In this edition: - The European Fruit Magazine

G.11 Photo: ACW
22
in 2006. M9T337 showed an average lesion length
of about 65% of the shoots, while the CG rootstocks
were only visibly infested by not more than
5 to 10% of the shoots in average. Rootstocks B9
and P16 were not included in this test. According
to information from foreign sources, they showed
susceptibility to fi re blight in artifi cial infection
tests. Though, fi re blight fi eld tests in the USA
with the Gala and McIntosh varieties indicated
that besides the CG rootstocks G.16, G.41, G.11,
also the B9 shows fi eld-tolerance to fi re blight.
In commercial orchards in the USA, these results
appeared to be confi rmed; it was stated that
the the amount of rootstock blight via the scion
variety was signifi cantly lower on B9 rootstock.
That is why B9 is recommended in the USA as a
replacement for M9 in fi re blight areas.
Discussion
In comparison with other European trial results,
the CG rootstocks are more vigorous in the ACW
trials. Vigor of the weakest CG rootstocks, G.41
and G.11 in Wädenswil was comparable to the
growth of M9Pajam2, which is the most vigorous
M9 type, while in other trials the growth of the
G.41 and G.11 lay, with a few deviations, between
the M9T337 and M9 Pajam2. With regard to overall
yield, the G.41 and G.11 performed better in
all the trials than the M9 standard. The relative
yield of the named rootstocks is comparable.
Growth and relative yields of the rootstocks B9
and M9T337 were at the same level. For many
years, the B9 has been upheld in Europe as an
alternative to the M9, with growth between the
M9Fleuren56 and the M9T337.
Conclusion and outlook
The rootstocks B9, G.11 and G.41 showed the best
agronomical production characteristics in the trials
together with the fi re blight susceptible standard
M9T337. If the fi eld tolerance observed in the
USA with regard to fi re blight will be confi rmed
in future tests carried out by ACW in Swiss commercial
orchards, B9 would be a suitable alternative
for M9. G.11 should be pursued due to its fi re
blight tolerance and its interesting production
characteristics. Unfortunately, G.41 cannot be
propagated well and is therefore not very likely
to establish itself in practice. Larger numbers
of trees with the Braeburn, Gala, Milwa (Diwa®)
and La Flamboyante (Mairac®) apple varieties
on the rootstocks B9 and G.11 will be planted in
spring 2009 in commercial orchard trials in eastern
Switzerland. Currently, we only recommend
these rootstocks for further experimentation on
commercial orchard level. More experience with
regard to their potential for reduction of fi re
blight pressure has to be gathered.