Agriculture Reference
In-Depth Information
with pear were subsequently found to be sensitive to a virus almost universally
present in pear cultivars (Cropley,
). In Europe, the main apple culti-
vars are tolerant of 'chat fruit' which has very severe effects on the cv. 'Lord
Lambourne'. The 'Malling' and 'MM.' apple rootstocks are tolerant of the
widely present stem-pitting virus whereas the cold-hardy rootstock 'Virginia
Crab' is not. Virus-induced incompatibility has been discussed in Chapter
(pp.
).
In common with most woody plants, apples and pears contain chemical in-
hibitors that inactivate viruses. The relative resistance of the trees may explain
why virus spread is usually much slower in them than in herbaceous crops.
The degree of virus infection is, however, increased by the practice of veg-
etative propagation which perpetuates infection from one generation to the
next. Heat treatment of shoot tips at
-
◦
C, producing shoots by tissue cul-
ture from meristems (which are generally free from virus), or a combination
of these methods is now generally used to provide virus-free source material
(Campbell,
).
Virus-free apple and pear trees grow more vigorously than those infected
with some of the common European viruses (apple chlorotic leaf spot virus,
Spy
decline, rubbery wood, stem pitting virus, stem grooving virus and
scaly bark virus for apple, and vein yellows for pear). They give heavier crops
but yields per unit tree size are similar to, or lower than, those of infected
trees (Posnette,
). The lower cropping efficiency
of virus-free trees may be a consequence of their greater size (cf. the relation-
ship between tree size and cropping efficiency shown in rootstock studies).
Virus-free trees of 'Golden Delicious' apple have smoother fruit skin with less
russeting (van Oosten
et al.
,
; van Oosten
et al.
,
).
Apple chlorotic leaf spot virus (ACLSV)
This closterovirus was first described from apple but is now known to occur
world-wide and to be common in all fruit tree and woody ornamental species
of
Rosaceae.
Most commercial apple cultivars are symptomlessly infected. Leaf mottle
and ring mosaic of pears are associated with ACLSV. Many pear cultivars
remain symptomless but 'Beurre Hardy', 'Beurre Bosc' and 'Williams'' are
susceptible and there are significant reductions in growth and yield on mixed
infection with pear vein yellows and quince sooty ringspot (Delbos and Dunez,
).
Apple chat fruit disease
The causal organism of this is not identified. It is restricted to apple. Symptoms
are seen only on the cvs. 'Lord Lambourne' and 'Tydeman's Early Worcester'
(Adams,
a). Fruits are small and discoloured.
