S. K. St. Martin

Effect of Partial Resistance on Phytophthora Stem Rot Incidence and Yield of Soybean in Ohio

Phytophthora root and stem rot of soybean commonly causes losses in both stand and yield in Ohio. Environmental conditions which favor the pathogen typically occur in many areas of the state during late spring and summer. This study examined the performance of 12 soybean cultivars with partial resistance, with or without Rps genes, to different populations of Phytophthora sojae and various levels of disease pressure. The soybean cultivars were evaluated in seven field environments with and without metalaxyl over 4 years. There was a highly significant genotype-environment interaction which was due in part to variable disease pressure. The incidence of Phytophthora stem rot in subplots ranged from 0 to 10 plants in the most susceptible cultivar, Sloan, while significantly less stem rot developed in cultivars with high levels of partial resistance or partial resistance combined with an Rps gene in three of the seven environments. Metalaxyl applied in-furrow had a significant effect on early and final plant populations as well as yield (P < 0.001) in two of the seven environments, and for yield (P = 0.05) in one environment. This indicates that at these two environments, 2001 Lakeview and VanBuren, early season Phytophthora disease was controlled with the in-furrow fungicide treatment. When diverse populations of P. sojae were present, yields from soybean cultivars with high levels of partial resistance were significantly higher than those with low levels of partial resistance. Soybean cultivars with specific resistance genes Rps1k, Rps1k + Rps6, or Rps1k +Rps3a had higher yields than plants with only partial resistance in environments where race determination indicated that the populations of P. sojae present were not capable of causing disease on plants with the Rps1k gene. However, in an environment with very low disease pressure, yields of soybean cultivars with partial resistance were not significantly different from those with single Rps genes or Rps gene combinations. These results demonstrate that genetic traits associated with high levels of partial resistance do not have a negative effect on yield. Soybean cultivars that had the most consistent ranking across environments were those with moderate levels of partial resistance in combination with either Rps1k or Rps3a.

Molecular Marker Analysis of Soybean Plant Introductions with Resistance to Phytophthora sojae.

ABSTRACT Molecular analysis of sources of resistance to plant pathogens should expedite and confirm novel gene discovery and consequently the development of disease resistant cultivars. Recently, soybean plant introductions (PIs) were identified that contain putative novel Rps genes for resistance to Phytophthora sojae. The number of resistance genes that confer resistance to P. sojae isolates OH17 (1b,1d,2,3a,3b,3c,4,5,6,7) and OH25 (1a,1b,1c,1k,7) was then determined in several of the PIs. The objective of this study was to determine if the Rps genes present in these PIs were associated with eight described Rps loci that have been mapped on soybean molecular linkage groups F, G, J, and N. Nine F(2:3) soybean populations were genotyped with simple sequence repeat (SSR) markers linked to previously mapped Rps loci. The nine PI populations all had SSR markers associated (P < 0.01) with resistance to P. sojae isolate OH17 in the Rps1 region. Rps1c is a likely candidate in eight PIs but novel genes may also be possible, while novel genes may confer resistance in one PI to P. sojae isolate OHI7. Two or more Rps genes, including some that are potentially novel, confer resistance to P. sojae isolate OH25 in eight of the populations. However, based on the response to these two isolates, virulence already exists for at least some of the novel genes identified in this study.

Soybean genotypes resistant to Phytophthora sojae and compensation for yield losses of susceptible isolines

Two isolines, with different alleles for resistance to Phytophthora sojae, of the soybean cultivars, Beeson, Century, and Williams were grown in replicated tests to assess yield losses attributable to this pathogen. Isolines susceptible to prevalent races of the pathogen had black seed coats as a marker trait; isolines resistant to prevalent races of the pathogen had yellow seed coats. Included in the tests were blends composed of equal numbers of seed of the two isolines for each cultivar. Tests were conducted at three locations in Indiana and one location in Ohio for three years. In six environments, where Phytophthora root and stem rot damaged soybean, isolines susceptible to prevalent races of the pathogen produced seed yields from 65 to 93% of the yields of isolines resistant to these races. In four of these environments, isoline blends produced yields equal to those of the resistant isoline. In two environments, where susceptible isolines averaged 65 and 69% of the yield of the resistant isoline; the blends averaged 89 and 83% of the yield of the resistant isoline. Where the pathogen reduced yields of susceptible isolines, yellow seeds of the isoline resistant to prevalent races of the pathogen contributed from 10 to 33% more seed than would be expected if resistant and susceptible isolines contributed equally to seed yield. The data demonstrate that plants of the resistant isoline were compensating for reduced productivity of the susceptible plants in the blend.

Genetic Analysis of Soybean Plant Introductions with Resistance to Phytophthora sojae

ABSTRACT Phytophthora sojae, which causes Phytophthora root and stem rot of soybean, is a serious disease worldwide and is managed primarily by deploying cultivars with resistance. Thirty-two soybean plant introductions (PIs), all but three of which were from South Korea, were proposed as new sources of single-gene resistance to P. sojae. The objective of this study was to characterize the inheritance of resistance to P. sojae in these PIs. Twenty-two soybean populations from crosses of these PIs and the susceptible cv. Williams were inoculated with P. sojae OH17 (vir 1b, 1d, 2, 3a, 3b, 3c, 4, 5, 6, 7), and OH25 (vir 1a, 1b, 1c, 1k, 7). These isolates were selected because they are virulent on soybeans with all known Rps genes and many Rps gene combinations. Thirteen of the twenty-two populations had consistent segregation responses following inoculations between the two generations. In two PIs, resistance was conferred by two genes to OH17 and three genes to OH25. Resistance to both isolates was conferred by a single gene in PI 398440 although the individual families were not resistant to the same isolates. The data suggest that six of the populations have three-Rps gene combinations as previously proposed, while another four may have either a novel Rps gene or a four-Rps gene combination. Based on this phenotypic analysis, novel and uncharacterized Rps genes may be present in this material. More importantly, these PIs may serve as sources of novel Rps genes that can be used to more effectively manage Phytophthora root and stem rot.

A testcross procedure for selecting exotic strains to improve pure-line cultivars in predominantly self-fertilizing species.

AbstractMethods for identifying germplasm carrying alleles with the potential to improve a particular single-cross hybrid have been proposed and discussed in recent years. There is a need for similar methods to be used in breeding crops for which pure-line cultivars, rather than hybrids, are the goal. The objective of this research was to develop a method to identify germplasm lines with the potential to contribute favorable alleles not present in a specified pure line or set of pure lines. Given a set of adapted pure lines (A1, A2 ..., Am) to be improved and a set of germplasm lines (P1P2 ..., Pf), the procedure consists of producing all f x m possible hybrids and evaluating them along with the parents. The testcross statistic Tij is defined by Tij=γ(Fij−Aj)+(1−γ) (Fij−Pi), where Aj, Pi, and Fij represent the performance of thejth adapted line, the ith germplasm line, and their hybrid, respectively. The statistic

Comparison of selection methods for improvement of the population hybrid

\bar T_i = \left( {{1 \mathord{\left/ {\vphantom {1 m}} \right. \kern-\nulldelimiterspace} m}} \right)\sum {\left( {T_{ij} } \right)}

Quantitative trait loci for partial resistance to Phytophthora sojae in soybean

is the mean value of Tij over all adapted parents Aj. If γ=(1/2)(1+d′), where d′ = the mean degree of dominance, then Tij measures the potential for alleles from Pi to improve Aj and