Lotus Newsletter (2004) Volume 34, 66 – 72. 66 Evaluation of tetraploid big trefoil (Lotus uliginosus Schkuhr.) for rust resistance SEBASTIÁN ARRIVILLAGA, SEBASTIÁN HERNÁNDEZ, JAVIER CILIUTI, MÓNICA REBUFFO, SILVIA GERMÁN and SILVINA STEWART. Instituto Nacional de Investigación Agropecuaria, INIA La Estanzuela, Colonia, Uruguay. Introduction Several species of Lotus are cultivated in Uruguay; however, the prevalence of diseases has only been thoroughly studied on birdsfoot trefoil (Lotus corniculatus L.; Altier, 1997). The widespread cultivation of susceptible cultivars almost always results in an increase of disease severity. The area of big trefoil (Lotus uliginosus Schkuhr.) has increased in recent years, mainly due to the good adaptation of the tetraploid cultivar Grasslands Maku. This synthetic cultivar developed in New Zealand is based on 13 elite plants selected after three generations of recurrent selection of an induced tetraploid developed by colchicine treatment (Charlton, 1983). Rust outbreaks caused by Uromyces spp. have been observed on this cultivar since 2000 and its severity ranged from trace to severe, where all the lower leaves were killed by the rust and upper leaves had open pustules (Ciliuti et al, 2003). Rust on birdsfoot trefoil develops mostly in summer, from February to April (Altier, 1997), whereas the disease has been observed on big trefoil from November onwards (Ciliuti et al, 2003). Mild winters allow overwintering of the pathogen in Uruguay; therefore the disease may appear at any time during warm and wet weather conditions. Host specificity experiments have identified two races of rust on big trefoil in Uruguay (Ciliuti et al., 2003). The recent appearance of rust on the main big trefoil cultivar utilized in Uruguay and the lack of information of the pathogen reaction within the tetraploid cultivar raised the need to study its genetic variability and to compare the effectiveness of field selection for rust tolerance with selection under artificial inoculation in greenhouse conditions. Materials and Methods Big trefoil germplasm utilized in this study came from seed stocks of tetraploid cultivar Grasslands Maku harvested either in New Zealand (NZ 1 to NZ 3) or Uruguay (UR 1, UR 2), and a sample of the diploid accession G4701 (D) from New Zealand that gave origin to the tetraploid cultivar (Charlton, 1983). An experiment was set on the Forage Nursery Experimental Field at La Estanzuela, Colonia, Uruguay. Plots of 25 plants, spaced 1 m apart, were distributed in a complete randomized block design with 10 replicates. Seedlings were established in the greenhouse in June 2000 and transplanted to the field on the second week of August 2000.