Agriculture Reference
In-Depth Information
reports of infection of resistant rootstocks on occasion there has been no serious
breakdown of resistance.
The rosy apple aphid (
Dysaphis plantaginea
Pass) causes leaf curling on young
shoots. Resistance is available in material derived from
Malus robusta
.
The rosy leaf curling aphid (
Dysaphis devecta
Wlk) also causes leaf curling and
galls. 'Fiesta', a resistant cultivar derived from 'Cox's Orange Pippin', carries
the
Sd
gene for resistance to biotypes
and
of the aphid (Roche
et al.
,
).
Suckers
Theapplesucker(
Psylla mali
Schmidberger)isusuallywellcontrolledbyroutine
sprays, but pear sucker (
Cacopsylla pyricola
Foerster) is a major pest. Infested
leaves turn brown and often drop, and the fruits drop prematurely or are
small and of poor quality. The feeding on foliage suppresses root growth and
reduces tree vigour. The psylla also transmits the MLO which is the cause
of pear decline. Pear psylla is controlled by chemical sprays, but appreciable
resistance to many insecticides has developed (Anon.,
b). Host resistance
to psylla has been identified in several species of Asian pear as well as in
P.
communis
but no genotype combines this resistance with resistance to Fabraea
leaf spot (
Fabraea maculata
Atk) and high fruit quality (Bell and van der Zwet,
).
Mites
The fruit tree red spider mite (
Panonychus ulmi
Koch) became a serious orchard
pestlargelybecauseofinsecticidalsprayprogrammeswhichkilleditspredators
and because of its own ability to develop resistance (Anon.,
c). The first
symptom is a minute speckling of the leaves; then the foliage loses colour and
becomes first dull green, then brownish green or bronze, and brittle. White
skins cast by mites while moulting are conspicuous on the underside of the
leaves.
Injury is caused by the mites sucking sap and damaging leaf cells. Infestation
checks shoot growth before it affects net CO
uptake per unit leaf area (Avery,
; Avery and Briggs,
). Reduction in CO
assimilation is slight until
% of the leaf is speckled and bronzing begins. The photosynthetic surface of
the plant is decreased by up to
% by infestations which cause reductions in
leaf size and number, and premature defoliation also decreases the leaf surface
area. The growth of the root systems is decreased before that of the shoots and
to a greater extent.
The multiplication of fruit tree red spider mites is greater on some cultivars,
e.g. 'Discovery' and 'Worcester Pearmain', than on others such as 'Cox'.
