SATrends Issue 82
September 2007
1. Selection of the fittest
Downy mildew infected pearl millet plants.

Downy mildew (DM) caused by Sclerospora graminicola is a dreaded disease of pearl millet in the semi-arid tropics. In India, the disease is quite severe on single-crops F1 hybrids and causes substantial yield losses. The on-farm DM surveys during 1994-2006 in the hybrid-intensive states of Maharashtra, Rajasthan, Gujarat, and Haryana revealed increased DM susceptibility of a hybrid when grown in the same field for more than three consecutive crop seasons, indicating the emergence of new virulences at different locations/environments over time. Greenhouse studies carried out at ICRISAT further confirmed the evolution of new virulent populations of DM fungus in India.

To manage the disease through host plant resistance it is imperative that resistance against new virulent populations is identified. Scientists took 129 pearl millet germplasm accessions from 16 countries that were identified as highly resistant (=10% incidence) in the downy mildew nursery at ICRISAT, India during 1990-93 and re-evaluated them against the new population of S. graminicola in the downy mildew nursery as well as in the greenhouse during 2006. Of the 129 accessions, 60 were found resistant in the field and 25 in the greenhouse screens. Twenty-one lines that showed resistance both in field and greenhouse screens were further evaluated in the greenhouse against 11 diverse virulent pathotypes of S. graminicola collected from different pearl millet growing areas in India and maintained at ICRISAT. Differential reactions to downy mildew resistance were observed among the accessions against the pathotypes. None of the 21 accessions showed resistance to all the 11 pathotypes. However, 3 accessions (IP 18295, P 1449-2 and YL-18) were resistant (=10% incidence) to 10 pathotypes. Two accessions, IP 18298 and IP8289 showed resistance to 9 pathotypes, whereas, IP 22396, YG-2 and YG-8 were resistant to 8 pathotypes. The germplasm lines with resistance against multiple pathotypes looked the most promising for use in the resistance breeding program.

Screening of pearl millet germplasm against downy mildew at ICRISAT in field and greenhouse.

Virulence changes over time in S. graminicola populations signifies the genetic potential of the pathogen to evolve virulent populations to match the resistance of the new host cultivars. ICRISAT is closely monitoring the virulence change in the pathogen populations and the resistance against new pathotypes available in the pearl millet germplasm.

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2. Power of a hybrid legume

Pigeonpea (Cajanus cajan) is a multipurpose food legume of the semi-arid tropics. It is a low input, high return crop, and is especially popular among the poor and marginal farmers. The productivity has been a major area of concern for this crop, which has remained discouragingly low to about 600-700 kg ha-1 over the past five decades. Scientists at ICRISAT accepted the challenge of breaking the yield barrier. They utilized a naturally existing phenomenon of out-crossing in this crop. ICRISAT succeeded in developing the worlds’ first genetic male sterility (GMS) based pigeonpea hybrid ICPH 8 in 1991. This gave 25-30% yield advantage over the popular checks.

A medium duration, high yielding hybrid.

However, scientists soon realized that the GMS based hybrid seed production suffered from technical bottlenecks. They searched for a more efficient cytoplasmic male sterility (CMS) based hybrid system. After 17 years the group struck gold, when a wild species C. cajanifolius (A4 cytoplasm) was used as a cytoplasm donor. This species produced nearly 100% male sterile plants, with nearly perfect maintenance and fertility restoration, all essential for a commercially viable hybrid.

Over a couple of years the A4 basedsystem of hybrid seed production technology has been very encouraging. The heterosis of the experimental hybrids ranged between 150-300%, ideal for bringing the next quantum jump in yield. Some hybrids have yielded more than 7 tons ha-1, heralding a new era of heterosis breeding in pigeonpea and demonstrating yield levels, hitherto never seen in this crop. Dr MS Swaminathan, eminent agricultural scientist, remarked, “hybrid pigeonpea technology is like dwarfing genes in wheat and rice and this will create a second green revolution”.

Two hybrids, ICPH 2671 (medium maturity group) and ICPH 2438 (short-maturity group) have been found very promising. These hybrids have given heterosis ranging between 20-80% and 30-100%, respectively over best checks at various locations. Besides high heterosis, ICPH 2671 has high resistance to wilt and sterility mosaic, which are prevalent in almost all pigeonpea growing areas.

ICRISAT scientists have demonstrated the commercial feasibility of hybrid seed production and are currently in the process of increasing the breeder seed of the parents and hybrid. Parental lines of ICPH 2671 were distributed to various ICAR institutes, SAUs and private seed companies for hybrid seed production. This is a step closer towards large-scale commercial utilization and bestowing the power of hybrid technology to the farmers of South Asia and Africa.

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