Agriculture Reference
In-Depth Information
spring are extremely important in the survival of
P. triticina
when snow cover is not
present. Positive average deviations of the fungal temperature equivalent function
and the cumulative rainfall indicate periods during which re-infections may occur in
the overwintering phase of the epidemic. The study of winter climatic conditions
permits evaluation of the possibilities of pathogen multiplication and partially
enables forecasting of epidemic changes in spring. Afterwards, leaf rust severity is
highly correlated with cumulative degree-days >20°C following inoculation (Subba
Rao
et al.,
1990).
15.3 SURVIVAL AND SEXUAL STATE
15.3.1 Oversummering
Both powdery mildew and cereal rusts oversummer on volunteer crops in the
asexual stage, infect the autumn-sown crop and, eventually, overwinter on the
volunteers to infect the crops in spring (Zadoks, 1961). Furthermore, stripe rust can
oversummer in high altitudes, as in the Alps, where harvest is late and the wind may
carry the inoculum to volunteer crops (Zadoks, 1961).
Puccinia striiformis
can
survive the non-crop season in mild climates on volunteer cereal plants, or on other
graminaceous hosts, although at a very low rate (Sharp and Hehn, 1963; Shaner and
Powelson, 1973; Stubbs 1985; Dennis and Brown, 1986; Line, 2002). In these mild
areas, volunteer wheat plants are abundant because of the relatively frequent
rainfalls in summer that promote the built-up of rust inoculum and are responsible
for the recurrence of epidemics (Park, 1990). In dry regions (Iran, South Africa),
oversummering can depend on grass species such as
Bromus
spp.
(catharticus,
arenarius
,
oxydon
),
Hordeum murinum
and
Poa trivialis
(Nazari
et al.
, 1996;
Boshoff
et al.,
2002). Survival of
P. graminis
f.sp.
tritici
is generally difficult during
the non-cereal growing season (Roelfs, 1985a). However, a few surviving local
uredinia can produce local inoculum.
Powdery mildew differentiates a sexual stage, which contributes to over-
summering. With the beginning of senescence of the lower leaves of cereal plants
in early summer, the formation of generative mycelium and cleistothecia of
B. graminis
is initiated starting on the lower leaves. This process can take place as
early as mid-May in wheat, especially in years with hot dry springs (Götz
et al
., 1996).
In the cleistothecia, 15 to 20 asci develop, each containing eight haploid ascospores
which are dispersed by wind, even under high humidity after rain, i.e. in conditions
which would be unfavourable for conidial dispersal (Götz
et al
., 1996). On wheat,
ascospores can develop at any time during the second half of the year; as they are
released at night and in the early morning hours with favourable infection
conditions, they can also contribute to the epidemic during the season (Felsenstein,
1996; Götz
et al
., 1996). Therefore, the sexual reproduction is more important for
powdery mildew on wheat than on barley.
Apart from ascospores, conidia from the summer crop can also infect volunteer
plants so that a mixture of ascospores and conidia forms the inoculum for the winter
crop. The spore population initiating the autumn epidemic originating from
ascospores was estimated to be 25% and 1.5-15% by Brown and Wolfe (1990) and
