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Rust: a disease damaging wheat crop
M. Ibrahim Khaskheli & M. Mithal Jiskani
WHEAT is the major staple food of the people of the country.
A large quantity of wheat is grown in the Indus River Valley
here. But every year there is shortage of yield because of
various factors. Most serious among them is the biotic
factor. Various diseases, including fungal, viral, plant
parasitic nematode, bacterial, phytoplasmal and parasitic
higher plant (witch weed) are among the biotic factor. In
our country 50, among the above mentioned diseases, are
reported which damage wheat crop, result in reduced yield
and poor grain quality.
 Although these diseases can effectively be controlled
through non-chemical and chemical treatments, most of
farmers are unable to follow the recommended practices due
to economic constraints, or health and environmental
hazards. Breeding for resistance varieties has been a
successful research activity and certainly is the best
method to prevent the crop from diseases in the long-term.
However, variation in pathogen may sometimes create a threat
to the crop production.
Wheat rusts are the most widespread and destructive among
all other diseases. The rusts are of three types on the
basis of infection and pathogen viz., black stem rust (Puccinia
graminis f. sp. tritici), leaf or orange rust (P. recondita)
and yellow or stripe rust (P. striiformis). These pathogenic
fungi belong to genus Puccinia, family Pucciniaceae, order
Uredinales and class Basidiomycetes. These rust fungi are
highly specialised plant pathogens with narrow host ranges.
These diseases occur in almost all wheat-growing areas of
the country. Record showed that in 1906-1908 there had been
severe attack of black stem rust in Mirpurkhas, Sindh, and
in 1978, yellow or stripe and orange or leaf rust were
severe all over wheat-growing areas. However, black stem
rust is not common now, because of introduction of
rust-resistant varieties.
Leaf rust is very common in most of the commercial varieties
and orange rust also prevail in almost all except two or
three varieties of wheat. Recent rain during current wheat
cropping season was favourable to spread of orange or leaf
and yellow or stripe rusts, which appears even on varieties
claimed as resistant against these diseases. But no
cumulative losses have occurred.
The recent rain actually warned the researchers to introduce
rust-resistant varieties, because such varieties are
susceptible to commonly occurring orange or leaf and yellow
or stripe rusts.
The fungus causing black stem rust or stem rust has a long
life-cycle with many pathogenic races. The fungus has five
distinct stages to cause the disease in wheat and in the
alternate host, barberry plant. Barley and other grasses are
also reported as alternate hosts, but wheat is the only
important host plant. This disease produces rust like orange
coloured pustules on leaves, stems and leaf sheaths. These
pustules later become dark brown or black. When fully
developed, it erupts through the epidermis of stems, leaf
sheaths and blades, glumes and awns.
Yellow rust or stripe rust is caused by pathogenic races of
fungi. The first sign of this disease is the appearance of
minute, yellow and elongated pustules in rows/lines on the
leaf surface. These rows look like a sewing machine
stitches. When these pustules mature, they break open the
epidermis to release a yellow-orange mass on the surface of
leaf. In many varieties, the pustules develop in long,
narrow, yellow stripes on leaves, leaf sheaths and heads.
The fungus causing leaf rust or orange rust is also
heteroecious, having a long life-cycle. Wheat is the major
host of the fungus. The first evidence of leaf rust is the
development of very small, round, bright orange coloured
pustules in scattered pattern over leaf blades and sheaths.
The pustules darken with age, becoming orange-brown to
black-brown. It occurs worldwide wherever wheat is grown.
The leaf rust organism survives on infected volunteer crops
and/or on early sown and late maturing wheat crops. The
pathogen can survive in almost any condition in which the
host leaf can survive. Leaf rust is disseminated by wind and
transported through rain water.
The factors responsible for the development of leaf rust
includes viable spores, susceptible or moderately
susceptible variety, moisture on leaves (six to eight hours
of dew), and favourable temperatures (between15-25 C). If
any one of these factors is not present, the rust infection
will not occur. Relatively cool nights along with warm days
provide excellent conditions for the spread of the disease.
Under favourable environmental conditions, rust spores
germinate and penetrate into the wheat leaf. The fungus
obtains nourishment from the leaf, and produces more spores
within a week, which erupt through the leaf surface
(epidermis). These newly produced spores are blown by wind
to other wheat leaves or fields. These conditions are
prevailing during the current cropping season due to long
cool season, cloudiness and rain fall.
There is a different reaction of resistant and susceptible
wheat variety to infection. If a variety is resistant,
reaction only develops within the leaf which may kill or
retard the fungus, and as a result infection is checked.
Resistant varieties may produce yellowish to white spots at
the site of infection and spore penetration. The moderately
resistant varieties develop small reddish to orange pustules
surrounded by a yellow to white ring (halo). The susceptible
varieties have no ability to overcome the fungal growth. The
fungus grows extensively and produces relatively large
pustules, which in term may give rise to thousands of spores
daily, each one of which is capable of re-infecting wheat.
During the growing season, millions of spores are produced
and these spores can move to great distances through wind
currents. Therefore, this disease can increase rapidly and
epidemics may occur whenever susceptible varieties are grown
and weather conditions are favourable for rust development.
The extent of yield losses caused by leaf rust depends on
the severity and infection period. If the crop is severely
rusted from the seedling stage to maturity, the greatest
losses can occur. Yield losses are related to percent
severity, especially on the top (flag) leaf. Wheat leaves
manufacture carbohydrates and other nutrients. The flag leaf
is a primary contributor to the nutrients necessary for
grain fill. The greater the flag leaf area damaged by rust,
the smaller the leaf area available to manufacture nutrients
needed for producing plump kernels.
It is a time to continue efforts for evolving new resistant
varieties, as all the available varieties are near about
susceptible to rusts. The growers are advised to ensure
timely sowing or as early as possible, because only early
sown crop mature earlier and that may be safe from rusts. On
the other hand, avoiding thick sowing and heavy nitrogenous
fertiliser applications and light irrigation with possibly
long intervals also help minimise rust infections and
incidence.
Courtesy: The DAWN
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