Wheat is the major and staple crop in Pakistan. It is cultivated on more than eight million hectares. Major limiting factor to the production of wheat is the diseases caused by fungus, nematodes, bacteria and other A-biotic factors. Wheat yellow rust is present worldwide causing more than 70% losses in wheat. Four major epidemics due to yellow rust were recorded in 1978, 1997-98 and 2005 and caused loses of US 244 million dollars, 33 million dollars and 100 million dollars to the Pakistan economy. The pathogen pucciniastriiformisf.sp. tritici infect the green tissues of the crop and other cereal crops. Infection can occur at any stage of the crop from single leaf stage to plant maturity. Symptoms appear after one week of infection as yellow to orange colored rust pustules known as uredium. These rust pustules get their name from the characteristic that resemble with the fresh rust on iron.
Uredia stripes formed on the leaves at mature stage of the planted give yellow and dusty appearance. The pathogen utilizes the water and nutrients of the plants ultimately weaken the plant.
Wheat is the primary host of pathogen but barberries are the alternative host plants for pucciniastriiformis. As in the other disease three factors in the disease triangle are required for the development of disease including the pathogen virulence, host susceptibility, and favourable environmental conditions. Moisture temperature and wind are the environmental factors essential for pathogen survival and dispersal.
The main means for the spread and dispersal of the rust is the wind. The spores (uredospores) are produced on the upper surface of the leaf in the pustules relatively in huge number. Spores travel through the wind some of them land on the other plants while some remains in the air until the sunlight will kill them; however large number of spores becomes air borne to land on the other living plants. During winter when the humidity is high , the spores in the air remains in the farm of clumps which become heavy in mass so travel over the short distance, resulting in hot-spot infection which can be seen in early spring.
During the summer when the humidity is very low then the single spores travel over the long distances which may result in the uniform distribution of the disease. Long distances mean when disease is in the one part of the wheat belt then it will spread to the other areas. Sometime the spores may also adhere with the clothes of travelers which result in the introduction of disease in to new state.
For establishment of the pathogen to cause infection optimum temperature and humidity is required. Infection occurs when the humidity is very high for 4-6 hours at 10 oc- 15 oc. The growth is rapid when the temperature is 12-15 oc and it will be stopped at 3oc and 25 oc. If the temperature become outside the range in any part of the day then sporulation stops but again resumes growth when the temperature become favorable. Sporulation is favoured by high humidity and similar temperature thus fresh spores of the yellow to orange color are usually seen in the morning, while it becomes duller by the afternoon.
For keeping the low level of rust different control strategies are required including growing of resistant varieties that meets agronomic and other disease resistance requirement, spray of fungicides, use of biocontrol agents. There are two farm of the resistance in the wheat variety first one is the seedling resistance and the other one is the adult plant resistance. Both of the resistance is race specific. Seedling resistance is usually controlled by single gene which is unfortunately overcome by the new races/pathotypes of the pathogen as happen with H45 wheat variety in 2003.
Different fungicides can be used to control the stripe rust in wheat. Tilt (Propiconazole) Quardis (Azoxystrobin) Quilt (Azoxystrobin, propiconazole) is available fungicides against rust in wheat. Moreover seed treatment with active ingredient like, flutriafol, triadimenol, triticonazole, are available under one trade name can also be done. These three products provide control for about 8 weeks after the sowing and delay the onset of disease.
Use of the rusticides is inhibited in Pakistan because high cost of chemicals, risk of health hazard moreover fungicides are not environment friendly. Different bio control agents can be used for control of wheat rust including Verticilliumlecanii, Beauveriabassiana, Metarrhiziumanisopliae. By the application of spore suspension of these biocontrol agents on the plants infected with yellow rust at early stages it will reduces the number of pustules on the plants. The colony growth of these biocontrol agents depends upon the optimum humidity level and temperature. More attention is required for the economic use of biocontrol agents at proper time. The use of these strategies will lead to disseminate the rust in wheat and ultimately increase the productivity.
The authors are from the University of Agriculture Faisalabad, Punjab, Pakistan.