Agriculture Reference
In-Depth Information
Apple mildew
Apple mildew, caused by
Podosphaera leucotricha
(Ell. & Ev.) Salmon, is currently
one of the most important diseases of apple in England and many other fruit-
growing countries. Its conidia and hyphae overwinter in buds. Floral and
foliar organs emerging in spring from infected buds emerge totally or partially
coated with powder mildew. This is primary mildew (Butt,
). Infected fruit
buds give dwarf leaves and the flower buds remain closed: these organs wither
in late spring. Infected vegetative terminal buds produce totally mildewed
shoots. Those not totally colonized in the previous year grow as 'white shoots'
for several weeks then shed leaves. During spring and summer new conidial
infections give colonies of secondary mildew, mainly on the undersurface of
leaves. The shoot tip, newly formed buds and, in some cultivars, e.g. 'Jonathan',
fruitlets, may also be infected.
The quantity of primary mildew depends on the intensity of bud infection in
the previous year and on winter temperatures. A few hours at
−
◦
to
−
◦
C,
−
◦
Cinlatewinter,canalmosteradicatetheoverwintering
fungus.Infectioninspringandsummerisfavouredbyhighhumidity,especially
at night.
Mildew reduces leaf area, photosynthesis and the number of fruiting spurs.
It also induces russet and reduces fruit size. Butt
et al.
(
−
◦
to
orevenat
) showed that an
increase from
%to
% of mildewed leaves in mid-summer resulted in a
reduction of
% in number of shoots per tree,
% in number of leaves per
shoot,
% in fruit size,
% in crop weight and
% in crop value for 'Cox's
Orange Pippin'.
There is considerable variation in resistance among commercial cultivars,
'Discovery' being highly resistant and 'Crispin' ('Mutsu') and 'Jonathan' highly
susceptible. Breeding programmes have involved the use of partially resistant
parents, e.g. 'Prima'. More closely targeted breeding involves the gene
Pl
derived from
M
.
×
zumi
open-pollinated (Knight and Alston,
) and
Pl
from
M
robusta
and
Pl
w
from a North American ornamental crab apple
'White Angel' (Laurens,
.
×
).
Control is by sprays of sulphur on cultivars which are not damaged by this, of
dinitrophenyl esters, bupirimate, fenarimol, nitrothal-isopropyl, triadimefon,
myclobutanil and penconazole. Sprays are generally applied, where needed,
every
days from before flowering until late summer when most
extension growth has ceased. It is important that the sprays do not damage
fruit skin, do not interfere with biological control programmes (some, e.g.
binapacryl, are incompatible with the use of predaceous mites to control red
spider mite) and do not check shoot growth excessively (e.g. triadimefon is an
ergosterol biosynthesis inhibitor). Control is also achieved by hand pruning to
remove infected shoots, fruits, etc. Use of a bud-penetrating surfactant, either
,
or
