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S V Agricultural College, ANGRAU (2003)

Pathogenic and genetic diversity in populations of Sclerospora granzinicola, the incitant of downy mildew in pearl millet

Pushpavathi, B

Titre : Pathogenic and genetic diversity in populations of Sclerospora granzinicola, the incitant of downy mildew in pearl millet

Auteur : Pushpavathi, B

Etablissement de soutenance : S V Agricultural College, ANGRAU.

Grade : PhD thesis 2003

Résumé partiel
Studies on pathogenic and genetic diversity, mating types, and inheritance of virulence in Sclerospora graminicoia (Sacc.) Schroet., the pearl millet downy mildew pathogen were undertaken at International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Andhra Pradesh, India. A total of 21 isolates from major pearl millet growing areas of India were selected from the collections maintained at ICRISAT in the form of oosporic inocula. From these oosporic inocula, asexual nocula were generated on a universally susceptible host genotype, 70428 and were denoted as parental isolates. During the establishment of parental isolates, the isolate, Sg 153 recorded the highest disease incidence (76.15%) with shortest latent period (6.00 days) and the isolate Sg 021 (1.97%) and Sg 004 (2.61%) recorded the lowest disease incidence with longest latent period (30.00 days). Isolates collected during 1997 recorded significantly higher disease incidence than those collected in the previous year. The 21 parental isolates were e.laluated for pathogenicity on a set of ten host differentials and were found highly variable for virulence, disease incidence, disease reaction, latent period, virulence index and oospore production potential. Among the parental isolates, the isolate Sg 139 was found highly virulent and Sg 110 the weakly virulent. Based on disease incidence, the parental isolates were classified into seven pathotype groups. A representative isolate from each group was identified and ten single-zoospore isolates (SZIs) from each representative isolate were established for further studies. Considerable variation was found among the SZls of Sg 139 and Sg 110 for virulence, disease incidence, disease reaction, latent period and virulence index on host differentials used. Among the SZIs of Sg 139, the isolate Sg 139-4 was found highly virulent while, the isolate of Sg 139-1 was the least virulent. In case of SZIs of Sg 110, the isolate Sg 110-3 was found highly virulent and the least virulent was Sg 110-9. A high level of polymorphism was detected among the parental isolates using AFLP analysis with three primer combinations. Presence or absence of few unique bands was observed in isolates Sg 004, Sg 025, Sg 026, Sg 139 and Sg 115. Based on similarity index, the isolates were classified into eight groups. The cluster composition varied for AFLP analysis and virulence analysis, and these two were found independent. Results of mating type study demonstrated the existence of two mating type groups designated as Mat A and Mat B. Of the 70 SZIs evaluated, 62 were found self-sterile and 8 self-fertile, indicating the predominant heterothallic nature of the fungus with rare occurrence of homothallism. Among the 70 SZls, the overall frequency of both the mating types was approximately equal.

Présentation : ICRISAT

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