Articles
EFFECT OF PRUNING AND OTHER ORCHARD FACTORS ON THE ROOTSTOCK PHASE OF FIRE BLIGHT
Thus, it is often recommended that fire blight infections be pruned out of young apple trees during the growing season to prevent rootstock infection.
The effectiveness of this practice was evaluated on field grown trees of Empire, Jonamac, and Liberty on M.26 rootstock and trees of Empire, Liberty, and Mutsu on M.9 rootstock.
Pruning out scion infections 3 weeks after blossom inoculation and then repeatedly during the growing season gave variable results in 3 years of experiments.
In 1995 and 1997 pruning had no beneficial effect on eventual fate of the rootstock, but in 1996 pruning did appear to reduce rootstock death.
However, 2 to 3 consecutive years of pruning greatly reduced fruit yield.
In comparison to non-inoculated trees, fruit yield on pruned inoculated M.9 trees (2 yrs of pruning) and M.26 trees (3 yrs of pruning) were reduced by over 60%. Despite these high yield reductions, an economic analysis indicated that pruning out fire blight infections was cost effective.
Accumulated net present value (NPV) 20 years after planting for a M.26 planting (672 trees /ha, central leader) without fire blight was $4,684, compared with -$47 for unpruned inoculated trees and $521 for pruned inoculated trees.
The improved profitability in the pruned treatment was a result of the reduced tree loss in 1996. The analysis also indicated that replanting, rather than pruning fire blight out of infected trees, may be cost effective if severe fire blight occured in trees in their 1st or 2nd leaf, however, losses from replanting greatly increased for trees in the 3rd, 4th, or 5th leaf (in the above trial M.26 trees were in their 3rd, 4th, and 5th leaf in 1995, 1996, and 1997, respectively). For example, if a fire blight epidemic required replanting 50% of the trees in the 2nd yr and 3rd yr, accumulated NPV was reduced to $3,191 and $2,001, respectively, whereas replanting in the 5th yr or 6th yr reduced NPV to -$6,360 and -$9,290, respectively.
Field inoculation of shoots of newly planted, non-bearing trees on M.26 indicated that bacteria moved systemically into the rootstock of 1st year trees, but did not result in rootstock infection.
In contrast, blossom inoculation of 3-yr-old trees on M.26 in their 2nd year of fruiting, and blossom inoculation of 2-yr-old trees on M.9 in their first year of fruiting, resulted in 11% and 5% rootstock infection, respectively.
This suggests that trees coming into bearing may be at greater risk of rootstock infection than non-bearing trees.
Field inoculation of 2-yr-old trees of Empire, Liberty, and Jonamac on M.26, indicated that infected rootstock suckers were strongly correlated with rootstock infection.
In addition, infections of the scion were very important, with risk of rootstock infection increasing with the number of scion infections.
The susceptibility of the scion cultivar, and the frequency of burrknots, did not seem to influence the frequency of rootstock infection.