A New Identification Method Reveals the Resistance of an Extensive-Source Radish Collection to Plasmodiophora brassicae Race 4

Abstract

Raphanus sativus, an important cruciferous vegetable, has been increasingly affected by clubroot disease. Establishing a stable and accurate resistance identification method for screening resistant germplasms is urgently needed in radish. In this study, the influence of inoculum concentration, inoculation methods, and pH of the substrate on disease occurrence was studied. The result showed that the disease index (DI) was highest at 2 × 108 spores/mL, the efficiency of two-stage combined inoculation methods was higher than others, and pH 6.5 was favorable for the infection of P. brassicae. By using this new method, DIs of 349 radish germplasms varying from 0.00 to 97.04, presented significantly different levels of resistance. Analysis showed that 85.06% germplasms from China were susceptible to P. brassicae, whilst 28 accessions were resistant and mainly distributed in east, southwest, northwest, and south-central China. Most of the exotic germplasms were resistant. Repeated experiments verified the stability and reliability of the method and the identity of germplasm resistance. In total, 13 immune, 5 highly resistant and 21 resistant radish accessions were identified. This study provides an original clubroot-tolerance evaluation technology and valuable materials for the development of broad-spectrum resistant varieties for sustainable clubroot management in radish and other cruciferous crops.

Evaluation of Germplasm and Development of Markers for Resistance to Plasmodiophora brassicae in Radish (Raphanussativus L.)

“…A recent study reported that the majority of the tested radish germplasm was susceptible to the clubroot pathogen, including 81 susceptible and 204 highly susceptible accessions based on the DIs of an extensive source collection of 349 accessions. Exotic radish germplasm has a higher degree of resistance to clubroot disease than local Chinese radish cultivars [29]. However, early studies indicated that the majority of radish cultivars and inbred lines possess a high degree of resistance [32,40], which may reflect differences in the tested germplasm and pathogen races.…”

“…Radish develops an enlarged taproot with few fibrous roots, which may result in infection symptoms that are initially inconspicuous and only subsequently become visible. Precise and reliable artificial inoculation of radish with P. brassicae is not straightforward compared with inoculation of other cruciferous crops [1,29]. Yang et al evaluated the clubroot resistance of a radish germplasm collection using a two-stage inoculation method combining bud injection and injury to the seedling root, which achieved superior stability and accuracy of infection compared to a single-inoculation method [29].…”

“…At 60 days after sowing, each radish seedling was uprooted, and the roots were washed and examined for clubroot symptoms. Symptoms were graded visually using the modified method of Kamei et al [30] and Yang et al [29]: grade 0, no symptoms; grade 1, a few small, separate, globular clubs on lateral roots; grade 2, a few small clubs on the taproot; grade 3, obviously enlarged clubs on the taproot; and grade 4, severe clubs on the taproot (Figure 1). The treatments were arranged in a completely randomized block design with three biological replicates and 10 seedlings for each replicate.…”

“…Sixty-eight radish cultivars and breeding lines were previously evaluated for clubroot resistance and most of the American radishes tested were moderately to highly susceptible; all of the Japanese and many of the Dutch cultivars were completely resistant [28]. Yang et al identified 13 immune, 5 highly resistant, and 21 resistant accessions from among 349 radish accessions [29]. A major clubrootresistance QTL (Crs1) has been identified, and synteny analysis suggests that this region in radish and the Crr3 region in B. rapa originate from the same ancestral genomic region [30].…”

Pathotyping Systems and Pathotypes of Plasmodiophora brassicae—Navigating toward the Optimal Classification

“…Pathotypes highlight the ability of P. brassicae to diversify and adapt to given conditions [ 17 , 18 ]. Due to genetic variability, numerous isolates of P. brassicae with different pathogenicity can be found worldwide [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ].…”