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Combating
rust in wheat
 RUSTS
are fungal diseases of plants called Puccinia (family
Puccinaceae). These are most destructive diseases worldwide
and have the ability to destroy the entire wheat crop. They
have the ability to form new races that can attack
previously resistant cultivars, and have the capacity to
move long distances with potential to develop rapidly under
optimal environmental conditions resulting in serious yield
losses.
Production of these plant parasites is affected by uncertain
climatic conditions. Wheat rusts are the most devastating
and harmful of all the wheat diseases. Leaf rust, stem rust,
and stripe rust are the three rust diseases of wheat.
Different ecological zones of the country are vulnerable to
different rust varieties. In the plains of Northern
Pakistan, leaf rust is the most common. Since 2003, the
incidence and severity of stripe rust has increased
affecting wheat production all over the country.
Wheat rust fungi are pathogens which disperse by wind over
long distances. The rust spores spread within and outside
the infected field in a short span of time. After every 7-10
days new pustules are formed, and in a favourable
environment, severe rusting occurs 30-40 days after the
initial infection. Weather plays a key role in the
development of rust epidemics. Cool nights followed by warm
days (55-80°F) and long periods of dew or a wet wheat canopy
are ideal conditions for rust to flourish. For all rusts,
six to eight hours of free moisture on the wheat canopy is
necessary for infection.
Controlling rust is a complicated job because of constant
changes in strains (races) of the pathogens. For example,
variet ies formerly rated as ‘resistant’ strains have, in
recent years, begun to show signs of susceptibility at
various locations. In many situations, the varieties
remained resistant for only three to four years. Crop
rotations and modified tillage practices in most cases are
not effective. Planting an early maturing variety may reduce
the impact of leaf rust and stem rust but too early
varieties are damaged by frost, especially in southern
Punjab.
Pathogenic activity of rusts can either be curtailed by
resistance varieties or by use of chemicals. Variety
resistance is the most economical method in controlling
these diseases. Two kinds of resistances are known to occur
in wheat as horizontal and vertical.
Durable or horizontal resistance, as defined by Johnson
(1978), remains effective in a cultivar during its
widespread cultivation for a long period of time in an
environment favourable to the disease. Partial resistances
rendered by minor genes furnish the plants with durable form
of resistance.
Slow rusting wheat exhibits moderate levels of
susceptibility to rust pathogen and equip the plants with
adult plant resistance. Field life of such varieties is
15-24 years. The varieties Era and Frontana are classical
examples of durable resistance. Lyallpur73, Pavon76,
Chakwal86, Rawal87, Pasban90, GA2002, Chapio, Tukuru, Kukuna
Tonichi81, Yaco, Opata85 and Parula are slow rusting
varieties.
Vertical resistance is explained by a theory known as the
‘gene for gene’ theory, which suggests that two genes
contribute to rust resistance, one in the plant and one in
the rust fungi. The plant must have the correct rust
resistance gene to fight the rust pathogen and the rust must
have a gene that identifies it to the plant as an unwanted
intruder. This type of complete resistance is contributed by
major gene. The resistance provided by these genes is of
short duration. Varieties with complete (vertical
resistance) resistance survive for a very short duration in
the field for about five years. Inqilab91, Chenab70, WL711,
Yacora70 are the varieties carrying vertical resistance.
When there is no single variety to ensure protection as well
as production growers should use the concept of variety
complementation. Variety complementation is employed when
adapted varieties that differ in parentage, maturity and
disease reaction are selected. Because there is no single
perfect variety, complementation allows the producer to
counter balance the potential weaknesses in each variety.
This compensation improves the opportunity for yield
stability of the entire production system. The four steps in
selecting complementary varieties are:
Identify the ‘work horse’ varieties that have a history of
good performance in the fields. Select varieties that differ
in parentage from the ‘work horse’ varieties and the other
varieties being considered. Select varieties that bloom
either earlier or later than your ‘work horse’ varieties to
spread the risk from weather and disease, and to spread out
harvest. Select varieties that have specific characteristics
needed for the production conditions or area. For example,
varieties that are leaf rust susceptible and stripe rust
resistant fit well in the NWFP and Northern Punjab, where
leaf rust is not a serious problem but stripe rust can be
devastating. In central Punjab, both leaf and stripe rusts
can reduce yields, so planting varieties resistant or
moderately resistant to both rusts is strongly encouraged.
Southern Punjab and Sindh are the areas of leaf and stem
rusts so the varieties must have adequate resistance against
them.
Use of appropriate fungicide is another effective but least
employed method of disease management. Chemical control is
more effective when rust diseases are identified on
susceptible varieties early in the growing season. In fields
planted with moderately resistant or resistant varieties, a
fungicide application may not be necessary even if some
disease occurs. Fields planted with moderately susceptible
or susceptible varieties should be scouted regularly, and
any sign of disease may warrant a fungicide application
(particularly in the case of stripe rust). In fields planted
with very susceptible varieties, two applications are
necessary to achieve a moderate level of control, but it is
better not to plant very susceptible varieties, particularly
to stripe rust.
The following criteria should be employed to decide whether
fungicide treatment is warranted: What is the variety’s
level of resistance to rust diseases? Varieties that are
moderately resistant (slow rusting) or resistant to stripe
and leaf rusts don’t need to be treated with fungicides.
Varieties that are moderately susceptible or susceptible to
one or both of the rusts should be closely monitored.
Rain-fed wheat should have a yield potential of 45-50 maunds
per hectare, and irrigated wheat should have a yield
potential of 70-75 maunds per hectare in order to justify
fungicide treatment.
If cool weather slows maturity, the window for disease
development is extended and the impact on the crop is
greater than if the crop is maturing early.
These diseases need moist conditions. If the forecast for
March is for above average rainfall, the disease risk is
higher. Dry conditions lessen the threat to rain-fed wheat
but may increase it for irrigated wheat because of the need
for more frequent irrigation.
In general, if trace amounts of rust are present on the flag
leaf in the early boot stage of development, and infection
below the flag leaf is moderate or moderately severe, it’s
likely that the flag leaf will become severely infected and
a fungicide application should be cost effective.
To maintain consistent and sustainable production of wheat
protection against rust pathogen is the dire need of this
era, as our major wheat varieties for instance Pak81,
Inqilab91, AS2002, Bhakkar2002 all became susceptible to
rust after their short stay in the field.
Amongst all the controlling strategies slow rusting wheat
are providing the most effective, long-term and cost
effective control over rusts. Slow rusting wheat varieties
should be developed and released for general cultivation. In
addition, extension system should be utilised to convince
the farmers to grow moderately resistant or slow rusting
wheat varieties. Slow rusting wheat is the only hope to
fight these harmful fungi.
Courtesy:
The DAWN
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