Inheritance and Genetic Mapping of Resistance to Pythium Damping‐Off Caused by Pythium aphanidermatum in ‘Archer’ Soybean

Abstract

Pythium damping‐off and root rot of soybean [Glycine max (L.) Merr.] causes poor stands and consequently reduces yields. Resistance to seedling diseases caused by Pythium spp. was reported in the soybean cultivar Archer and was described to be associated with the Rps1k gene for resistance to Phytophthora sojae. To characterize the inheritance of Pythium damping‐off resistance in Archer and to determine if this resistance is linked to Rps1k, the cross of Archer × ‘Hutcheson’ (susceptible parent) was made and F2:4 lines generated. Parents and F2:4 lines were screened with Pythium aphanidermatum using a hypocotyl inoculation technique. Five days after inoculation, plant survival was scored. The F2:4 lines and parents were also inoculated with race 7 of P. sojae to assess the association of resistance to Pythium spp. with the Rps1k gene. Data from the F2:4 lines fit the model for a single dominant gene in Archer that confers resistance to P. aphanidermatum. The genetic linkage test showed that resistance to P. aphanidermatum was independent of the Rps1k gene in Archer. To identify the genomic location of the Pythium resistance gene, F2:4 lines were screened with simple sequence repeat (SSR) markers from all the soybean molecular linkage groups (MLGs). Two SSR markers, Satt510 and Satt114 on MLG F, which were polymorphic between the parents and the resistant and susceptible bulks, were shown to be associated with Pythium resistance. The Pythium resistance gene, named Rpa1, was located 10.6 cM from Satt510 and 26.6 cM from Satt114.

Resistance to Pythium Seedling Disease in Soybean

“…When seed exudates were examined, total sugar and organic acid concentrations were less in seed exudates of the Pythium resistant cultivar Archer than those in the susceptible cultivar Hutcheson (Table 9) (Nanayakara, 2001). The levels of these exudates increased as the seed quality of each cultivar was reduced, but those increases were greater with (Rosso et al, 2008).…”

“…ultimum in Phaseolus vulgaris. The location of Rpa1 was determined using 88 SSR primers from the 20 major linkage groups (MLG) in soybean (Rosso, et al 2008). These primers were tested against the parents, Archer and Hutcheson, and a resistant bulk made up of 12 of the resistant F 2:4 lines and a susceptible bulk composed of 10 of the susceptible F 2:4 lines.…”

“…Plant resistance to Pythium damping-off is rare and has not been actively pursued in breeding programs; however we have identified a cultivar with high levels of resistance to Pythium damping-off that may be useful to reducing the impact of this disease in soybean (Rosso et al, 2008). In 1996, we began investigating the effect of cultivars on stand when subjected to early flooding and which pathogens were favored by these saturated conditions.…”

Pathogenicity of Pythium species causing seed rot and damping-off in soybean under controlled conditions

“…Typical symptoms of infection by Pythium spp. include soft and decayed seed before germination, pre-or postemergence damping-off in the seeding stage, and hypocotyl discoloration and root rot in advanced growth stages (Rosso et al 2008;Yang 1999).…”

“…Pre-emergence damping-off is commonly assessed using a pot assay, in which inoculum is mixed with soil or another growing medium and seeds are subsequently planted in the medium (Ali-Shtayeh et al 2003;Bates et al 2008;Broders et al 2007;Kirkpatrick et al 2006aKirkpatrick et al , 2006bZhang and Yang 2000). Alternatively, a plug of Pythium isolate can be placed directly on the hypocotyl to induce infection (Rosso et al 2008;Thomson et al 1971). …”