Many Calathea species in the family Marantaceae are beautiful ornamental plants with variegated foliage. Among them, C. picturata ‘Argentea’, an evergreen perennial that has pale green leaves with dark green margins and a red underside, is a popular houseplant in Taiwan. In 2004, a new foliage disease that caused leaf blight of C. picturata ‘Argentea’ was first observed in a nursery in southern Taiwan. Initial symptoms were tiny, brown spots that appeared on the leaves of all ages, which quickly enlarged and coalesced. These necrotic lesions spread to cover the entire leaves in high temperature and moisture conditions and caused leaves to shrivel and eventually die. A dematiaceous hyphomycete with multicelled conidia was consistently isolated from the diseased leaves after being surfaced sterilized with 10% Clorox and placed on vegetable juice agar (10% V8 juice, 0.02% CaCO3, and 2% agar [VJA]). Pathogenicity of the isolate was tested by spraying ‘Argentea’ calathea leaves with a conidia suspension (1.6 × 105 conidia/ml) prepared from a culture grown on VJA at 28°C for 7 days. Plant leaves sprayed with distilled water were used as a control. Three pots of 15-cm high ‘Argentea’ calathea plants were inoculated with 10 ml of a conidia suspension and the experiment was conducted twice at 28°C and 90% relative humidity in a growth chamber. Tiny, brown spots started to show on all inoculated leaves 5 days after inoculation and the progression of symptom development was similar to that observed in nature. Control leaves remained asymptomatic. The same dematiaceous hyphomycete fungus was reisolated from 13 of 16 disease tissues taken from four symptomatic leaves. A colony of the calathea isolate was olive green when grown on potato dextrose agar (PDA) and conidia production was observed 7 days after incubation in darkness. The conidiophores were either branches from or the ends of normal mycelium, some of them geniculate with conidium produced at each bend measuring 142 to 602 (340) × 3 to 6 (4) μm on disease tissues and 51 to 150 (103) × 3 to 5 (4) μm on PDA. Conidia were multicelled with protruding hilum at the base, terminal cells thickened, olivaceous brown or golden brown in fusiform shape with blunt tips, 5 to 11 septate on disease tissues and 6 to 11 septate on PDA, measuring 46 to 166 (95) × 8 to 19 (13) μm on disease tissues and 58 to 145 (94) × 6 to 15 (11) μm on PDA, germinating by producing germ tubes semiaxially from each end. Morphological characteristics of the calathea isolate fit the description of the genus Exserohilum (2). Comparison of rDNA internal transcribed spacer (ITS) sequence of the calathea isolate with those in GenBank revealed that it shared 99.5% (549 of 552) similarity with a published sequence (GenBank Accession No. EU571210) (3) and Exserohilum rostratum was its closest species. ITS sequence analysis was done as previously described (1). Morphological and molecular data identified the pathogen as E. rostratum (Drechs.) Leonard & Suggs (= Bipolaris rostrata (Drechs.) Shoemaker). To our knowledge, this is the first report of leaf blight caused by E. rostratum on C. picturata in Taiwan.
References: (1) L. L. Chern et al. Plant Dis. 94:1164, 2010. (2) K. J. Leonard. Mycologia 68:402, 1976. (3) R. Sappapan et al. J. Nat. Prod. 71:1657, 2008.