Agriculture Reference
In-Depth Information
haustorium formation. Mildew conidia penetrate the host plant with an infection peg
through the epidermis. Rust urediniospores need to be hydrated to germinate and the
germ tube tends to grow at right angles to the long axis of the leaf and then to reach
closest stomata through which penetration takes place. The germ tube terminates
in a swelling to form an appressorium in the cases of B. graminis, P. triticina and
P. graminis f.sp. tritici; P. striiformis f.sp. tritici does not differentiate appressoria.
The establishment of a haustorium in the plant cell, which serves to take up
nutrients, terminates the metabolically independent phase of spore infection.
Blumeria graminis forms haustoria in the epidermal cell and Puccinia spp . in
adjacent mesophyll cells. The mycelium is exophytic, i.e. on the leaf surface, for
powdery mildews and endophytic (in the intercellular spaces within the leaf) for
cereal rusts. Average infection efficiency under optimal conditions is similar (20-
45%) for these pathogens (Table 15.1).
The germination and penetration of B. graminis and Puccinia spp. spores are
mainly influenced by temperature, relative humidity and wetness conditions. The
cardinal temperatures for infection (Table 15.1) reflect the relatively wide adaptation
of B. graminis and P. triticina compared with that of P. striiformis f.sp. tritici
(which is restricted to cooler temperatures) and P. graminis f.sp. tritici (warmer
temperatures). Light is not required for substomatal vesicle formation for B.
graminis , P. triticina and P. striiformis f.sp. tritici but is reported to be necessary for
P. graminis f.sp. tritici (Burrage, 1970).
Blumeria graminis germination can take place over a wide range of relative
humidities, even near 0%. Germination rate increases with relative humidity,
reaching the highest levels at 97-100% (Friedrich and Boyle, 1993). Very high
vapour pressure deficits of the air, which also reflect dry conditions, have negative
effects on mildew infection (Friedrich, 1995c). The high water content of the conidia
(63-70%) is considered as a reason for the primary germ tube forming and attaching
itself onto the leaf in the absence of external water (Friedrich and Boyle, 1993). For
the growth of the appressorial germ tube in dry conditions, the water has to be taken
up from the host by the primary germ tube (Carver and Bushnell, 1983). The role of
adhesion to the cereal surface in the infection process has been described by
Nicholson (1996).
Many contradictory statements exist concerning the effect of liquid water on the
mildew germination process. While several workers have reported a deleterious
effect of water on conidia (e.g. Pauvert and de la Tullaye, 1977), Merchán and
Kranz (1986a) stated that leaf wetness up to 72 h did not affect germination and
appressorial formation. The negative effect of rain on infection was mainly due to
the washing-off of conidia and not so much due to the inhibition of germination
(Merchán and Kranz, 1986b). Conidia of B. graminis f.sp. hordei can retain their
ability to grow normally on leaves after a period on or in water (Sivapalan, 1993).
This may be why inoculation of barley plants by spraying conidia suspended in
water is possible (Lumbroso et al., 1982). A distinct inhibition of infection was,
however, observed in the presence of guttation droplets and liquid excretions of leaf
pieces (Merchán and Kranz, 1986a).
Free water on the leaf surface in the form of dew droplets (usually formed during
the night) is essential for the germination of rust urediniospores. The minimum
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