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First Report of the Giant Stem Nematode, Ditylenchus gigas, from Broad Bean in Iran

The giant stem nematode, Ditylenchus gigas (Nematoda: Tylenchida) has been recorded from several European and African countries mainly bordering the Mediterranean Sea (2). This nematode causes considerable yield loss of broad bean, Vicia faba, and it may induce more severe damage than the typical faba bean race of D. dipsaci. Spread of infestation through seed limits export of broad bean and has made these nematodes quarantine pests in many countries (2). Broad bean is cultivated in the north, west, southwest, and central parts of Iran. Although D. dipsaci has been reported from different crops and regions in Iran, there is no record of broad bean infection by this nematode. A survey of broad bean fields was conducted in the north and west provinces in a continuation of a study on different populations of D. dipsaci in Iran in May to July of 2007 and 2008 and resampling from some farms in June 2012. The sampling was performed at flowering stage and after. The aboveground plant samples were collected and cut into pieces of 2 to 3 cm, then incubated for 5 to 6 h in Whitehead trays. Morphological and molecular analysis of isolated nematodes from Kermanshah and Lorestan provinces revealed the presence of D. gigas in the samples. Of the 23 plant samples of cv. Barekat collected from Mazandaran and Golestan provinces in the north, 47.8% were infected with stem nematode, mostly with high population density of over 20,000 nematodes per 5 plant stems. The percentage of infected samples of broad bean cv. Shakhbozy collected in Lorestan and Kermanshah provinces in the west was 76.5%. The symptoms of infection were observed as necrotic lesions on the stem surface and reduction of internode distances in severe infection. The giant stem nematode population from Kermanshah showed the following characters: females (n = 20), L = 1,650 ± 140 (1,270 to 1,875) μm; b′ = 8.6 ± 0.6 (7.7 to 10.0), c = 19.0 ± 1.3 (19.2 to 21.2), c′ = 4.7 ± 0.5 (1.1 to 5.3), stylet = 11.6 ± 0.5 (11 to 12) μm; post vulval sac = 96 ± 16 (58 to 140) μm; vulval-anus distance = 217.0 ± 21.0 (178 to 272) μm, tail = 86.4 ± 9.4 (66 to 102) μm; males (n = 10), L = 1,495 ± 148 (1,236 to 1,636) μm; b′ = 7.7 ± 0.3 (7.3 to 8.1), c = 17.3 ± 0.7 (16.3 to 18.6), stylet = 11.3 ± 0.5 (11 to 12) μm, tail = 86.5 ± 8.5 (71 to 95) μm, spicules = 24.8 ± 1.7 (23 to 28) μm. The morphological and morpohometric features were generally in agreement with those published for D. gigas (2). The morphological identification of D. gigas from Iran was supported by the analyses of the ITS rRNA and the D2-D3 expansion segments of the 28S rRNA gene sequences. The rRNA gene of D. gigas from broad bean and D. dipsaci from garlic were amplified and sequenced using two primer sets: (i) the TW81 and AB28 for the ITS-rRNA and (ii) D2A and D3B for partly 28S rRNA gene, as described by (2). New sequences were deposited in the GenBank under accession numbers KC310732 through KC310735. The Iranian D. gigas sequences showed 100% similarity with those of the Italian D. gigas isolates (ITS rRNA: HQ219231, HQ219232; D2-D3 of 28S rRNA: HQ219217 and HQ219216). The identification was further supported by PCR with species specific SCAR (sequence characterized amplified region) primers for this species (1). The specimens from broad bean generated a specific fragment ∼200 bp for D. gigas, whereas the samples with D. dipsaci from garlic and alfalfa produced one fragment ∼250 bp specific for this species. To our knowledge, this is the first report of D. gigas from broad bean in Iran.

References: (1) M. Esquibet et al. Genome 46:1077, 2003. (2) N. Vovlas et al. Plant Pathol. 60:762, 2011.