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.

In recent decades, ‘Candidatus Liberibacter spp.’ have emerged as a versatile group of psyllid-vectored plant pathogens and endophytes capable of infecting a wide range of economically important plant hosts. The most notable example is ‘Candidatus Liberibacter asiaticus’ (CLas) associated with Huanglongbing (HLB) in several major citrus-producing areas of the world. CLas is a phloem-limited α-proteobacterium that is primarily vectored and transmitted among citrus species by the Asian citrus psyllid (ACP) Diaphorina citri. HLB was first detected in North America in Florida (USA) in 2005, following introduction of the ACP to the State in 1998. HLB rapidly spread to all citrus growing regions of Florida within three years, with severe economic consequences to growers and considerable expense to taxpayers of the state and nation. Inability to establish CLas in culture (except transiently) remains a significant scientific challenge toward effective HLB management. Lack of axenic cultures has restricted functional genomic analyses, transfer of CLas to either insect or plant hosts for fulfillment of Koch’s postulates, characterization of host-pathogen interactions and effective screening of antibacterial compounds. In the last decade, substantial progress has been made toward CLas culturing: (i) three reports of transient CLas cultures were published, (ii) a new species of Liberibacter was identified and axenically cultured from diseased mountain papaya (Liberibacter crescens strain BT-1), (iii) psyllid hemolymph and citrus phloem sap were biochemically characterized, (iv) CLas phages were identified and lytic genes possibly affecting CLas growth were described, and (v) genomic sequences of 15 CLas strains were made available. In addition, development of L. crescens as a surrogate host for functional analyses of CLas genes, has provided valuable insights into CLas pathogenesis and its physiological dependence on the host cell. In this review we summarize the conclusions from these important studies.

Phytoplasmas ('Candidatus Phytoplasma' spp.) are insect-transmitted, cell wall-less plant pathogenic bacteria that cause economically important crop diseases. Because phytoplasma are difficult or impossible to culture in vitro, they are taxonomically classified according to the convention used for unculturable microorganisms. The first coherent scheme of classification of phytoplasmas, based on the RFLP pattern of the 16S rRNA-encoding gene generated with seventeen restriction endonucleases, has been updated several times until the development of the iPhyClassifier. iPhyClassifier is an interactive online tool capable of determining the 'Candidatus Phytoplasma' species, group, and subgroup, of unknown samples using the 16S F2nR2 sequence. Latin America, an important geographic area in relation to food production, has a high incidence of plant diseases caused by phytoplasmas. However, many phytoplasmas associated with these diseases were not properly classified. An extensive literature review and the use of iPhyClassifier allowed us to identify six tentative new 16SrI subgroups, six 16SrII, one 16SrIII, one 16SrVII, one 16SrIX, one 16SrXII, two 16SrXIII, and two new tentative groups (16SrXXXIII-A and 16SrXXXIV-A) among the Latin American strains that were either previously unclassified or misclassified.

Plasmodiophora brassicae (Wor.) is an obligate intracellular plant pathogen affecting Brassicas worldwide. Identification of effector proteins is key to understanding the interaction between P. brassicae and its susceptible host plants. To date, there is very little information available on putative effector proteins secreted by P. brassicae during a secondary infection of susceptible host plants, resulting in root gall production. A bioinformatics pipeline approach to RNA‐Seq data from Arabidopsis thaliana (L.) Heynh. root tissues at 17, 20, and 24 d postinoculation (dpi) identified 32 small secreted P. brassicae proteins (SSPbPs) that were highly expressed over this secondary infection time frame. Functional signal peptides were confirmed for 31 of the SSPbPs, supporting the accuracy of the pipeline designed to identify secreted proteins. Expression profiles at 0, 2, 5, 7, 14, 21, and 28 dpi verified the involvement of some of the SSPbPs in secondary infection. For seven of the SSPbPs, a functional domain was identified using Blast2GO and 3D structure analysis and domain functionality was confirmed for SSPbP22, a kinase localized to the cytoplasm and nucleus.

Clubroot is an economically important disease affecting Brassica plants worldwide. Plasmodiophora brassicae is the protist pathogen associated with the disease, and its soil-borne obligate parasitic nature has impeded studies related to its biology and the mechanisms involved in its infection of the plant host. The identification of effector proteins is key to understanding how the pathogen manipulates the plant’s immune response and the genes involved in resistance. After more than 140 years studying clubroot and P. brassicae, very little is known about the effectors playing key roles in the infection process and subsequent disease progression. Here we analyze the information available for identified effectors and suggest several features of effector genes that can be used in the search for others. Based on the information presented in this review, we propose a comprehensive bioinformatics pipeline for effector identification and provide a list of the bioinformatics tools available for such.

Plant pathogen effector proteins are key to pathogen virulence. In susceptible host Brassicas, the clubroot pathogen, Plasmodiophora brassicae, induces the production of nutrient-sink root galls, at the site of infection. Among a list of 32 P. brassiae effector candidates previously reported by our group, we identified SSPbP53 as a putative apoplastic cystatin-like protein highly expressed during the secondary infection. Here we found that SSPbP53 encoding gene is conserved among several P. brassicae pathotypes and that SSPbP53 is an apoplastic protein able to directly interact with and inhibit cruciferous papain-like cysteine proteases (PLCPs), specifically Arabidopsis XYLEM CYSTEINE PEPTIDASE 1 (AtXCP1). The severity of clubroot disease is greatly reduced in the Arabidopsis xcp1 null mutant (AtΔxcp1) after infection with P. brassicae resting spores, indicating that the interaction of P. brassicae SSPbP53 with XCP1 is important to clubroot susceptibility. SSPbP53 is the first cystatin-like effector identified and characterized for a plant pathogenic protist.