Finger millet (Eleusine coracana) is a tetraploid plant species grown majorly in semi-arid regions of India and Africa. The grains of finger millet have a rich nutrient composition in terms of good quality protein, high content of fibre and a variety of minerals especially calcium. Finger millet is a hardy crop that grows in marginal lands with poor nutrient composition. It is mainly grown as a rainfed crop and is often considered as a poor man’s cereal.
The major limiting factor for finger millet production in the cultivated areas is the occurrence of ragi bast. Blast of finger millet was first reported in the Tanjore area of Tamil Nadu almost a century ago. The disease is reported to cause complete harvest loss if it occurs prior to grain formation. The average loss due to blast is reported to be around 28 to 36 percent.
Blast is a fungal disease caused by Magnaporthe grisea. The same pathogen causes the devastating blast disease of rice.
Magnaporthe grisea is an ascomycete fungus that is a biotroph and is highly diverse in nature. Even though the pathogen has a broad range of hosts among grasses and sedges, a particular strain has only a limited number of hosts.
Magnaporthe spreads as air borne conidia (asexual spores) which upon reaching an ideal host enters the cells through stomata and germinates under high relative humidity (more than 90%) and congenial temperatures (25 to 280C). Symptoms generally appear as lesions on the 4th to 5th day of conidial entry. Frequent rains and cloudy skies are congenial for the conidial germination due to high relative humidity and presence of water droplets that facilitates germination.
The fungus infects in different stages of the crop leading to leaf blast, neck blast and finger blast.
The symptoms appear as circular lesions that are pointed towards either ends. The centre of the spots appear greyish and the borders become brownish. In susceptible genotypes, several of such spindle shaped spots coalesce together, leading to drying of the entire leaf. Resistant genotypes develop specks on the leaves and necks.
When the fungus infects the neck region, a few inches of neck just below the finger turns brownish black ultimately leading to breakage of the peduncle.
Maximum damage is caused by neck blast as it prevents grain formation or shrivelling of grains.
The disease is controlled through cultural methods, chemical methods and also by the use of resistant varieties.
Blast epidemic is a serious concern in all finger millet growing areas as the pathogen is highly variable and the build-up of inoculum of new virulent race occurs frequently. There is a necessity to understand the host-pathogen interaction in a detailed manner so that more durable resistant varieties can be generated, better management measures developed and the devastating disease can be kept in control.
Source : Jinu Jacob, Deepika Cheruku, Swarna Ronanki, ICAR-Indian Institute of Millets Research, Hyderabad
Image source : Diseases of Millets - a ready reckoner