Articles
VARIETAL DIFFERENCES IN INCIDENCE OF INFECTION OF APPLE BLOSSOMS AND SHOOTS BY ERWINIA AMYLOVORA (FIRE BLIGHT)
In this research we examined the incidence of infections of both blossoms and shoots following artificial inoculation in the field.
The experiments were carried out on 6-year-old trees of 37 cultivars on M.7 rootstock.
The cultivars were divided into early, mid, and late blooming groups.
Each group was inoculated when most of the trees were close to full bloom.
Some cultivars were included in more than one group.
Since previous research had indicated that open blossoms were more susceptible than unopened blossoms or blossoms with petal fall, individual clusters that had at least one blossom open but none with petal fall were tagged just prior to inoculation.
The trees of each cultivar were divided into low and high dose treatments to ensure the development of an appropriate number of infections for comparisons to be made between cultivars.
The tagged clusters were inoculated by atomizing a suspension of 0.7 x 105 (low dose) or 0.7 x 107 (high dose) cells of a virulent isolate of E. amylovora onto the blossoms using compressed nitrogen.
Four weeks after inoculation the tagged clusters were scored for infection.
The incidence of infection was expressed as the proportion of inoculated clusters that became infected.
Since statistical analysis indicated that there were significant differences in the incidence of infection on the same cultivars in different bloom-time groups, cultivars could only be compared directly within each bloom-time group.
Significant differences in incidence of blossom infection occurred between cultivars in each group.
Vegetative shoots of the same trees were inoculated 6 weeks after bloom by bisecting the youngest unfolded leaf on the shoot with scissors dipped in a suspension of 108 or 106 cells/ml of E. amylovora.
Three weeks after inoculation, the proportion of inoculated shoots that became infected was recorded.
Significant differences in incidence of infection occurred between cultivars.
The incidence of blossom and shoot infections of cultivars was compared with the severity of infection of those cultivars as determined by the inoculation of vegetative shoot tips with high doses of E. amylovora.
There was a significant correlation between a cultivar’s susceptibility to the severity of infection and its susceptibility to the incidence of shoot infection.
There was also a significant correlation between the susceptibility to the severity of infection and the susceptibility to the incidence of blossom infection among the early bloom cultivars, although the correlation for the mid or late bloom cultivars was nog significant.
Complete results of this research will be published elsewhere.