First Report of Kalanchoe Leaf and Stem Spot Caused by Corynespora cassiicola in Denmark

K. Madriz-Ordeñana, H. J. L. Jørgensen, K. L. Nielsen, Hans Thordal-Christensen

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Abstract

Kalanchoe (Kalanchoe blossfeldiana Poelln.) is a succulent plant of great importance to the flower industry in Denmark. Denmark accounts for nearly one fourth of the European production of ∼200 million plants per year. In October 2015, we observed stems and leaves of Kalanchoe affected by what appeared to be a fungal disease in a commercial nursery in Denmark. It is estimated that up to 5 to 6% of the plants were affected at different growth stages. The symptoms were dark brown, sunken, necrotic lesions at the base of the plants, expanding to the stem and lower leaves. Severely affected plants showed extensive crown rotting and defoliation. The causal fungal pathogen was isolated from lesions on potato dextrose agar (PDA). Cultures were whitish-gray to light brown and darker underneath. Older cultures became uniformly dark brown. Microscope observation revealed cylindrical conidiophores, straight or slightly curved, and pale to light brown. Conidia had 2 to 19 septa and appeared light brown, straight or curved, cylindrical, and occasionally obclavate with a length of 15.9 to 239.1 µm (avg. 53.9 µm, n = 171) and a width of 4.9 to 10.9 µm (avg. 7.5 µm, n = 100). Based on this morphology, the fungus was identified as Corynespora cassiicola (Berk. & Curt.) Wei (Ellis and Holliday 1971). An isolate, CP2285, was deposited in our culture collection. The ribosomal ITS region of this isolate was sequenced (GenBank accession no. KX458107) and found to have over 99% nucleotide identity to ribosomal ITS sequences of several reported strains of C. cassiicola from different hosts, thus confirming its identity. Pathogenicity tests were carried out using cultivar Tender White. For this, four cuttings, with leaf and petiole, were placed in separate petri dishes containing filter paper soaked in sterile water. Inoculum (8 × 10 4 conidia·ml –1, 0.05% Tween 20) was obtained from 3-week-old PDA cultures. Three 80-µl drops of the inoculum were placed separately on each leaf and the petiole. Another four cuttings were mock-treated with water with 0.05% Tween 20. Initial lesions appeared in all inoculum-treated tissues after 3 days of incubation at room temperature (18 to 22°C), while extensive lesions occurred after 10 days, producing a symptom pattern identical to originally diseased plants. No symptoms appeared in the controls. To test whether C. cassiicola can infect Kalanchoe by spreading through the soil, we treated 14 newly planted, symptom-free cuttings by drenching the soil (pathogen-free commercial sphagnum) with 1 ml inoculum (8 × 10 4 conidia·ml –1, 0.05% Tween 20). Another 14 cuttings were mock-treated with water with 0.05% Tween 20. After 4 weeks of cultivation in the greenhouse, 9 out of the 14 inoculated plants showed characteristic leaf and stem spot lesions. None of the control plants were symptomatic. Microscopy of samples prepared from these experiments showed the presence of fungal structures characteristic of C. cassiicola, thus fulfilling Koch’s postulates. To our knowledge, this is the first report of leaf and stem spot caused by C. cassiicola in Kalanchoe in Denmark. Given the commercial importance of Kalanchoe, and the severity and wide distribution of C. cassiicola (Dixon et al. 2009), we consider this disease of particular significance for the successful production of Kalanchoe in Europe.

Original languageEnglish
JournalPlant Disease
Volume101
Issue3
Pages (from-to)505
Number of pages1
ISSN0191-2917
DOIs
Publication statusPublished - Mar 2017
Externally publishedYes

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