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Survival of Phakopsora pachyrhizi on volunteer soybeans in Entre Ríos province, Argentina

Presenter: Ing. Agr. A. Norma Formento, Factores Bióticos y Protección Vegetal. INTA EEA Paraná
Coauthor(s): J. de Souza, J. and I.G. Vicentin, Factores Bióticos y Protección Vegetal. INTA EEA Paraná (Entre Ríos) Argentina

The Asian soybean rust fungi (ASR) is an obligate parasite and some legumes have been reported to be host in other areas. In Argentina it was first detected in Misiones province during 2001-02 growing season in soybean crops and kudzu (Pueraria lobata), but the epidemiological importance of volunteer soybean plants is not well understood. The objective of this study was to determinate the importance of volunteer soybean plants that could serve as overwintering sources of the pathogen and provide primary inoculum for soybean crops in Entre Ríos province. Subfreezing daily air (0.05 height) temperatures were registered in the INTA EEA Paraná meteorological station. During 2004, erumpent uredinia of P. pachyrhizi were observed on the abaxial surface of the leaflets from V2 to R3 development stages on volunteer soybean plants surviving after 22 below-freezing temperatures (-0.1 to -7.4°C) until 11 July. After that no uredinia were observed till the next growing season (2 March, 2005). Since April until 27 October 2005 erumpent uredinia were found in volunteer plants in different development stages, while first planting dates began. During this period, the number of days with below-freezing temperatures was 21 (0.0 to -4.9°C) and the coldest temperature was -4.9°C (21 May). Since other hosts are rare or absent in the region, we concluded that volunteer soybean plants may provide a "green bridge" and therefore be a good disease reservoirs in Entre Ríos between seasons.

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