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