RESUMEN
Grafted tomato (Solanum lycopersicum L.) is widely used to manage soil-borne diseases (Lee et.al 2010). In Taiwan, grafting on eggplant (S. melongena L.) rootstock have been extensively used to reduce bacterial wilt in tomato production. In July 2019, wilting plants were found at a cherry tomato farm (~ 0.5 ha) located at Miaoli County. About 10% tomatoes of cv. 'Mint Shine' grafted onto the eggplant rootstock displayed wilt symptoms. Numerous leaflets with chlorosis, inter-vein yellowing, V-shaped necrotic lesions and withered leaves were observed on the affected plants. Some plants eventually wilted and died. A cut at the grafting site revealed the vascular discolorations on both scion (tomato) and rootstock (eggplant). A fungus with a compact whitish colony was consistently isolated from the symptomatic vascular tissue by using acidified potato dextrose agar (PDA) plates. Two isolates, Ve2 from eggplant and Ve4 from tomato, grown on PDA plates were characterized. Both Ve2 and Ve4 grow slowly (ca. 2.6 mm/day at 28 oC) and shared almost identical cultural and morphological characteristics. They first showed whitish mycelium and cream color in reverse within 1 week. Later, numerous microsclerotia developed evenly over the colony and the reverse color turned dark black. Microscopic observations revealed hyaline hyphae with black, elongated, irregularly spherical microsclerotia measuring 31.3 to 71.5 × 16.8 to 49.0 µm (average 50.4 × 28.5 µm) on a 3-week-old PDA culture. Abundant hyaline, single-celled, ellipsoida conidia measuring 2.7 to 4.7 × 0.9 to 3.2 µm (average 3.7 × 1.9 µm) and verticillate conidiophores were observed. The fungus was identified as Verticilium dahliae based on the consistent morphological characteristics (Hawksworth et. al 1970). To confirm the identity, the internal transcribed spacer regions of ribosomal DNA, amplified by PCR with universal primers ITS4/ITS5 (White et.al 1990), were sequenced. Both strains shared the same sequences (GenBank MZ734460; MZ736637), and BLASTn searching was 100% identical to many records of V. dahliae including an ex-epitype CBS130341. Pathogenicity was tested on 3-week-old seedlings of tomato cv. 'Bonny best' and eggplant cv. 'Longship' by a root dip method (Bhat & Subbarao 2007). Eighteen plants arranged into three replications were inoculated for each host-isolate combination, and incubated in the greenhouse at 25±3â. The pathogenicity test was repeated two times, with the result that both isolates were pathogenic to tomato and eggplant. Both isolates induced wilt symptoms in all inoculated plants within 14 days post-inoculation (DPI). Severe leaf drop, wilting and vascular discoloration in all inoculated eggplant whereas slight yellowing and mildly stunt growth in tomato were observed at 21 DPI. Koch's postulates were fulfilled by re-isolating the same fungus from both infected tomato and eggplant. All uninoculated plants remained health and no V. dahliae was isolated from them. To our knowledge, this is the first report of V. dahliae and Verticilium wilt of grafted tomato caused by this pathogen in Taiwan. This pathogen affects over 400 plant species and has resulted in significant economic losses in many regions of world (Subbarao 2020). It is important to investigate the distribution and extent of damage caused by this emerging pathogen on Solanaceous or other crops.
RESUMEN
Mungbean (Vigna radiata L.) is routinely grown in the experimental fields at the headquarters of the World Vegetable Center (23°6'30.88"N, 120°17'51.31"E) for breeding, research and germplasm multiplication. In a spring 2016 mungbean trial, about 50% of the plants were affected with powdery mildew. The white, powdery-like patches first appeared on the upper leaf surfaces, and soon developed to grey patches on both sides of the leaves. Purple to brown discoloration appeared on the underside of the infected leaf. Microscopy examination revealed that the causal organism was not Erysiphe polygoni, which had previously been documented as the powdery mildew pathogen on mungbean in Taiwan (Hartman et al. 1993). The fungus produced typical structures of the powdery mildew Euoidium, anamorph of the genus Podosphaera. The mycelium consisted of septate, flexuous hyphae with indistinct appressoria. The erect conidiophores arising from superficial hyphae varied from straight or slightly curved to curled. Three to ten conidia were borne in long chains with crenate edges. Foot-cells were straight, cylindrical and measured 30 to 52 µm long. Conidia were hyaline, ellipsoid-ovoid to barrel-shaped, with fibrosin bodies, and measured 27 to 33 (mean = 30.4) × 15 to 20 (mean = 16.6) µm. Germ tubes were clavate and occasionally forked, and were produced from the lateral sites of the conidia. No chasmothecia were found in the samples. The morphological characteristics were consistent with P. xanthii (Castagne) U. Braun & Shishkoff (Braun & Cook 2012). To confirm the identity, the internal transcribed spacer (ITS) region of rDNA and partialß-tubulin gene (TUB2) for the isolate MG3 were amplified with the primers ITS4/ITS5 (White et al. 1990) and BtuF5/BtuR7a (Ellingham et al. 2019), respectively. BLASTn analysis revealed the ITS sequence (MN833717) was 100% identical to many records of P. xanthii whereas the TUB2 sequence (MW363957) was 100% identical to a record of P. fusca (syn. P. xanthii; KC333362) in NCBI GenBank. A pathogenicity test was conducted by dusting conidia from an infected leaf onto six healthy four-week-old mungbean plants (cv 'Tainan No. 3'). Another three plants were not inoculated and were used as control. All the plants were maintained in a greenhouse at 25 to 28°C. All inoculated plants developed powdery mildew symptoms after 10 days, whereas the control plants remained symptomless. To our knowledge, this is the first report of P. xanthii causing disease on mungbean in Taiwan. P. xanthii also has been reported on mungbean in Thailand (Meeboon et al. 2016), while other records referring to E. polygoni infecting Vigna spp. are from Brazil and Fiji (Farr & Rossman 2020). Although both P. xanthii and E. polygoni have now been reported as causing powdery mildew on mungbean in Taiwan, which species predominates or is more important remains unclear. A comprehensive survey with accurate species identification is required to develop effective management of the disease, particularly for resistance breeding.