Lee I-M, Bottner K D, Sun M
Molecular Plant Pathology Laboratory, USDA-ARS, Beltsville, MD 20705.
Potato Laboratory, Montana State University Extension Service, Bozeman 59717.
Plant Dis. 2009 Sep;93(9):970. doi: 10.1094/PDIS-93-9-0970B.
Potato purple top (PPT) is a devastating disease that occurs in the United States, Canada, Mexico, Russia, and elsewhere causing great economic loss to the potato industry through substantially reduced tuber yield and quality. Chips and fries processed from infected tubers often develop brown discoloration, greatly reducing their marketability. At least seven distinct phytoplasma strains belonging to five different phytoplasma groups (16SrI, 16SrII, 16SrVI, 16SrXII, and 16SrXVIII) have been reported to cause purple top and related symptoms in potato (3). During an unusual drought in 2007, a newly emerging potato disease with extensive yellowish or reddish purple discoloration of terminal shoots and leaves, similar to PPT symptoms, was observed in isolated potato fields in Montana where over 50% of plants exhibited symptoms. Shoot tissues were collected from three symptomatic plants and 17 tubers randomly collected from 17 other symptomatic plants. The tubers were cold treated to induce sprouting and then planted in the greenhouse. All tubers produced plants of which seven exhibited PPT symptoms including severe stunting. Total nucleic acid was extracted from leaf veinal tissue, stolons, or tubers of 10 symptomatic and 10 asymptomatic plants (both field-collected and greenhouse samples) as previously described (3). A nested-PCR assay, using universal primer pair P1/16S-SR followed by R16F2n/R16R2n, was performed as previously described (2,3) to detect phytoplasmas in these samples. Phytoplasma strains were detected in all symptomatic plants. Restriction fragment length polymorphism (RFLP) analyses of nested-PCR products (approximately 1.2 kb) with seven key restriction enzymes (AluI, MseI, HhaI, Tsp509I, HpaII, RsaI, and BfaI) indicated that all samples contained a very similar or identical phytoplasma most closely related to reference strain MW1 (belonging to subgroup 16SrIII-F) (1). Analysis of cloned 16S rDNA sequences confirmed the identity of this new phytoplasma and sequences of three representative PPT-MT strains were deposited in GenBank with Accession Nos. FJ226074-FJ226076. Computer-simulated RFLP analyses of 1.2-kb 16S rDNA sequences of this new phytoplasma and representative members in the peach X-disease phytoplasma group (16SrIII) available in GenBank indicated the strain is distinct and represents a new subgroup, 16SrIII-M (4). This study also indicated that the phytoplasma is tuber transmissible since approximately 35% of plants produced from infected tubers collected in this study developed symptoms. Transmission via infected tubers may pose a potential threat for disease spread by planting uncertified seed potatoes. To our knowledge, this is the first report of 16SrIII group phytoplasmas-associated diseases in potato. A phytoplasma closely related to the PPT-MT strains has recently been detected in potato seedlings exhibiting purple top, rosette, and stunting in Alaska (GenBank Accession No. FJ376628). References: (1) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (2) I.-M. Lee et al. Int. J. Syst. Evol. Microbiol. 54:337, 2004. (3) I.-M. Lee et al. Int. J. Syst. Evol. Microbiol. 56:1593, 2006. (4) W. Wei et al. Int. J. Syst. Evol. Microbiol. 58:2368, 2008.
马铃薯紫顶病（PPT）是一种毁灭性病害，在美国、加拿大、墨西哥、俄罗斯及其他地区均有发生，通过大幅降低块茎产量和品质给马铃薯产业造成巨大经济损失。用感染病菌的块茎加工成的薯片和薯条常常会出现褐色变色，严重降低其市场价值。据报道，至少有七种不同的植原体菌株属于五个不同的植原体组（16SrI、16SrII、16SrVI、16SrXII和16SrXVIII），可导致马铃薯出现紫顶病及相关症状（3）。在2007年一场异常干旱期间，在蒙大拿州一些孤立的马铃薯田块中观察到一种新出现的马铃薯病害，顶梢和叶片出现大面积淡黄色或红紫色变色，类似于紫顶病症状，超过50%的植株表现出症状。从三株有症状的植株上采集了茎尖组织，并从另外17株有症状的植株上随机采集了17个块茎。对块茎进行冷藏处理以诱导发芽，然后种植在温室中。所有块茎都长出了植株，其中七株表现出紫顶病症状，包括严重矮化。按照之前描述的方法（3），从10株有症状和10株无症状植株（包括田间采集和温室样本）的叶脉组织、匍匐茎或块茎中提取总核酸。采用巢式PCR检测法，先用通用引物对P1/16S-SR，然后用R16F2n/R16R2n，按照之前描述的方法（2,3）对这些样本中的植原体进行检测。在所有有症状的植株中均检测到植原体菌株。用七种关键限制性内切酶（AluI、MseI、HhaI、Tsp509I、HpaII、RsaI和BfaI）对巢式PCR产物（约1.2 kb）进行限制性片段长度多态性（RFLP）分析，结果表明所有样本均含有一种与参考菌株MW1（属于16SrIII-F亚组）关系极为密切或相同的植原体（1）。对克隆的16S rDNA序列进行分析，确认了这种新植原体的身份，并将三个代表性PPT-MT菌株的序列存入GenBank，登录号为FJ226074-FJ226076。对该新植原体1.2 kb的16S rDNA序列以及GenBank中桃X病植原体组（16SrIII）的代表性成员进行计算机模拟RFLP分析，结果表明该菌株是独特的，代表一个新的亚组，即16SrIII-M（4）。本研究还表明，这种植原体可通过块茎传播，因为在本研究中，从感染病菌的块茎长出的植株中约有35%出现了症状。通过感染病菌的块茎传播可能对种植未经认证的种薯导致病害传播构成潜在威胁。据我们所知, 这是关于马铃薯中与16SrIII组植原体相关病害的首次报道。最近在阿拉斯加表现出紫顶病、莲座状和矮化症状的马铃薯幼苗中检测到一种与PPT-MT菌株密切相关的植原体（GenBank登录号为FJ376628）。参考文献：（1）I.-M. Lee等人，《国际系统细菌学杂志》48:1153，1998年。（2）I.-M. Lee等人，《国际系统与进化微生物学杂志》54:337，2004年。（3）I.-M. Lee等人，《国际系统与进化微生物学杂志》56:1593，2006年。（4）W. Wei等人，《国际系统与进化微生物学杂志》58:2368，2008年。
