Brazil occupies an important position worldwide as a soybean producer and exporter. Nowadays Brazil is the second largest exporter of grain and bran and the third of soybean oil (Embrapa, 2000). Among the main factors limiting soybean production are the diseases that attack the plants.
A limitedincidence of powdery mildew was verified in the 1996/97 crop. The disease is caused by the fungus Microsphaera diffusa in diverse cultivars, and is favored by rainy weather and mild temperatures. Today the disease reaches all the production regions in Brazil, with losses ranging from 30 to 40% (Yorinori, 1997).
The most typical symptom of powdery mildew is the presence of a fine layer of mycelium and powdery spores (conidia), white or grayish-brown in color covering the aerial part of the plant, being less typically observed in the pods. This covering layer, indicating serious infection, prevents photosynthesis, causing leaf dryness and premature fall. Moreover, the plants present a grayish-brown to bronze coloration that looks like the dryness (desiccation) caused by herbicides (Sartonato and Yorinori, 2001).
Studies about the adult-plant resistance in soybean have reported that the plants, especially their leaves, may present differences in the level of infection with powdery mildew during their different stages of development. Mignucci and Lim (1980) observed that 1-parted leaves were found to be more susceptible in plants inoculated at the V1 stage than in those inoculated at the V3 stage and that the lower leaves of young plants were more susceptible than those in higher positions. They also observed that some time after the inoculation a reduction of the symptoms occured in all types of leaves some time after the inoculation, showing that the mildew develops better in young plants. Therefore, it can be seen that either the pathogen or the presentation of the symptoms are affected by the cultivar, the position of the leaves and the age of the plant at the time of inoculation.
Previous studies by Grau and Laurence (1975) and Mignucci and Lim (1980) showed that resistance to powdery mildew is inherited in a Mendelian way, i.e., it is controlled by major genes, although with a varying influence on the intensity and severity of the disease. Under field conditions three types of reactions can be observed under field conditions: susceptibility, adult-plant resistance, and full resistance.
Studies carried out in other countries (Lohnes and Bernard, 1992) have shown that in some soybean populations the entire resistance is controlled by an Rdm-c gene with a dominant effect, allelic to genes that confer adult-plant resistance (Rdm) and susceptibility (rdm). The Rdm-c gene originates from the CNS cultivar and is associated with the Rps2 gene which confers resistance to Phytophthora root rot. In contrast, adult-plant resistance is controlled by a pair of genes (Rdm, rdm), with the Rdm allele having a dominant effect and acting on adult-plant resistance and the rdm allele conferring susceptibility (Mignucci and Lim 1980). In Brazil there are no reports of studies related to the inheritance of soybean resistance to powdery mildew.
For the development of new cultivars resistant to powdery mildew it is of great importance to have previous knowledge about the genetic control of resistance in segregating populations, as well as about the existence of variability. As a result, a selective process can be applied, resulting in significant genetic gains. Thus, the objective of the present study was to develop segregating populations (a total of eight) derived from crosses between cultivars resistant and susceptible to powdery mildew in order to study the inheritance of soybean resistance to the disease.