Eight soybean [Glycine max (L.) Merr.] cultivars previously identiifed as resistant to soybean mosaic virus (SMV) and three susceptible cultivars were used in a study to determine the inheritance of reaction to SMV. Field‐grown F2 and F3 populations were inoculated with virus strain SMV‐1. Greenhouse studies were also conducted in which the stock was inoculated and the reaction of homozygous and heterozygous scions determined. A variant of SMV‐1, designated SMV‐l‐B, was isolated and used to inoculate cultivars showing differential reactions to SMV‐1. Two types of resistance were recognized. The highest level of resistance gave complete protection against both SMV‐1 and SMV‐l‐B. The lesser level of resistance gave protection against SMV‐1 in the homozygous condition, but some of the heterozygous plants were necrotic. All homozygous plants became necrotic after inoculation with SMV‐l‐B. We proposed that the highest level of resistance be symbolized Rsv Rsv, the lesser level of resistance as rsvt rsvt, and the susceptible as rsv rsv. Rsv, rsvt, and rsv form an allelomorphic series with Rsv dominant to rsvt and rsv; and rsvt dominant to rsv.
Soybean rust (SBR) caused by Phakopsora pachyrhizi Syd. is currently the most threatening fungal disease of soybean [Glycine max (L.) Merr.] in the Americas. Development of resistant or tolerant cultivars is a major goal in several soybean breeding programs. Four loci, all carrying dominant alleles that confer a resistant phenotype, have been described. We investigated the genetic basis of the resistance in PI 200456 and PI 224270 by crossing each of them with a susceptible cultivar (CD 208). Phenotypic segregation ratios for F2 plants and F2:3 lines showed that the resistance in each resistant parent was controlled by a single recessive gene. A test for allelism demonstrated that these genes are non‐allelic. This is the first report of recessive genes controlling SBR resistance in soybean and may represent a different type of resistance for breeding programs aimed at development of more durable resistance.
ELI SIDNEY LOPES e ANTÔNIO ROBERTO GIARDINI, Seção de Microbiologia doSolo, e ROMEU A. S. KIIHL ( 2 ), Seção de Leguminosas, Instituto Agronô-mico
SINOPSEFoi conduzido um ensaio em solução nutritiva sem nitrogênio, com condições assépticas, usando-se como inoculantes para as plantas amostras de solos LR e LB coletadas em locais já cultivados e não com soja, com o objetivo de verificar a presença de Rhizobium capaz de nodular o cultivar santa-rosa.Verificou-se que solos ainda não cultivados com a leguminosa são desprovidos de Rhizobium japonicum, e também que essa bactéria é encontrada em solos previamente cultivados, fixando nitrogênio em quantidades equivalentes à de uma estirpe selecionada.
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