe phaseolorum var. caulivora was quantified on resistant (Bay), intermediate (DPL105, W550), and susceptible (Bedford) soybean cultivars in the field during 1986 and 1987. Secondary production of spores arising from perithecia or pycnidia of D. p. caulivora was not detected during either growing season. Disease progress was described by the Weibull model. The shape parameter (c) of the model was equal to the idealized c value of a monocyclic disease for Bedford. Infection rates (k) declined as resistance levels of cultivars increased (Bedford, k = 0.12; W550, k = 0.09, Bay, k = 0.02 per week). An exponential model fit the disease gradients. An increase in stem canker spread in 1987 compared with 1986 was accompanied by a reduced gradient steepness. Increasing levels of cultivar resistance generally increased gradient steepness, particularly when severity was used to measure disease. Yield losses in intermediate and susceptible cultivars were linearly related to stem canker incidence with regression lines having similar slopes. No yield loss occurred in Bay as lesions failed to elongate or cause severe necrosis. The greater spread of stem canker in 1987 compared with 1986 was correlated with cumulative rainfall. Differences in directional spread were attributed to water movement patterns in plots