Light (LE) and scanning electron microscopy (SEM) were used to study the development of Phakopsora pachyrhizi from urediniospore germination to colonization on soybean. Drops of 50 µL of a spore suspension (3 x 10(4) urediniospores.mL-1) were inoculated on detached leaflets of seven cultivars, besides the genotypes PI 230970 and PI 459025. After inoculation, leaflets were stored in BOD at 23 ± 2 ± 2 ºC for incubation. Samples were taken from four to 240 hours after the inoculation (h.a.i.), submitted to the process of fixation for scanning electron microscopy (SEM) and decolorized into a solution of absolute ethanol/acetic acid (1:1, v/v). In decolorized samples differences were observed among cultivars in percentage of germination, appressorium formation and consequently in the penetration. On PI 429025 the lowest percentage of germinated urediniospores (20%) and of appressoria formation (11 %) was observed, and the cultivar BRS 154 presented a higher percentage of both germination (85.5 %) and appressoria formation (66.5 %). Among other cultivars the percentage of germination was from 42% to 73.5 % and from 21.5 to 60.5 % for appressoria formation. Several events in the colonization process observed by SEM were different from those in the literature.
Asian soybean rust; Glycine max; ultrastructure
