BACTERIAL CANKER OF STONE-FRUITS III. INOCULUM CONCENTRATION AND TIME OF INOCULATION IN RELATION TO LEAF-SCAR INFECTION OF CHERRY

Leaf scars on the fruiting spurs of the cherry varieties Roundel (high resistance) and Napoleon (low resistance) were inoculated with Pseudomonas mors-prunorum on four separate occasions in the autumn, using, on each occasion, the same range of five different inoculum concentrations. The results, recorded the following year, showed that the percentage diseased spurs (disease incidence) and the severity of the disease symptoms (disease severity) both increased with inoculum concentration. The median threshold concentration of inoculum (T.C.50), defined as the concentration necessary to give 50 % diseased spurs, varied with time of inoculation, but on all occasions was considerably higher for Roundel than for Napoleon. In another experiment the leaf scars at the nodes of the current year's growth, inoculated at weekly intervals throughout the autumn, were found to be susceptible from the beginning of September to the latter part of October. During this period disease incidence varied considerably with time of inoculation. There was evidence that this variation was related to two factors which influenced the numbers of bacteria penetrating into leaf scars, namely, (1) the rate of evaporation of the infection drop, and (2) the rate of suction of inoculum into the vessels of the leaf traces. The experiments provided evidence of a long infection period beginning early in the autumn. It is suggested, therefore, that the timing of bactericidal sprays in the autumn be advanced and that the present concept of 'protective' sprays in disease control be replaced by one based on the eradication of external sources of inoculum.