2.4.4 Symptoms and Extent of Damage by Leaf Rust


On susceptible wheat varieties it forms large
uredinia without causing chlorosis or necrosis in the host tissues, while on
resistant wheat varieties characterized by various responses from small
hypersensitive flecks too small to moderate size uredinia that may be
surrounded by chlorotic and/ or necrotic zones. It frequently lacks abundant
teliospore production as of stem rust at the end of season, resulting in brown
leaf lesion rather than a black stem lesion that occurs in stem rust. In severe
cases it leads to reduced root and shoot growth, reduced tillering and some
time even lead to death of plant. The loss in yield is mainly due to reduced
seed set and shrivelled grain formation due to moisture stress. The impact of
leaf rust on yield reduction in wheat is well documented globally, which ranges
from 10% under moderate to 65% under intense epidemics (Prabhu et al.,
2003), in recent past India experienced two successive epidemics in the
northwest part of the country in 1971-72 and 1972-73. The calculations on
widely grown cultivars Kalyansona, Sonalika, K68 and Pbc306 showed the yield
losses were 0.8-1.0 Mt and 1.3-1.5 Mt in the respective years (Joshi et al., 1975). Factors responsible for
this was favourable infection conditions with frequent spells of high relative
humidity during the crop season, large amount of inoculum generated by the
epidemics and large areas under susceptible varieties.


2.4.5. Epidemiology of Leaf Rust in

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Earlier studies of Mehta (1940) had shown that the
alternate host Thalictrum spp. did
not play any role in the epidemiology of disease in India. However, the
urediospores could oversummer in the cooler mid-low altitudes of the Himalayan
ranges on wild grasses and not in the Indo-Gangetic plains where temperature
was too high. The most important oversummering area is central Nepal, and
occasionally the Shiwalik, Hindukush Mountains and North Western Province also
served as the sources of inoculum. Later in 1977, Nagarajan et al. concluded that urediospores were
dispersed up to a distance of 270 km from central Nepal to parts of the
northern India. The isolated oversummering along the northwestern parts of
Himalayas is least important, because few spores are produced there. In Nepal,
well established infection occurs by November-December and in the eastern parts
of the Indo-Gangetic plains by the end of January. Urediospores from these well
established foci spread towards west through western disturbances and are
deposited along with the associated rains over northwestern India. The major
focus of inoculum built up is in the Nilgiri hills of southern India, where
urediospores survive all through the year. From there, the urediospores
disperse to central and northern India during November with tropical cyclones.
The defined dispersal route from south to north is called “Puccinia path” of
India (Nagarajan
and Joshi, 1984). The spread from the Nilgiris to central India is
an independent event and there is no feedback of inoculum to the Nilgiris (Nagarajan and
Singh, 1990).


2.4.6. Leaf rust resistance gene in wheat


The component of resistance and their phenotypic and
genetic characteristics can be determined by genetic analysis. Biffen (1905)
was the first to study the genetics of resistance to a pathogen. He showed that
resistance against stripe rust in the wheat variety Rivet was controlled by a
single dominant gene showing a Mendelian 3:1 segregation in the F2
generation of a cross between Rivet and a stripe rust susceptible variety.
Genetic studies of leaf rust resistance in wheat have been conducted by wheat
researchers world-wide. In the first of these studies, Mains et al. (1926) determined that the wheat
cultivars Malakof and Webster each had a gene that conditioned leaf rust
resistance, later designated as Lr1
and Lr2, respectively (Ausemus et al., 1946). Soliman et al. (1964) mapped Lr genes by identifying the chromosomes
that carried leaf rust resistance genes Lr1,
Lr3 and Lr11. Dyck and Samborski
(1968) demonstrated allelic variation in Lr
genes when they determined the presence of three alleles at the Lr2 locus. Leaf rust resistance genes
designated Lr1 to Lr61 have been described (McIntosh et
al., 2008). These genes have been characterized in common hexaploid wheat,
tetraploid durum wheat and many diploid wild wheat species. In hexaploid wheat,
leaf rust resistance genes are widely distributed across the genome, being
present on nearly every one of the 42 chromosome arms. Most leaf rust
resistance genes condition effective resistance to specific races of P. triticina. Race-specific resistance
is usually manifested by a hypersensitive response (HR) of rapid cell death
that occurs at the interface between fungal haustoria and host cells in the
epidermal and mesophyll layers. Race-specific Lr genes are effective in seedling
plants and remain effective in the adult plant stage. However, the resistance
conditioned by some genes, such as Lr12,
Lr13, Lr22a, Lr34 and Lr48  is best expressed in adult plants.

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