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

The effects of initial inoculum levels on spatio-temporal development in bl ack rot of cabbage from artificial sources, one per plot, were evaluated in replicated field experiments. The results support the hypothesis that blac k rot is a polycyclic disease. Its development was inoculum-dependent becau se the progress rate of epidemics and spatial spread were both positively c orrelated with the strength of the source. Fast disease development was rel ated to the number of rain days. The spread of black rot, associated with t he 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 prim ary focus. Maximum distance of black rot symptoms from the source of the fo cus 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 r esulted in faster outward spread of black rot, and differences between low and high levels were generally significant. Under the conditions of the exp eriments, performed during three relatively dry seasons, a single source of infection measuring 0.5x0.5 m was not capable of spreading disease over al l the plants in a plot of 6.5x6.5 m. The results imply that severe disease where whole crops are infected, as observed regularly in The Netherlands, c an only originate from a large number of small foci per field.