Agriculture Reference
In-Depth Information
Apple proliferation disease
The first symptoms of this disease are the growth of upright secondary shoots
to give 'witches brooms' at the ends of branches through suppression of apical
dominance. Average fruit weight can be reduced by
%. Infected young
treesgrowpoorlyandalthoughtheremaybesomerecoveryinlateryearsyields
and fruit size usually remain depressed. In 'recovered' trees with normal fruit
size crop losses can still be
-
%. The disease is widespread in central and
southern Europe under climatic conditions suitable for wine growing. 'Golden
Delicious', 'Jonathan' and 'Cox's Orange Pippin' are more sensitive than other
cultivars but damage is possible on all cultivars.
Trees with the symptoms of apple proliferation contain mycoplasma-like
organisms (MLOs) in the sieve tubes. These are thought to be the causal
organisms and leaf sucking insects are probably vectors (Kunze,
-
).
Apple rubbery wood disease
Affectedappletreesarestunted.Shootsandbranchesupto
yearsoldshowin-
complete lignification and are often bent to the ground. Strong shoots may de-
velop from the base of the trunk. Sensitive cultivars include 'Lord Lambourne'
(the standard indicator), 'Golden Delicious' and 'Gala'. Even cultivars like
'Cox's Orange Pippin' that do not show the characteristic symptoms have re-
duced vigour and cropping. The causal organism is not known with certainty.
No vector has been identified and spread in the orchard is very slow or ab-
sent. The use of healthy planting material provides practical control (Adams,
b).
Pear decline
Trees infected with pear decline MLO may show quick decline, slow decline
or leaf curl. Trees with quick decline wilt suddenly and die within a few days
or weeks. This is most evident in hot dry weather and on
P. ussuriensis
and
P.
pyrifolia
rootstocks. It may also occur on
P. communis
rootstocks but the inci-
dence is less and the trees often show red-leaf symptoms before wilting. Slow
decline is characterized by progressive and fluctuating weakening of the trees.
Terminal growth and leaf production is restricted and the leaves, which are
small, leathery and light green with uprolled margins, may redden and drop
prematurely. Trees may die within a few years of infection or live for many
years. Leaf-curl symptoms may be shown where the decline is less severe and
on relatively tolerant rootstocks, e.g.
P. communis
,
P. calleryana
and
P. betulifolia
(Seemuller,
).
Although quince rootstocks are much less susceptible than
P. communis
root-
stocks, to the extent that their use was recommended as a control measure,
