Between 2012 and 2015, 49 new clubroot‐infested fields were identified in 12 German federal states. Clubroot disease incidence varied within these fields from 22% to 92%. Field information revealed that in 85% of fields, oilseed rape was grown in rotation once every 2 or 3 years. Frequency of OSR in the rotation was significantly correlated with the incidence and prevalence of clubroot disease. The disease was detected in fields with soil pH ranging from 5.1 to 8.3, and a significant negative correlation was found between soil pH and the disease incidence of infested fields. Furthermore, more cases of disease and severe incidences were observed in sandy loam and loamy sand as compared with other soil types. Pathotype classification of the 49 Plasmodiophora brassicae populations was conducted on two differential sets, the European Clubroot Differential set and the set of Somé (1996). Additionally, the degree of virulence of the collected isolates was analysed on the clubroot‐resistant oilseed rape cv. Mendel. The results showed variation in pathotype distribution in different regions in Germany. The majority of isolates according to Somé were pathotypes 1 and 3, respectively, with pathotypes 2 and 5 in the minority. Detailed classification according to Buczacki showed the dominance of 16/31/31, 16/14/30 and 16/14/31 populations among 20 distinct virulence patterns of collected isolates. From all populations tested for virulence on cv. Mendel, 15 isolates were found to be moderately or highly virulent. These virulent populations were not restricted to a small geographical area in the country.
Background Plasmodiophora brassicae is the causal agent of clubroot disease of cruciferous plants and one of the biggest threats to the rapeseed (Brassica napus) and brassica vegetable industry worldwide. Disease symptoms In the advanced stages of clubroot disease wilting, stunting, yellowing, and redness are visible in the shoots. However, the typical symptoms of the disease are the presence of club‐shaped galls in the roots of susceptible hosts that block the absorption of water and nutrients. Host range Members of the family Brassicaceae are the primary host of the pathogen, although some members of the family, such as Bunias orientalis, Coronopus squamatus, and Raphanus sativus, have been identified as being consistently resistant to P. brassicae isolates with variable virulence profile. Taxonomy Class: Phytomyxea; Order: Plasmodiophorales; Family: Plasmodiophoraceae; Genus: Plasmodiophora; Species: Plasmodiophora brassicae (Woronin, 1877). Distribution Clubroot disease is spread worldwide, with reports from all continents except Antarctica. To date, clubroot disease has been reported in more than 80 countries. Pathotyping Based on its virulence on different hosts, P. brassicae is classified into pathotypes or races. Five main pathotyping systems have been developed to understand the relationship between P. brassicae and its hosts. Nowadays, the Canadian clubroot differential is extensively used in Canada and has so far identified 36 different pathotypes based on the response of a set of 13 hosts. Effectors and resistance After the identification and characterization of the clubroot pathogen SABATH‐type methyltransferase PbBSMT, several other effectors have been characterized. However, no avirulence gene is known, hindering the functional characterization of the five intercellular nucleotide‐binding (NB) site leucine‐rich‐repeat (LRR) receptors (NLRs) clubroot resistance genes validated to date. Important Link Canola Council of Canada is constantly updating information about clubroot and P. brassicae as part of their Canola Encyclopedia: https://www.canolacouncil.org/canola-encyclopedia/diseases/clubroot/. Phytosanitary categorization PLADBR: EPPO A2 list; Annex designation 9E.
A series of greenhouse experiments was conducted to evaluate the effect of Plasmodiophora brassicae virulence on clubroot development and propagation of resting spores in 86 plant species from 19 botanical families. Plants were artificially inoculated with two isolates of P. brassicae, which were either virulent on clubroot-resistant oilseed rape cv. Mendel (P1 (+)) or avirulent on this cultivar (P1). Clubroot severity and the number of resting spores inside the roots were assessed 35 days post inoculation. Typical clubroot symptoms were observed only in the Brassicaceae family. P1 (+)-inoculated species exhibited more severe symptoms (2 to 10–fold more severe), bigger galls (1.1 to 5.8 fold heavier) and higher number of resting spores than the P1-inoculated plants. Among all Brassica species, Bunias orientalis, Coronopus squamatus and Raphanus sativus were fully resistant against both isolates, while Camelina sativa, Capsella bursa-pastoris, Coincya momensis, Descurainia sophia, Diplotaxis muralis, Erucastrum gallicum, Neslia paniculata, Sinapis alba, S. arvensis, Sisymbrium altissimum, S. loeselii and Thlaspi arvense were highly susceptible. Conringia orientalis, Diplotaxis tenuifolia, Hirschfeldia incana, Iberis amara, Lepidium campestre and Neslia paniculata were completely or partially resistant to P1-isolate but highly susceptible to P1 (+). These results propose that the basis for resistance in these species may be similar to that found in some commercial cultivars, and that these species could contribute to the build-up of inoculum of virulent pathotypes. Furthermore, the pathogen DNA was detected in Alopecurus myosuroides, Phacelia tanacatifolia, Papaver rhoeas and Pisum sativum. It can concluded that the number and diversity of hosts for P. brassicae are greater than previously reported.
The present study investigated the changes in total and individual glucosinolates (GSLs) in roots and leaves of different clubroot-resistant and -susceptible oilseed rape cultivars following artificial inoculation with Plasmodiophora brassicae isolates with different virulence. The results showed significant differences in clubroot incidence and severity as well as in the amount of total and individual glucosinolates between oilseed rape cultivars in response to virulence of the pathogen. Single among with total aliphatic and total indolic glucosinolate contents were significantly lower in leaves of susceptible cultivars compared to resistant ones due to the infection. Similarly, single and total aliphatic as well as indolic glucosinolate contents in roots were lower in susceptible cultivars compared to resistant cultivars analyzed. The different isolates of P. brassicae seem to differ in their ability to reduce gluconasturtiin contents in the host. The more aggressive isolate P1 (+) might be able to suppress gluconasturtiin synthesis of the host in a more pronounced manner compared to the isolate P1. A possible interaction of breakdown products of glucobrassicin with the auxin receptor transport inhibitor response 1 (TIR1) is hypothesized and its possible effects on auxin signaling in roots and leaves of resistant and susceptible cultivars is discussed. A potential interplay between aliphatic and indolic glucosinolates that might be involved in water homeostasis in resistant cultivars is explained.
Clubroot of oilseed rape (OSR), caused by Plasmodiophora brassicae, is a disease of increasing economic importance worldwide. Previous studies indicated that OSR volunteers, Brassica crops and weeds play a critical role in the predisposition of the disease. To determine the effect of timing of foliar application of the herbicide glyphosate or mechanical destruction of OSR volunteers in reduction of clubroot severity and resting spore production, a series of studies was conducted under controlled conditions with a susceptible OSR cultivar and an isolate of P. brassicae. Plants were inoculated by injecting a spore suspension beside the root hairs at growth stage 11–12 (BBCH scale) and were terminated at 7 (early) or 21 (late) days post‐inoculation (dpi). Under controlled conditions, the first symptoms on roots were observed as early as 7 dpi. The early application of glyphosate as well as early mechanical destruction resulted in significant (P ≤ 0.05) reduction in the development of clubroot symptoms, root fresh weight and the number of resting spores⁄g root. Furthermore, the effect of volunteer management on clubroot severity in the succeeding OSR was studied by inoculating plants with the resting spores obtained from treated clubbed roots. Inoculated OSR exhibited root clubs similar to the initial symptoms after 35 dpi. Plants that were inoculated with spore suspension from early treated roots resulted in significant reductions in clubroot incidence and severity. Conversely, plants inoculated with the spore suspension from the late treated roots displayed levels of clubroot similar to the plants inoculated with the spore solutions of positive controls.
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