Asian soybean rust (ASR) caused by Phakopsora pachyrhizi was initially reported in the Province of Entre Ríos, Argentina in April 2004 (1). During the 2004-2005 growing season, ASR was observed in the main soybean-producing (Glycine max) region of the country (4) and it was observed on kudzu (Pueraria lobata) in Misiones and Santa Fe provinces (2). Of the environmental factors affecting rust survival over the winter, temperature is probably the most important one with no germination occurring below 7°C (3). The objectives of this study were to analyze the subfreezing daily air temperatures in the presence of new erumpent uredinia and the germination of P. pachyrhizi urediniospores. Ten sites with volunteer plants close to the meteorological station were found in the Paraná Research Station (31°51′S, 60°31′W). Weekly, from June 2004 through December 2005, sites were randomly sampled for volunteer plants (n = 15). The presence of the ASR was confirmed with a polymerase chain reaction (PCR) assay by SINAVIMO (4). The ASR incidence (ASRI) and erumpent uredinia incidence (EUI) was estimated as the proportion of affected plants. Uredinia were classified as: new erumpent with colorless spores; mature cinnamon with pale cinnamon-brown spores; and dead, empty, and dark without spores. The disease density was estimated as an average of the diseased leaflets according to the following scale: light (number of lesions 1 to 100), moderate (101 to 500), and heavy (>500). The ability of urediniospores from the erumpent pustules to germinate was tested during July 2004 and September 2005 on 1.5% of water agar and kept at 25 ± 2°C for 2 days. The subfreezing daily air (0.05 m height) temperature was registered. During the complete evaluation period, surviving plants from the Vc to R6 stages were observed. However, plants with ASR were only observed from June to July 2004 and May to September 2005. Locally, first planting dates begin in October. New uredinia were observed close to mature and dead uredinia on unifoliate and trifoliate leaflets, and petiols and stems in plants from the V2 to R5 stages. There were 13 days with below freezing temperatures from 1 June to 31 July 2004 (-0.1 to -7.4°C), and ASRI and EUI was 100%. The ASR mean density was light. The coldest temperature was -7.4°C on 11 July 2004, and thereafter, no uredinia were observed until the next growing season. From 25 April to 16 September 2005 there were 20 days with below freezing temperatures (-0.1 to -4.9°C). The ASRI and the EUI were 92.3 (76.9), 75.0 (58.3), 59.1 (32.6), 50.0 (40.9), and 36.7 (23.3)% in May, June, July, August, and September, respectively. The incidence of plants with a moderate to heavy disease level was 50.0, 41.7, 28.6, 29.5, and 10% respectively. Germination rate of urediniospores collected in July 2004 was 11% and 28% in September 2005. Low temperatures do not seem to be a limiting factor for the survival of P. pachyrhizi, and urediniospores could survive on volunteer plants until new soybean plants grow. Since another host is rare or absent in the region, volunteer soybean plants may provide a reservoir of inoculum for the next season.
References: (1) A. N. Formento. Roya de la soja en Entre Ríos. INTA-EEA Paraná. On-line publication. INTA, 2004. (2) A. N. Formento and J. de Souza. INTA-EEA Paraná. Serie Extensión No. 32, 2004. (3) M. Marchetti et al. Phytopathology 66:461, 1975. (4) SINAVIMO. Sistema Nacional Argentino de Vigilancia y Monitoreo de Plagas, Roya de la soja. On-line publication. SENASA, 2004.
