Date of Graduation

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Plant Science (PhD)

Degree Level

Graduate

Department

Plant Pathology

Advisor/Mentor

Correll, James C.

Committee Member

Clark, John R.

Second Committee Member

Bluhm, Burton H.

Third Committee Member

Jia, Yulin

Fourth Committee Member

Wamishe, Yeshi

Keywords

Biological sciences; Blast diseases management; Magnaporthe oryzae; Pathogen diversity; Rice blast

Abstract

Rice blast caused by Magnaporthe oryzae (= Pyricularia oryzae) B. Couch, is a leading disease of rice. Magnaporthe oryzae exhibits a high degree of diversity. The diversity of isolates of M. oryzae from Africa and the U.S. were examined using vegetative compatibility and virulence phenotyping as well as determination of variation in the avirulence gene AVRPiz-t in isolates from Africa. Also, evaluation of blast resistance genes in the interspecific rice germplasm “New Rice for Africa” NERICA was done using F2 progeny of the cross of U.S. susceptible cultivar M204 and NERICA 12. The U.S. isolates were in three vegetative compatibility groups (VCGs), whereas isolates from Africa were in one to four VCGs that were unique to each country. Among isolates from the U.S, four pathotypes were distinguished based on differentials with the CO39 genetic background whereas 10 were distinguished based on the differentials with the LTH background. The R gene Pi9 was the most effective against isolates from both Africa and the U.S. Also, interspecific rice cultivars were highly resistant against isolates from both the U.S. and Africa. No single genotype or R gene was effective against all isolates from both Africa and the U.S. Blast control in the U.S could be achieved by introgression of Pi9 and Pi11 into rice cultivars. In West Africa, blast control could be achieved by introgressing Pi9, Pita2 and Pik-m into the rice cultivar F6-36. Alternatively, Pi9, Pik-s and Pik-m, could be introgressed into the rice cultivar FKR62N. For blast control in East Africa, one could introgress Pi9 into NERICA 12 or Pi9 and Piz-5 into NERICA 2. Variation in 10 of the 70 open reading frame (ORF) sequences of AVRPiz-t examined were observed. From the ORF sequences eleven haplotypes were observed with one haplotype comprising 86% of both virulent and avirulent isolates. Six virulent isolates had single nucleotide substitutions, insertions or deletions which altered the amino acid sequences of the ORF that could have caused isolates to be virulent. Based on PCR markers, two R genes, Pib and Pita2 were found in NERICA 12, but NERICA 12 could possibly contain other R genes.

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