Biology Reference
In-Depth Information
Fig. 2 Venturia
inaequalis
, the apple-scab
fungus. A one-celled dark
conidia of
Fusicladium
stage; B perithecium with
two-celled ascospores
spore content. When a spore, carried by wind,
arrives on a young leaf or bud, it penetrates the
cuticle with a germ tube and develops a layer of
branching mycelium just under it. The scab spot
is evident in about 10 days, when brown
conidiopores bearing olive brown, one-celled,
somewhat pointed spores appear on the surface.
Secondary infection occurs when these conidia
are carried to new infection courts.
The expulsion of ascospores proceeds in
a series of discharges over a rather long period,
up to 3 months, starting in February, on the West
Coast, but a shorter period, beginning in April, in
New York. Germination and infection take place
from 41
to 79
F. Length of wetting period
necessary for primary infection decreases as the
temperature rises
fungus. Nitrogenous fertilizers increase yield of
the fruit but also susceptibility to scab.
Protective spraying, having a chemical film on
blossom, fruit, or foliage at all times when
weather makes infection probable, is the only
real answer to scab. This may mean more than
a dozen applications in a wet year and a minimum
of five any season, a program more suited to the
commercial grower than to the amateur. Timing
is all-important, and most states have a spray
warning service that tells of imminent discharge
of ascospores. Any spray schedule must be tai-
lored for the locality, the season, and apple vari-
eties grown. The apple grower gets this specific
help from his county agents.
Venturia pyrina (Anamorph,
Fusicladium
pyrorum
).
Pear Scab
, general on pear, also on
quince, similar to apple scab. The pear species of
Venturia
overwinters in fallen leaves and also in
affected twigs; the perithecia mature somewhat
later than those of apple scab. Conidia are formed
on pear twigs and washed to leaves and fruit.
Pear scab is not serious except on such varie-
ties as Flemish Beauty, Winter Nelis, Seckel,
Anjou, Bosc, and Duchess. Bartlett pears are
rather resistant.
13 to 18 h of continuous wet-
ting at 43
F and only 4 to 6 at 70
F. Secondary
infection from conidia continues all season in
rainy periods and even in storage scab may
show up on apples infected just before picking.
Control
No varieties are immune to scab. Resis-
tance varies with the season and the part of the
country. McIntosh apples are very susceptible;
Baldwins are fairly resistant but may scab badly
some years. There is more than one strain of the
