Agriculture Reference
In-Depth Information
periods can allow more efficient fungicide use. Captan and dithianon will
stop germ tube growth before infection is established and systemic fungi-
cides can be effective up to
hours after infection (Byrde,
; Richter,
a).
The main commercial cultivars are susceptible to scab. Genes for resis-
tance to this have been identified, notably the dominant allele
V
f
from
Malus
floribunda
which was used to produce cultivars including 'Prima',
'Priscilla', 'Liberty', 'Freedom', 'Priam', 'Primicia' and 'Goldrush'. The
M
×
floribunda
donor has totally unacceptable fruiting characteristics and,
despite back-crossing programmes to produce these cultivars, none has yet
gained widespread acceptability. 'Antonowka' is used as a polygenic source of
field resistance to all races of apple scab and 'Laxton's Fortune', 'Dulmener
Rosen' and 'Discovery' appear to have some breeding value ( Janick
et al.
,
.
×
;
White and Bus,
).
Pear scab
Venturia inaequalis
may be found on pears but pear scab is generally caused
by
V. pirina
Aderhold. This fungus attacks only pear where it causes scab of
shoots, leaves, fruits and buds. Shoot infections can give swellings which later
burst. Infected fruits are deformed and deeply cleft. Pear scab development is
facilitatedbysurfacewetnessandhightemperature(Richter,
b).
V. nashicola
is an important pathogen of Nashi pear,
P. pyrifolia
(Burm.) Nakai, in Japan
(Kajiura,
).
Apple canker
This is caused by
Nectria galligena
Bres. The fungus invades injuries to the bark
such as pruning wounds, scab lesions and, especially, leaf scars. It causes a
sunken area of bark, often surrounded by a ridge of tissue only partly invaded.
The cankers increase in size and may girdle the stem, killing the part above
the girdle. A rot around the eye of the fruit can also occur. Control is by cutting
out infected wood and restricting nitrogen supply. Fungicidal paints may be
applied to the cankers and benomyl and related fungicides are effective when
sprayed in spring and summer (Byrde,
). Canker is readily carried over
on young trees at planting time and nursery trees should be treated, e.g. with
copper compounds, to ensure clean orchard planting material.
Collar rot and fruit rot
These are caused by
Phytophthora cactorum
(Leb and Cohn) Schroet. and
P.
syringae
Kleb. which attack the rootstock or scion cultivar around soil level.
