GERMINATION AND INFECTION OF Phakopsora pachyrhizi UNDER SIMULATED ENVIRONMENT OF THE CENTRAL AREA OF SANTA FE, ARGENTINA

Abstract

Soybean asian rust (SAR) caused by the fungus Phakopsora pachyrhizi was confirmed in Argentina for the first time during the 2002-2003 growing season. Then, the disease spread to the Northern and Eastern provinces, reaching the central area of Santa Fe, which is one of the main agricultural areas of the country. However, the disease has been usually detected at the end of the growing season (late March to early April), causing little or no impact on grain and seeds production. The objectives of this study were: i) characterize, molecularly and morphologically, the isolates of P. pachyrhizi from this area; ii) evaluate the effect of two environmental combinations (a typical for the region and other optimal as a control) on the germination of urediniospores in laboratory conditions; and iii) assess the infecting capacity of the inoculum obtained and the disease progress promoted under greenhouse conditions. Morphologic, molecular and pathometric characteristics of uredinia and urediniospores allowed verifying P. pachyrhizi identity. Uredospores exposed to daily temperatures ranging from 35 to 25°C, combined with a photoperiod and humidity conditions similar to those typical of the central area of Santa Fe during the crop cycle, decreased germination and appressoria formation and, consequently, SAR symptoms and uredinia production. These biological and epidemiological features would explain the erratic occurrence of SAR in the central producing area of Santa Fe, Argentina. Therefore, these could be used as references to design crop management strategies aimed at decreasing disease impact in other production areas.Key words: Phakopsora pachyrhizi, inoculum, viability, environmental conditions. ii) evaluar el efecto de dos combinaciones ambientales (una típica de la región y otra óptima como control) sobre la germinación de las urediniosporas en laboratorio; y iii) evaluar la capacidad infectiva del inóculo obtenido y el progreso de la enfermedad generado bajo condiciones de invernadero. La caracterización molecular y morfológica de uredinios y urediniosporas permitieron verificar la identidad de P. pachyrhizi. Las esporas expuestas al rango térmico diario 35/25°C con fotoperíodo y humedad similares a las condiciones típicas del centro de Santa Fe, disminuyeron la germinación y formación de apresorios y, en consecuencia, la aparición de síntomas y producción de uredinios de RAS. Los resultados biológicos y epidemiológicos obtenidos podrían explicar la baja frecuencia de ocurrencia de RAS en la región central de Santa Fe, Argentina, y pueden ser tomados como referencia al diseñar estrategias de manejo de cultivo orientadas a minimizar el impacto de la enfermedad en otras áreas productivas.Palabras clave: Phakopsora pachyrhizi, inóculo, viabilidad, ambiente.