Magnaporthe oryzae populations adapted to finger millet and rice exhibit distinctive patterns of genetic diversity, sexuality and host interaction.

In this study, host-specific forms of the blast pathogen Magnaporthe oryzae in sub-Saharan Africa (SSA) were characterised from distinct cropping locations using a combination of molecular and biological assays. Finger millet blast populations in East Africa revealed a continuous genetic variation pattern and lack of clonal lineages, with a wide range of haplotypes. M. oryzae populations lacked the grasshopper (grh) element (96%) and appeared distinct to those in Asia. An overall near equal distribution (47-53%) of the mating types MAT1-1 and MAT1-2, high fertility status (84-89%) and the dominance of hermaphrodites (64%) suggest a strong sexual reproductive potential. Differences in pathogen aggressiveness and lack of cultivar incompatibility suggest the importance of quantitative resistance. Rice blast populations in West Africa showed a typical lineage-based structure. Among the nine lineages identified, three comprised ~90% of the isolates. Skewed distribution of the mating types MAT1-1 (29%) and MAT1-2 (71%) was accompanied by low fertility. Clear differences in cultivar compatibility within and between lineages suggest R gene-mediated interactions. Distinctive patterns of genetic diversity, sexual reproductive potential and pathogenicity suggest adaptive divergence of host-specific forms of M. oryzae populations linked to crop domestication and agricultural intensification.

Gliocladium catenulatum in Association with Sclerotium cepivorum on Onion Leaves in Ghana.

During a regular survey of diseases in farmers' fields of onion (Allium cepa L.) in the Upper East Region of Ghana in March 1997, bulbs infected with white rot disease were collected for identification. A portion of the white mycelium present on the onion scale was cultured on potato dextrose agar (PDA). After 2 days of incubation, white fluffy mycelium was observed that later developed black sclerotia on the colony surface to confirm the identity of Sclerotium cepivorum. Ten days later, an unknown fungus was observed growing from the original inoculation point of the S. cepivorum mycelium. Pure cultures of the unknown fungus grown on PDA were sent to CAB International, UK, and identified as Gliocladium catenulatum Gilman and Abbott (1). G. catenulatum grew along the mycelia of S. cepivorum and completely suppressed its growth. In culture, G. catenulatum colonized sclerotia of S. cepivorum and rendered them mushy and easy to crush, whereas the uncolonized sclerotia remained firm and difficult to crush. The uncolonized sclerotia when transferred onto PDA produced the white fluffy mycelial growth typical of S. cepivorum; the colonized did not grow at all. G. catenulatum also reduced colony growth, sclerotia formation, and maturation of S. cepivorum. Cultures of S. cepivorum inoculated with G. catenulatum measured 3 cm in diameter after 3 days of incubation and exhibited sparse mycelial growth, while cultures of S. cepivorum measured 5 cm in diameter after 3 days of incubation and produced a fairly even sheet of abundant, fluffy mycelial growth. Sclerotia were produced profusely in pure cultures of S. cepivorum whereas those of G. catenulatum and S. cepivorum in combination produced only two sclerotia after 6 days and these were later colonized by G. catenulatum. A striking characteristic of G. catenulatum was the production of a yellow pigmentation around the colony margins. Species within the genus Gliocladium are known to be antagonistic to, and parasitic on, other fungi. The antagonistic and/or parasitic nature of G. catenulatum on S. cepivorum suggests a possible role in biological control. This is the first report of G. catenulatum in association with S. cepivorum on onion in Ghana. Reference: (1) J. C. Gilman and E. V. Abbott. Iowa State College J. Sci. 1:225, 1927.