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Taiwan Agricultural Research Institute, Council of Agriculture, Executive Yuan--Publication-Special Publication of TARI-No.184

No.184

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Occurrence and Control of Impatiens Downy Mildew in Taiwan

Occurrence and Control of Impatiens Downy Mildew in Taiwan

Huang, Jin-Hsing1,* and Li, Pei-Ju1

Abstract

    In February 2013, a new disease of Impatiens walleriana occurred in nurseries in Changhua county, central Taiwan. The infected leaves showed pale green to yellow without distinct lesions, the dorsal surface showed white mildew, and the number of flower was obviously reduced. At the late disease stage, the leaves became brown and fell off. Plants collapsed rapidly especially at high humidity. When young plants and seedlings were infected, they generally didn’t survive. Microscopical observations of the infected leaves disclosed the hyaline, tree-like sporangiophores, with three apical branchlets. Sporangia were hyaline, oblong, and could release several to more than ten zoospores. The DNA sequences encoding to the large ribosomal subunit (LSU rDNA), being amplified using primers NL1 and NL4 and then sequenced, were identical to sequences of several Plasmopara obducnes isolates deposited in GenBank. Pathogenicity was confirmed by inoculation of detached leaves and potted plants with sporangia suspension of P. obducens, and the symptoms were similar to those in the field after 7 days inoculation at 20℃. Based on morphological, molecular and pathogenic characters, the pathogen was identified as P. obducens. The disease develop optimally at 12-20℃. The host range tests showed that the pathogen only infected certain I. walleriana and I. balsamina, but not I. hawkeri and other not impatiens potted plants. In the field tests, mancozeb+metalaxyl, benalaxyl, and phophorous acid + potassium hydroxide effectively controlled the disease after 4 times spraying at one week interval. However, the disease occurred again 2-3 weeks after stop spraying chemicals.

Key words: Impatiens walleriana, Plasmopara obducnes, control


1 Plant Pathology Division, Taiwan Agricultural Research Institute, COA
* Corresponding Author; e-mail: JHHuang@tari.gov.tw

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