Evaluation of short season soybean cultivars for Rps6 resistance to Phytophthora sojae.

Phytophthora root rot caused by Phytophthora sojae (Ps) is an important disease of soybean on poorly drained soil. The resistance gene Rps6, which provides resistance to a number of races, was reported in the cultivar Altona in 1982. A line, 840-7-3 (PI 436.477) has a gene for resistance and has been used in the development of a number of short season cultivars in Canada but the gene for resistance has not been previously identified. Inoculation of the original lines and crosses derived from PI 436.477 with a limited number of Phytophthora races indicated that Rps6 was present in a number of these lines and cultivars either alone or in combination with other genes for resistance to Phytophthora root rot. These lines may be useful sources of the resistance gene Rps6 in future breeding programs. Introduction: The Rps6 gene for resistance to Phytophthora sojae (Ps) in the soybean cultivar Altona was reported by Athow and Laviolette (1982). Altona resulted from a cross between O-52-903 (PI 194.654) (Rps6) and Flambeau (rps,rps). O-52-903 is from Sweden and another Swedish line, 840-7-3 (PI 436.477), that has been used in soybean breeding, has been resistant to hypocotyl inoculations with Ps in prior trials at the Greenhouse and Processing Crops Research Centre, but it was not clear if the line contained Rps6. Results obtained in 1994-95 from inoculations of the cross 840-7-3 (Rps6?)/HARO 6272 (Rps6,Rps7) with Ps race 1 did not show segregation for resistance and susceptibility so it was concluded that Rps6 was present in 840-7-3. However, the F2 population was small (38 plants) and limited testing was done in the F3 and F4. This study was conducted to identify the gene for resistance in 840-7-3 (PI436.477) and to identify cultivars that may have resistance derived from 840-7-3. Materials and Methods: Because the number of reported Ps races is increasing and the difficulty of maintaining Ps cultures with the correct virulence formula, it has become increasingly difficult to include all races in the characterization of putative resistance genes. As a compromise, we inoculated the Swedish lines and progeny derived from 840-73 with 7 races of Ps to compare their responses. The pedigrees of the 20 cultivars that have 840-7-3 in their parentage and genotype are listed in Table 1. The pedigree and genotype for Cabot and Zephyr is incomplete. The lines and cultivars were inoculated by the hypocotyl method (Anderson and Buzzell, 1992) with isolates B135R3.40, D45R5.10, C20R10.3, D35R17.2, C25R21.6, and B17R31.16 obtained from Dr. A. Dorrance, OARDC, OH., which correspond to Ps races 3, 5, 10, 17, 21, and 31, respectively (Hartman et al., 1999). Rps7 which is found in Harosoy, HARO 1272 (Harosoy 63) and HARO 6272 is susceptible to all of the above races so the presence or absence of this gene could not be verified in any of the soybean lines screened. Results and Conclusions: Expected results were obtained with the control lines/cultivars containing Rps6 (Table 2). Harosoy (rps6, rps1) was susceptible to all races and HARO 1272 (Rps1a, Rps7) was distinguishable from Rps6 by a resistant response with Races 17 and 31. Inoculation of O-52-903 and 840-7-3 resulted in a similar response pattern suggesting both contain Rps6. Of the cultivars evaluated, 5 contained no resistance, 6 contained Rps6 and Rps1a, 4 contained Rps6 and two contained Rps1a based on responses to the Ps races used for screening. Inoculation of 3 cultivars with Races 17 and 31 resulted in a variable response. Results indicate that Rps6 is present in 840-7-3 and some of the progeny derived from this line and that these progeny could be used as a source for Rps6 in breeding programs. Inoculation of Commander resulted in intermediate responses to races 3, 17, 21, and 31; therefore, it was not clear if Rps1a or Rps6 were present. Inoculation of Galaxy and RCAT Bobcat with race 17 resulted in a susceptible response which was not expected if Rps1a were present which suggests the presence of a resistant gene other than Rps1a. The presence of Rps6 in these varieties is assumed but cannot be confirmed. References: Anderson, T. R. and Buzzell, R. I. 1992. Inheritance and linkage of the Rps7 gene for resistance to Phytophthora rot of soybean. Plant Dis. 76:958-959. Athow, K. L. and Laviolette, F. A. 1982. Rps6, a major gene for resistance to Phytophthora megasperma f. sp. glycinea in soybean. Phytopathology 72:1564-1567. Hartman, G. L., Sinclair, J. B. and Rupe, J. C. 1999. Compendium of Soybean Diseases. 4 Ed. APS Press. Pp.39-42. Table 1. Soybean cultivars and lines, source/pedigree, and Rps genes detectable in this study. Cultivar/Line Source/Pedigree Rps genes Harosoy rps Altona O-52-903 x Flambeau Rps6 O-52-903 PI 194.654 Rps6 HARO6272 (Harosoy [7] x Altona) Rps6 L89-1581 (Williams [6] x Altona) Rps6 HARO 1272 (Harosoy 63) Rps1a 840-7-3 PI 438.477 Rps6? Cultivars from crosses involving 840-7-3 (Rsp6?) Maple Amber [Harosoy 63 x Altona] x 840-7-3 [Rps1-a x Rps6] x Rps6? Maple Arrow Harosoy 63 x 840-7-3 Rps1-a x Rps6? Maple Presto [Amsoy x Portage] x 840-7-3 [rps x rps] x Rps6? Maple Belle [Evans e3 x 840-7-3] x Evans e3 [Rps1a x Rps6?] x Rps1-a AC 2001 [Harosoy e3//840-7-3/2*Evans] x Conrad [[rps x Rps6?] x Rps1-a] x rps Maple Glen BD22115 x Premier (B216) [Rps6?//rps x rps] rps Zephyr FLSL-line x Maple Glen ? x Rps6? Cultivars from crosses involving Maple Arrow (Rps6? Rps1-a) AC Bravor Maple Arrow x Wayne Rps6?Rps1-a x rps Commander Evans x Maple Arrow Rps1-a x Rps6?Rps1-a KG 41 B220 x Maple Arrow Rps1-a x Rps6?Rps1-a Maple Donovan Maple Arrow x Harcor Rps6?Rps1-a x Rps1-a Marathon McCall x Maple Arrow rps x Rps6?Rps1-a KG30 McCall x Maple Arrow rps x Rps6? Rps1-a OAC Eclipse Maple Arrow x Williams Rps6?Rps1-a x rps OAC Frontier Pioneer 1677 x Maple Arrow rps x Rps6?Rps1-a Galaxy McCall x Maple Arrow rps x Rps6?Rps1-a RCAT Bobcat T8508 x OAC 86-07 rps x [Rps6? Rps1-a x rps] Cultivars from crosses involving Maple Presto (Rps6? rps1) Alta Amsoy 71 x Maple Presto Rps1-a x Rps6? Cabot Maple Presto/Evans // I-1/M.Presto) x FLSL-line [Rps6? x Rps1-a]x? x Rps6? x ? AC Harmony [Maple Presto x Williams] x Weber [Rps6? x rps] x rps Note: B220 [Hark x Amsoy 71], BD22115 [840-7-3//Portage x Amsoy], OAC86-07 [Maple Arrow x Williams], P1677 [Corsoy x (Corsoy x Rampage)], T8508 [S1346 x Calland]. Table 2. Soybean cultivar summary of responses to 7 races of P. sojae Cultivar P. sojae race reactions* Rps genes present** 3 5 8 10 17 21 31 Harosoy S S S S S S S rps1-a, rps6, Harosoy 63 S S S R R S R Rps1-a O-52-903, Altona, 840-7-3 HARO6272, L89-1581 R S S R S R S Rps6 AC2001, AC Harmony, KG30, Maple Glen, Zephyr S S S S S S S rps1-a rps6 Alta, Cabot, Maple Amber, Maple Presto R S S R S R S Rps6 Maple Belle, OAC Frontier S S S R R S R Rps1-a AC Bravor, KG41, Maple Arrow, Maple Donovan, Marathon, OAC Eclipse R S S R R R R Rps1-a, Rps6 Commander I S S R I I I Rps1-a? Rps6 Galaxy, RCAT Bobcat R S S R S R R Rps? Rps6? * R = 70% of plants killed, I = 31-69% of plants killed. ** Based on reactions to the isolates used.

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