Susceptibility of broccoli cultivars to bacterial head rot: in vitro screening and the role of head morphology in resistance

Head rot is a major disease of broccoli caused by the soft rot pathogens Ps eudomonas fluorescens and Erwinia carotovora. Two in vitro pathogenicity te sts were evaluated as a methods to identify broccoli cultivars susceptible or resistant to bacterial head rot. One test used mature heads excised from the plant and inoculated with squares of cotton lint which had been soaked in a bacterial suspension. The other test involved stab-inoculating axenic ally grown seedlings. With the excised head test, susceptible cultivars sho wed a black soft rot, whilst less susceptible or moderately resistant culti vars showed only watersoaking, or browning and slight softening of the tiss ue. No cultivar was completely resistant. Ten cultivars were tested, and th eir susceptibility ratings corresponded with previously recorded field data , with one exception. This laboratory test could be used to screen for susc eptibility to head rot in broccoli breeding programmes. The seedling test d istinguished differences in aggressiveness among bacterial isolates but not cultivar susceptibility. Increasing head size correlated negatively with d isease resistance. Head shape, i.e. cultivars which showed a domed shape ra ther than a flat shape, was positively correlated with disease resistance. Thus small domed heads are more resistant to head rot than large flat heads . Other morphological characteristics, viz. floret prominence and number, a nd sepal stomatal number were not correlated with host resistance.