Evaluation of Australian Native Species of<i>Glycine</i>for Resistance to Soybean Rust (1981)

sprayed with distilled water, covered with ABSTRACT plastic film, and incubated at 24 C in a BURDON,J.J.,and D. R. MARSHALL. 1981. Evaluationof Australian native species of Glycine growth cabinet lit at 70 Wm2 for 16 for resistance to soybean rust. Plant Disease 65:44-45. hr/day. Leaves of soybean cultivar Extensive screening of six Australian native species of Glycine uncovered variation in reaction to Dare were included in all cases as Phakopsora pachyrhizi, the causal agent of leaf rust of soybeans. Some of these native species control. represent a potentially valuable source of resistance genes for soybean rust resistance breeding For all accessions four epidemiprograms. ologically related characters were assessed: incubation period (sensu [15]; the number of days from inoculation to Soybeans are affected by a wide range to promote germination and planted in visible symptoms such as flecking), latent of pests and diseases (1,4,11). The most individual pots in a heated glasshouse. period (number of days from inoculation important in the eastern hemisphere Eight weeks after emergence, plants were until the first pustule [uredium] erupted), (2,12) is leaf rust caused by Phakopsora screened for resistance to P. pachyrhizi. number of active pustules 5 days after the pachyrhizi Syd. In years favoring Because this survey was designed to first pustule erupted, and intensity of development and spread of this pathogen, determine the extent of resistance in the pustule development 5 days after the first the total soybean production of individual six species, only one plant was tested per pustule erupted (scale of 0, no southeast Asian countries has been accession. development, to 3, large, highly productive reduced by as much as 30%, and losses up Levels of disease resistance were pustules). to 90% have been reported in individual determined by using a detached leaf Accessions were then categorized as fields (2,6,10). This pathogen was technique (16). Ten newly expanded highly resistant, intermediate, or highly recently discovered in Puerto Rico leaves from each plant were placed, susceptible. In highly resistant plants, (13,14) and seriously threatens soybean abaxial surface up, on a 0.4% water agar disease failed to develop beyond visible production in the United States and support medium containing 5 ppm flecking. Plants in the intermediate and South America. gibberellic acid. Leaves were inoculated highly susceptible categories showed a The vulnerability of the American in a 1.2-m Perspex settling tower by continuum in response to infection by P. soybean crop to this pathogen is compressed air injection of 6 mg of P. pachyrhizi as measured by the three illustrated by a lack of resistance to the pachyrhizi spores through a modified characters of the latent period, the virulent and highly aggressive races surgical syringe into the top of the tower. number of pustules per square centimeter, found in Asia. Of 215 cultivars listed by For 5 min spores were allowed to settle, and pustule intensity. Although many Hymowitz et al (5) for which rust and then each agar gel was removed from values differed significantly (Table 1), resistance has been assessed, 205 (95.4%) the tower. Inoculated leaves were then categorization of intermediate accessions are highly susceptible, 8 (3.7%) are susceptible, and only 2 (0.9%) show intermediate resistance. Some resistance is . .. a mediapparen istat hogen risolates fm Table 1. Range of responses of six Australian native species of Glycinea to infection by Phakopsora apparent to pathogen isolates from pachyrhizi Puerto Rico (3). This vulnerability is also true in Asia where extensive disease Species Incubation period Latent period Pustules screening tests have demonstrated only Accessionb (days) (days) (no./ct) Intensity