Agriculture Reference
In-Depth Information
the conidia often exhibit low viability (Holb
et al.
, 2004). An exception to the sexual
infection paradigm occurs in areas of Israel where winter temperatures are
unfavourable for sexual reproduction. Here,
V. inaequalis
has asexual infection
cycles only; spring epidemics arise from conidia produced in diseased leaves that
persist on shoot tips through the mild dormant period (Boehm
et al.
, 2003).
The sexual infection cycle arises from ascospores released from pseudothecia.
The distinctive ascospores of
V. inaequalis
are recognized based on their colour,
morphology and size. Mature ascospores (11-15 x 5-7 µm) are two-celled,
yellowish-green to tan, with smooth walls. The upper cell is shorter and wider than
the lower cell. The ease of recognizing these spores has been very useful and
important to studies on the epidemiology of apple scab. Mature conidia (12-22 x
-
6 9 µm) are usually one-celled, yellowish-olive, and ovate to lanceolate.
Venturia inaequalis
overwinters in infected fallen leaves on the orchard floor. In
late autumn and early spring, pseudothecia are initiated in these dead leaves.
Maturation of pseudothecia in late winter and early spring is favoured by alternating
periods of wetness and dryness. Normally, some pseudothecia produce mature
ascospores, beginning around the green-tip stage of flower bud development
(Gilpatrick and Szkolnik, 1978). Maturation and discharge of ascospores usually
lasts for 9 to 12 weeks. Peak ascospore maturation and discharge normally occurs
around the bloom period (Gilpatrick and Szkolnik, 1978; Villalta
et al.
, 2002). When
the leaf litter becomes wet, ascospores are forcibly ejected about 3 mm (range
-
concentrations of ascospores are highest close to the ground and decrease rapidly
with height above the ground (Aylor, 1995; Aylor and Qiu, 1996).
Germination begins, provided a film of moisture is present, as soon as
ascospores and conidia land on the host. About 9-17 days are required from
inoculation to the appearance of the olive-green, velvety scab lesions (Mills and
LaPlante, 1951; Tomerlin and Jones, 1983). The average temperature after
penetration governs the length of the latent period, provided humidity is not limiting
(Tomerlin and Jones, 1983).
Conidia within new scab lesions are disseminated by splashing rain and by air.
Interestingly, dry removal of conidia from lesions is common, and the frequency of
conidia in orchard air on days without rain is often diurnal; more conidia are trapped
in a volumetric spore trap in the afternoon and evening than in the morning (Sutton
0.1 8.1 mm) from the leaf surface (Aylor and Anagnostakis, 1991). Aerial
et al.
, 1976). Conidial germination and infection occurs under similar conditions
as those for ascospores (Keitt and Jones, 1926; Moore, 1964; Schwabe, 1980).
Secondary infection on fruit can occur in autumn but visible symptoms may not
develop until after the fruit has been stored for several months. Due to the loss of
ontogenetic resistance, the pathogen can also build up on apple leaves, particularly
on the under surface, late in the season (Kollar, 1996; Li and Xu, 2002; Sutton
et al.
,
1976). Infections on old leaves may either develop as small faint lesions with diffuse
colonies during the autumn or as symptomless infections consisting of mycelium
and conidia when examined microscopically (Li and Xu, 2002). Such lesions may
give rise to pseudothecia for the next growing season.
