Gram is an important and valuable crop particularly in the Barani tract of Pakistan. Blight is a serious disease of this crop in the North Western part of Punjab province. It frequently appears in an epidemic form and causes heavy losses. Total destruction of the crop has been reported in severe cases.
Symptoms :The fungus attacks all the above ground parts of the plant. The disease appears in the form of circular spots on leaves and pods and elongated spots on petioles and stem. On leaflets, the spots are round, sometimes elongated and bear irregularly depressed brown dots and are surrounded by brownish red margin. These spots may coalesce and entire leaf turns brown presenting a scorched appearance.
On green pods, the spots are usually circular with dark margins and have pycnidia arranged in concentric circles. Seeds within pods may also show lesions. On petiole and stem, lesions are elongated (3-4 cm), brown and bear black dots. Lesions may girdle the affected portion causing drooping and wilting of the above portion. If main stem is girdled at the base, the entire plant dries. With the advancement of disease patches of drooping and wilting plants become prominent in the field, later involving the whole field.
Casual Organism :Ascochyta rabiei Perfect stage: Mycosphaerella rabiei
Disease Cycle:The fungus survives as pycnidia on diseased plant debris and seeds. Badly affected seed normally fail to germinate. Pycnidia can survive in debris for about two years at a temperature of 10-35ºC, if RH on soil is 0-3%. The pycniospores (conidia) are exuded from the pycnidia only during and after rains. They are disseminated through the agency of raindrops of flowing rainwater. The conidia germinate by germ tubes, form aspersoria and enter the host through cuticle or stomata by means of infection pegs.
Epidemiology :Cloudy, moist and rainy weather favours the disease during any part of the growing season of the gram crop particularly during the last three months i.e., January-March. High rains during the preceding summer season and low winter rains and dry weather conditions during winter decrease chances of epidemics. Low summer rains and high and frequent winter rains during February to April encourage epidemics.
The following conclusions have been arrived at by Late Dr. Sattar as a result of analysis of weather and disease incidence data during fifty years (1911-61):
- Areas with an average winter rain of less than 3.5 inches will not suffer from blight epidemics.
- Areas with an average winter rain of 3.5 inches to 6 inches will occasionally suffer from epidemics.
- Areas with an average winter rain of above 6 inches are exposed to frequent epidemics.
- Areas with high summer rains do not suffer from epidemics.
- Areas with low summer rains are likely to suffer frequently from epidemics.
- Use of healthy seed collected from blight free areas
- Seed treatment with Benlate, Calixin-M etc.
- Foliage spraying is the most satisfactory control method. Bordeaux Mixture and some new fungicides such as Acrobat MZ, Ridomil Gold MZ and Banko (0.2 – 0.25%) give good results.
- 3. Elimination of diseased plant debris by:
- Harvesting crop by pulling out with hand instead of cutting with a sickle.
- Sweeping the threshing floors and burning or burying the collected debris,
- Ploughing the field with a furrow turning plough, after the first shower of rain in order to burry the remnants of diseased plants
- Disallowing bhusa stacking in the field.
- Mixed cropping of gram with wheat, barley and taramira, etc.
- Use of blight resistant varieties: CM -44, CM -72, CM -88, Punjab -91, Paidar etc.