The persistence of Rhizobium etli strains CE3 and Ph 163 was studied in two soil types representing major French bean growing areas in Egypt. Clay soil from MENOUFIA and silty loam soil from ISMAILIA were planted by bean Cultivars; Bronco and Giza 6. The inoculation with strain Ph 163 in the first bean cultivation was significantly higher in nodule biomass and number; whereas, the strain CE3 was significantly higher in plant biomass accumulation (MOAWAD et al. 2004). The persisting inocula strains seem to perform differently in the two soils in terms of nodulation, biomass accumulation and N-uptake by the two cultivars as compared with their performance with the first inoculation. CE3 strain persisting in the soil performed better than Ph.163 strain. The nodule occupancy by the persisting inoculant rhizobial was determined by two approaches; fluorescent antibody (FA) technique and other Rep-PCR fingerprinting. Both techniques were close in the evaluation of persisting inoculant strains which nodulated beans in the second planting season without inoculation. The results obtained showed that both strains are good survivors in the two soils.
The common bean (Phaseolus vulgaris L.) is one of the important food crop legumes in moderate region (BOTTOMLEY 1991). This legume exhibits less nitrogen fixation during pod filling under low mineral nitrogen conditions (PEOPLES and HERRIDGE 1990) as compared to other legumes (LARUE and PATTERSON 1981). In order to overcome this shortcoming, a practice of inoculation with selected Rhizobium strains exhibiting high levels of nitrogen fixation can lead to increased yields (STREETER and STEWART 1998). However, the inoculation often fails, with no effective symbiosis being formed between the host plant and the inoculant strain. This frequent failure may be due to the inability of the inoculant strains to compete with less symbiotically efficient indigenous strains (THIES et al. 1991).
Arid lands in Egypt represent about 96% of the total area of the country, where dry climate prevails. The high temperature, low relative humidity, high evaporation, and scanty rainfall (1.4 to 5.3 mm/year) all contribute to the fact that the greater part of Egypt is barren (ABDEL-GHAFFAR 1989). The limited acreage of agricultural land makes the intensive agriculture the only reliable approach to produce food and fiber for the increasing population in Nile Delta and Valley (ZAHRAN 1999). With this intensive agriculture, the application of mineral fertilizers including nitrogen is the common practice for improving soil fertility. However, the negative effects of nitrate and other combined nitrogen sources on the symbiosis of Rhizobium and legumes are well documented (ABDEL-WAHAB and ABD-ALLA 1995). Recent reports demonstrate the potential of breeding genotypes of legumes, e.g. pea, bean, and soybean, with improved nodulation and N 2 fixation in the presence of combined nitro-