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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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