Differential Orchard Fireblight Susceptibility of Young Apple Stions1

Abstract

Abstract Marked variations in the number of fireblight infections were observed on 3-year old ‘Double Red Jonathan’ stions grafted on 8 size-controlling clonal rootstocks. The average number of fireblight infections per tree ranged from 33 with the MM 109 rootstock to 0 with the Alnarp 2 rootstock. The number of fireblight infections was correlated with the number of flower clusters on the stions (r= .70**) but was not correlated with stion vigor as expressed by trunk circumference (r=-.18). The number of fireblight infections on non-flowering shoots also was correlated with the number of flower clusters on the stion (r= .60**) but was not correlated with trunk circumference (r= -.20). The susceptibility of non-flowering shoots to fireblight was influenced by the number of flower clusters on the stion, but flowers were not an essential avenue of bacterial infection, even when present. Within the Merton-Mailing rootstock series, the number of flower clusters was not correlated with the size of the stion.