03001naa a2200145 a 450000100080000000500110000800800410001910000160006024501200007626000090019652025260020570000180273170000160274977300900276516519142004-04-16       1998    bl uuuu     u00u1 u    #d1 aALVAREZ, E.  aDNA polymorphism and virulence variation of Phytophthora population isolated from cassava Manihot esculenta Crantz.  c1998  aAbstract: Cassava (Manihot esculenta Crantz) is the fourth most important source of dietary carbohydrate in developing countries. A major constraint to cassava production in Latin America is rot, a destructive and widespread disease. To effectively breed for root-rot resistance, researchers must understand the nature of root rotcausing pathogens in general and the virulence patterns of Phytophthora in particular. The overall objective of this research was to study the genetic variation of Phytophthora isolates in terms of virunlence and DNA polymorphism. One hundred twenty two Phytophthora isolates were obtained using a direct palting method and a baiting technique in which fragments of cassava plantlets propagated in vitro, were used as bait. All isolates were characterized by morphology, pathogenicity and PCR-RAPD analysis. Seven reference isolates of different Phytophthora species were also included in the molecular analysis(I). We also identified the isolates by enzyme-linked immunosorbent assay (ELISA) and DNA hybridization, using a Phytophthora specific probe(2). Virulence variation was determined by inoculating through wounding, cassava sprouts that represented 10 varietal differentials. The inocula were 30 selected isolates from different Colombian regions. Most isolates were not clearly differentiated on PDA, V8A and oatmeal agar by colony type or growth. Identification of the isolates by morphology was difficult because not all isolates produce fungal structures.Amplification of the ITS (internal Transcribed Spacer) region of the rDNA was obtained with template DNA from 122 isolates using extracted DNA. The amplified product for the ITS region of all species was 900 bp. Restriction digest with Aiul, Mspl and Hindl of the product amplified for the ITS region showed different restriction paftems, which corresponded to the different species tested. Five radom 10 mer primers (Operon Technologies, Inc.) gave reproducible bands. Polymorphisms with these single primers differentiated the isolates. Colombian Phytophthora populations are highly diverse and our isolates grouped into 12 clusters. All the isolates were detected by the DNA hybridization probe, a more sensitive method than the ELISA test for identifying the pathogens. The sysptoms caused by individuals were very similar, although sysmptom severity differed among isolates. Virulence was observed within the Colombian group of isolates, but no correlation between virulence variation and DNA polymorphism was observed.1 aCHACON, M. I.1 aSANCHEZ, J.  tRevista Brasileira de Mandioca, Cruz das Almasgv. 17, p. 37, nov., 1998. Suplemento.