At least 12 avirulence genes have been genetically identified and mapped in Phytophthora sojae, an oomycete pathogen causing root and stem rot of soybean. Previously, the Avr4 and Avr6 genes of P. sojae were genetically mapped within a 24 kb interval of the genome. Here, we identify Avr4 and Avr6 and show that they are actually a single gene, Avr4/6, located near the 24-kb region. Avr4/6 encodes a secreted protein of 123 amino acids with an RXLR-dEER protein translocation motif. Transient expression of Avr4/6 in soybean leaves revealed that its gene product could trigger a hypersensitive response (HR) in the presence of either Rps4 or Rps6. Silencing Avr4/6 in P. sojae stable transformants abolished the avirulence phenotype exhibited on both Rps4 and Rps6 soybean cultivars. The N terminus of Avr4/6, including the dEER motif, is sufficient to trigger Rps4-dependent HR while its C terminus is sufficient to trigger Rps6-mediated HR. Compared with alleles from avirulent races, alleles of Avr4/6 from virulent races possess nucleotide substitutions in the 5′ untranslated region of the gene but not in the protein-coding region.

Phytophthora sojae causes root and stem rot, one of the most devastating diseases of soybean worldwide. In Heilongjiang and Fujian provinces in China, serious cases of Phytophthora stem and root rot have occurred and caused heavy losses in the past several years. To determine the current population status of this pathogen, we investigated the pathogen's distribution, pathotypes, and metalaxyl sensitivity in both provinces. P. sojae was baited and isolated from 258 soil samples in both provinces using the soybean leaf bait method. The pathotypes of all isolates were characterized on 13 differential soybean cultivars using the hypocotyl slit inoculation method, and the sensitivity of all isolates to metalaxyl was tested in vitro. In all, 75 isolates were recovered from 75 fields in 33 counties; of these, 31 counties were in Heilongjiang Province and 2 counties were in Fujian Province. Thirty-five new pathotypes were identified compared with the previously defined races. Less than 5% of the isolates were virulent to cultivars with individual Rps genes 1a, 1c, or 1k. No metalaxyl-resistant isolates were found; the half maximal effective concentration values of all isolates ranged from 0.04 to 0.22 μg ml^–1. These results suggest that effective management of the disease in both provinces can be accomplished through the use of resistant cultivars with Rps genes 1a, 1c, or 1k and the fungicide metalaxyl.

Phytophthora sansomeana E.M. Hansen & Reeser is a newly described species and infects Douglas-fir, alfalfa, and soybean (1). Soybean production is an important part of the local economy in Yili State in Xinjiang Uygur Autonomous Region, northwest China. Unfortunately since 2005, root and stem rot disease has emerged on a number of farms. To identify the causal agent, plant samples with symptoms, including whole plant wilting or yellowing and stunting, were collected from fields during 2005 and 2008. Tissue from the edges of stem lesions was placed on selective lima bean agar (LBA) at 20°C for 3 to 4 days (2,3). Four single zoospore isolates of Phytophthora were obtained and maintained on LBA or 10% V8 juice liquid medium for examination of morphological and physiological characteristics. The colonies on LBA were aerial and rosaceous. The isolates were homothallic, and oogonia and oospores were readily produced in culture after 7 days on LBA plates. Oogonia averaged 38 μm and oospore width ranged from 23 to 48 μm and averaged 31 μm. Antheridia were approximately 15 × 12 μm and predominantly amphigynous in V8 juice. Sporangia were terminal or paragynous on persistent sporangiophores, nonpapillate, ovoid to obpyriform, and measured 52 × 35 μm with an average length/breadth ratio of 1.5. Hyphal swellings were produced in V8 juice 2 days after inoculation. The optimum temperature for growth was approximately 25°C and none occurred at 0 or 35°C. The internal transcribed spacer (ITS) sequence of this Phytophthora species (GenBank FJ966880) agreed 100% with sequences of P. sansomeana isolates deposited in GenBank (GQ853880 and EU925375). Pathogenicity tests were performed by hypocotyl inoculation method (2) using isolate Yili71 and potted soybean cv. Williams. Plants were grown in a growth chamber for 10 days before inoculation in 16-cm-diameter pots (2). Plants were inoculated with 2- × 2-mm plugs of mycelium grown for 4 days on LBA at 25°C, the plugs were adhered to the sides of wounded lower hypocotyls. As controls, plants were inoculated with LBA agar plugs without mycelium (2). Inoculated plants were maintained in a growth chamber at approximately 25°C with a 10-h dark/14-h light cycle and 50% relative humidity and symptom development was monitored daily for 1 week. Wounded stems inoculated with mycelium developed water-soaked lesions, which were similar to those seen on naturally infected plants. A Phytophthora sp. was reisolated from the margins of expanding lesions on wounded stems. To our knowledge, this is the first report of P. sansomeana infection of soybean in China and the threat it may pose to soybean production is unclear. References: (1) E. M. Hansen et al. Mycologia 101:129. 2009. (2) Z. Y. Wang et al. Fungal Genet. Biol. 43:826, 2006. (3) X. B. Zheng. Methods in Phytophthora. Chinese Agriculture Press. Beijing, China, 1995.

Areca palm yellow leaf (AYL) disease caused by the 16SrI group phytoplasma is a serious threat to the development of the Areca palm industry in China. The 16S rRNA gene sequence was utilized to establish a rapid and efficient detection system efficient for the 16SrI-B subgroup AYL phytoplasma in China by loop-mediated isothermal amplification (LAMP). The results showed that two sets of LAMP detection primers, 16SrDNA-2 and 16SrDNA-3, were efficient for 16SrIB subgroup AYL phytoplasma in China, with positive results appearing under reaction conditions of 64oC for 40 min. The lowest detection limit for the two LAMP detection assays was the same at 200 ag/μl, namely approximately 53 copies/μl of the target fragments. Phytoplasma was detected in all AYL disease samples from Baoting, Tunchang, and Wanning counties in Hainan province using the two sets of LAMP primers 16SrDNA-2 and 16SrDNA-3, whereas no phytoplasma was detected in the negative control. The LAMP method established in this study with comparatively high sensitivity and stability, provides reliable results that could be visually detected, making it suitable for application and research in rapid diagnosis of AYL disease, detection of seedlings with the pathogen and breeding of disease-resistant Areca palm varieties.

The areca palm, Areca catechu L., family Arecaceae is an important herbal medicine which has potential for the treatment of parasitic diseases, digestive function disorders and depression (Peng et al. 2015). Yellow leaf disease (YLD), caused by phytoplasma, is a destructive disease of Areca catechu. In 1981, the YLD was first discovered in Tunchang, Hainan, China. According to the investigation in 2020, the occurrence area of YLD was 32 102.38 hm2 in Hainan, China, resulting in 50%-60% yield loss. Previous researchers based on 16S rDNA gene PCR amplification analysis showed that YLD in Hainan was caused by 16SrI group phytoplasma (Che et al. 2010). In August, 2022, yellow leaf symptoms were observed on middle and lower leaves of Areca catechu. Forty symptomatic plants and three asymptomatic samples were collected in Wenchang, Hainan, China (19°33′9″N, 110°48′5″E). Forty-three samples (0.1g each) were used to extract total DNA (TIANGEN plant genomic DNA extraction kit). Phytoplasma universal primers named P1/P7 (Schneider et al. 1995) and R16F2n/R16R2 (Gundersen and Lee 1996) for 16Sr DNA and primers named fTuf1/rTuf1 and fTufu/rTufu (Schneider et al. 1997) for tuf genes were used for amplifying phytoplasma sequences from isolated DNA samples by nested PCR. No fragment was amplified in asymptomatic plants and four out of forty symptomatic samples could amplify target fragment. R16F2n/R16R2 amplicons (1 248 bp) and fTufu/rTufu amplicons (845 bp) from four symptomatic Areca catechu samples were sequenced in BGI (https://genomics.cn/). The 16Sr DNA GenBank accession numbers of four positive strains (named HNWC5, HNDZ1, HNDZ3 and HNDZ6) were OQ586072, OQ586085, OQ586086, OQ586087, respectively and the tuf GenBank accession numbers were OQ595209, OQ595210, OQ595211, OQ595212, respectively. Sequence alignment showed that the 16S rDNA and tuf sequence of HNDZ1, HNDZ3 and HNDZ6 were 100% consistent. 16S rDNA of HNWC5 was 99.96% consistent with HNDZ1 and tuf of HNWC5 was 98.31% consistent with HNDZ1. Interestingly, blast search based on 16S rDNA gene of HNWC5 showed 100% sequence identity with that of 16SrII group phytoplasma such as 'Eclipta prostrata' phytoplasma strain Ep1(MH144204.1), 'Aeschynomene americana' phytoplasma isolate AA1(MH231157.1) and 'Acacia confusa' witches'-broom phytoplasma isolate HK6(ON408364.1). Blast search based on tuf gene of HNWC5 showed 98.7% sequence identity with that of bamboo witches'-broom phytoplasma (FJ853160.1) and 91.02% sequence identity with that of 'podocarpus nagi' fasciation phytoplasma (KR633146) and 90.78% sequence identity with that of 'Musa acuminata' elephantiasis disease phytoplasma (MF983708). The phylogenetic tree was constructed based on 16Sr DNA gene by MEGA 7.0 employing neighbor-joining (NJ) method with 1000 bootstrap value (Kumar et al. 2016). The result indicated that the HNWC5, HNDZ1, HNDZ3 and HNDZ6 phytoplasma strains clustered a subclade in 16SrII group. The virtual RFLP analysis based on the 16Sr DNA gene sequence was performed by the online phytoplasma classification tool iPhyClassifier (Zhao et al. 2009) using restriction endonucleases of AluI, BamHI, BfaI, BstUI, DraI, EcoRI, HaeIII, HhaI, HinfI, HpaI, HpaII, KpnI, Sau3AI, MseI, RsaI, SspI and TaqI. The result indicated that HNWC5 was most similar to the reference pattern of peanut witches'-broom phytoplasma (16SrII-A subgroup, GenBank accession: L33765) and the pattern similarity coefficient of HNWC5 is 1.00. However, the HpaII restriction endonuclease pattern of HNDZ1, HNDZ3 and HNDZ6 was different from L33765 and the similarity coefficient was 0.97, which indicated this strain may represent a new subgroup within the 16SrII group. To our knowledge, this is the first report of 16SrII group related phytoplasma associated with YLD on Areca catechu in China. Our study contributes to understanding the polymorphism of phytoplasma causing YLD and provides an important reference for pathogen specific detection.