FIRST REPORT OF POWDERY MILDEW CAUSED BY ERYSIPHE CRUCIFERARUM ON BRASSICA JUNCEA IN VIETNAM
Ngày đăng : 29/08/2019


L. T. T. Tam; P. N. Dung and N. V. Liem

        Brassica juncea (L.) Czern., belonging to the Brassicaceae, is usually consumed as a raw, fresh vegetable in the diet of Vietnamese people during the winter and spring seasons. In February 2014, powdery mildew was first observed on 80 to 100% of the area under B. juncea cultivation in Ha Noi and Lang Son. Within a week, symptoms appeared as irregular white patches on both sides of the leaves, and entire plants were affected. Economic loss was unavoidable as farmers were unable to safely spray chemicals in time to suppress the disease. Two representative specimens were deposited in the Plant Protection Research Institute Herbarium (Accession Nos. PPRI-PM033 and PPRI-PM034). Mycelia on pods, stems, and leaves were amphigenous, effuse or in patches, hyphal cells were thin-walled, smooth, hyaline. Appressoria on the hyphae were well developed, lobed, and mostly positioned in pairs. Conidiophores were cylindrical, and composed of one to four cells, and measured (81.8-)111-179.5 (-184.5) µm in length. Foot-cells were curved or straight, cylindrical, and measured 25.0 to 37.5 × 7.5 to 10.0 µm. Conidia were produced singly, were oval to cylindrical or ellipsoid, and measured 35.0 to 42.5 × 12.5 to 15.0 µm (length/width ratio of 2.6 to 3.4), and fibrosin bodies were absent. Conidia exhibited pseudoidium-type germination. No chasmothecia were found. The morphological characteristics were consistent with descriptions of Erysiphe cruciferarum Opiz ex L. Junell (Braun and Cook 2012). To confirm the identity of the fungal causal agent of the disease, the complete ITS regions of rDNA from PPRI-PM033 and PPRI-PM034 were amplified by the universal primer pairs HF1/HR4 and directly sequenced. The resulting sequence was deposited in GenBank (Accession Nos. KM260717 and KM260718). A GenBank BLAST search using the present data revealed that the ITS sequences had 100% identity with that of E. cruciferarum on Brassica rapa subsp. pekinensis in China (Accession No. KC878683) (Zhao et al. 2014). Pathogenicity was confirmed through an inoculation test by gently pressing affected leaves of B. juncea onto young leaves of 10 potted three-week-old seedlings of B. juncea. Ten noninoculated seedlings were used as controls. Plants were maintained in a greenhouse at temperatures ranging from 18 to 25°C. Inoculated leaves developed symptoms after 8 days, whereas the control plants remained symptomless. The fungus observed on inoculated leaves was morphologically identical to that observed on the original diseased leaves, and the sequence of the amplicon from PCR using primers HF1/HR4 was the same, fulfilling Koch’s postulates. Erysiphe cruciferarum is also known to infect numerous host species belonging to various genera of the Capparidaceae, Fumariaceae, Papaveraceae, Resedaceae, and Brassicaceae and is almost circumglobal in range, including Africa, North and South America, Asia, Australia, Europe, New Zealand (Braun and Cook 2012). To our knowledge, this is the first report of E. cruciferarum on B. juncea in Vietnam (Farr and Rossman 2014). Climatic conditions from February to May annually are especially favorable for development and outbreak of powdery mildews. Therefore, this disease on B. juncea will be a serious threat to the agricultural production of spring-season vegetables in Vietnam.

Nguồn; apsjournals.apsnet.org

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