Agriculture Reference
In-Depth Information
Comparative low temperature hardiness of
rootstocks
Hardiness as measured by minimum survival tempera-
ture with no effect on growth (MST) and field hardiness.
Table
.
Mean MST
(
◦
C)
Rootstock
Hardiness class
'M.'
−
. a
Very sensitive
'M.'
−
. b
Moderately sensitive
'A.'
−
. c
Hardy
'J.'
−
. cd
Moderately hardy
'B.'
−
. cd
Hardy
'O.'
−
. cde
Hardy
'P.'
−
. e
Hardy
Means followed by the same letter are not significantly
different by the DMR test at the . level.
Hardiness classes on the scale developed from Quamme
().FromQuammeandBrownlee().Reproduced
with permission.
tolerance. Rom and Brown (
) found 'M.
' and 'M.
' to be relatively
tolerant and 'MM.
' least tolerant.
Although there are differences in drought sensitivity between rootstocks,
with 'MM.
' being
intolerant, this is not usually an important factor in rootstock choice because
of the widespread use of irrigation. However, it does influence the choice of
planting density because trees on 'MM.
' being relatively tolerant and 'M.
', 'M.
' and 'MM.
' are much smaller, relative to those
on other rootstocks, when grown on sandy soils under rainfed conditions.
Resistance to pests and diseases
Different plant diseases are of variable importance in different apple growing
regions depending on local environmental conditions. Fire blight, caused by
the bacterium
Erwinia amylovora
(see Chapter
), is of major importance
in warm, humid apple growing areas such as the eastern United States. Crown
rot and collar rot, caused by species of
Phytophthora
fungi especially
cactorum
and
syringae
, are most severe where there are recurrently wet and waterlogged
soils. Each of these diseases can be of such severity that trees on intolerant
(sensitive) rootstocks die. Differences between rootstocks in their sensitivity to
latent viruses are particularly important in cases where the scion graftwood
may be contaminated and graft unions fail, and where the virus can be spread
by nematodes as is the case with apple union necrosis and decline due to
tomato ringspot virus (Ferree and Carlson,
,p.
).
