Spatio-temporal development of black rot (X. campestris pv. campestris) in cabbage in relation to initial inoculum levels in field plots in The Netherlands

The effects of initial inoculum levels on spatio-temporal development in black rot of cabbage from artificial sources, one per plot, were evaluated in replicated field experiments. The results support the hypothesis that black rot is a polycyclic disease. Its development was inoculum-dependent because the progress rate of epidemics and spatial spread were both positively correlated with the strength of the source. Fast disease development was related to the number of rain days. The spread of black rot, associated with the primary gradient, at first resulted from allo-infection, later followed by a phase during which allo-infection was complemented by auto-infection. Three-dimensional maps of disease severity showed the dominance of the primary focus. Maximum distance of black rot symptoms from the source of the focus was limited to a few metres so that damage to cabbage by focal inoculum was limited to the plants close to the source. Spatio-temporal development and initial inoculum were related. High inoculum levels in point sources resulted in faster outward spread of black rot, and differences between low and high levels were generally significant. Under the conditions of the experiments, performed during three relatively dry seasons, a single source of infection measuring 0.5 × 0.5 m was not capable of spreading disease over all the plants in a plot of 6.5 × 6.5 m. The results imply that severe disease where whole crops are infected, as observed regularly in The Netherlands, can only originate from a large number of small foci per field.