Gustavo Adolfo Prado

The double life of Ceratobasidium: orchid mycorrhizal fungi and their potential for biocontrol of Rhizoctonia solani sheath blight of rice

Ceratobasidium includes orchid mycorrhizal symbionts, plant pathogens and biocontrol agents of soilborne plant pathogens. It is not known to what extent members of the first guild also can participate in the others. Ceratobasidium spp. were isolated from roots of Colombian orchids and identified by phylogeny based on nrITS sequences. Phylogenetic grouping of Ceratobasidium spp. isolates corresponded to orchid host substrate (epiphytic vs. terrestrial). Isolates were tested for virulence on rice and for biocontrol of Rhizoctonia solani, causal agent of sheath blight of rice. All Ceratobasidium spp. isolates caused some signs of sheath blight but significantly less than a pathogenic R. solani used as a positive control. When Ceratobasidium spp. isolates were inoculated on rice seedlings 3 d before R. solani, they significantly reduced disease expression compared to controls inoculated with R. solani alone. The use of Ceratobasidium spp. from orchids for biological control is novel, and biodiverse countries such as Colombia are promising places to look for new biocontrol agents.

Sheath-blight resistance QTLS in japonica rice germplasm

Sheath blight (SB), caused by Rhizoctonia solani, is a serious disease of cultivated rice (Oryza sativa L.) for which genetic resistance is in demand by breeders. With the goal of resistance (SBR)-QTL discovery in U. S. japonica breeding material, 197 doubled-haploid lines from a cross between MCR10277 (resistant) and Cocodrie (susceptible) were evaluated in field and greenhouse assays with U. S. and Colombian pathogen isolates and genotyped at 111 microsatellite marker loci. Four SBR QTLs from MCR10277 were identified, together accounting for 47% of field genetic variation. In all trials the strongest effect was provided by a chromosome-9 QTL, qsbr_9.1, but some QTLs differed for U. S. and Colombian R. solani isolates. SBR QTLs coincided with only two of several height or heading-time QTLs, suggesting that the relationship between these developmental traits and SBR is not simple. For the U. S. isolates, a microchamber greenhouse assay revealed the same QTLs as did field inoculation.

Population Dynamics of the Rice Blast Pathogen in a Screening Site in Colombia and Characterization of Resistance

Rice blast disease caused by Pyricularia grisea Sacc., the anamorph of Magnaporthe grisea (Hebert) Barr, is the main rice production constraint in Colombia. Development of resistant cultivars has been the preferred means of controlling this disease; however, development of resistant cultivars with durable blast resistance has been a very difficult task, especially in areas where rice is grown continuously such as in Colombia, and where the climatic conditions are highly conducive to blast. In general, blast resistance is defeated by the pathogen shortly after cultivar release (Correa-Victoria and Martinez, 1994). Major efforts are being made at CIAT to understand the high pathogen variation observed, often reported as the main cause of resistance breakdown (Correa-Victoria et al., 1994). An extensive study of P. grisea diversity in Colombia was initiated in 1990 by analyzing the population structure in the country. We have analyzed extensively the virulence diversity and virulence spectrum of several hundred isolates collected from different rice cultivars. Isolates have been inoculated under greenhouse conditions onto rice cultivars with known resistance genes, rice commercial cultivars released in Colombia over a period of 30 years, sources of durable blast resistance, the international set of host differentials, and a set of near isogenic lines carrying known resistance genes.