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Title:   Author  item
                                            DIAZ-VALASIS, M - INIFAP, MEXICOitem
                                            CADENA-HINOJOSA, M - INIFAP, MEXICOitem
                                            FERNANDEZ-PAVIA, S - INST INV AGROPECUARIAS,MXitem
                                            GUZMAN-PLAZOLA, R - INST FITOSANIDAD, MXitem
                                            Grunwald, Niklaus

                                                         - *Nik*
  Submitted to: Mexican Journal of Phytopathology
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 10/27/2003
                                    Publication  Date: N/A
                                    Citation: N/AInterpretive Summary: We conducted research on the biology of oospores, which are sexual reproductive structures produced by the potato late blight pathogen Phytophthora infestans. These oospores have a double wall that makes them resistant to desiccation and frost and are thought to thus provide a means of surviving the winter when incorporated in the soil with the debris of potato crop. Oospores form naturally at the center of origin of the potato late blight pathogen. We studied abundance and concentration of oospores in different soils that had cultivated or wild potato species in the central highlands of Mexico. Our results support the idea that oospores of P. infestans remain viable over the winter in both wild and cultivated soils and that they can initiate epidemics in subsequent cropping seasons.
                                Technical Abstract: Presence of oospores in soil as a source of initial inoculum increases the epidemiological potential of Phytophthora infestans and makes the control more complicated. In this work, an estimation of the incidence and density of oospores of this fungus in wild and cultivated soils was done by using: 1) A bioassay in humid chamber in the greenhouse, 2) a technique using a 24-well multiplate, and 3) a direct extraction of oospores based on the separation of the silt fraction from other soil particles by washing and sedimentation. In 1999, all potato leaflets incubated in soil from Huiloapan, Texcoco, were infected. In soils from Ocozacapa, Texcoco, la Alcantarilla and Camino Rojo, Calpulalpan, the percentage of infected disks were 20, 23 y 20%, respectively. In soils from el Huarache and Huiloapan J.V. disk infection of P. infestans  was not detected. In most of the soils from wild potatoes, infection was not detected with any of the methods used. Only in Camino Rojo-La Herradura, la Cañada, Huiloapan and with a lower percentage Oyamel-Zacatón showed some infection, but it was lower than in cultivated soils. The same situation is expected with oospore concetration. It was observed that infective soils under greenhouse conditions also were infective under bioclimatic chamber conditions. Nevertheless, soils from Oyamel-Zacaton were infective in greenhouse humid chamber, but not in the bioclimatic chamber. In bioclimatic chamber experiments, the highest frequency of disk infection was observed in Ocozacapa with a 71.3% infected disks and a concentration of 96.8 oospores per gram  of soil. In soil from Huiloapan a 27.3% infected disks was observed with  an oospore concentration of 61.4 per gram of soil, and in Camino Rojo a 5.5% of infected disks were registered and an oospore concentration of 41 per gram of soil. A direct relation between oospore concentration and percentage of infected disks was observed. Our results support the idea that oospores of P. infestans remain viable and some of them survive from one growing season to another in wild and cultivated soils, and when they germinate start another epidemics.


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