First Report of Cylindrocladium pseudonaviculatum Causing Leaf Spot of Pachysandra terminalis
Authors: LaMondia J. A. , Li D. W. , Marra R. E. , Douglas S. M. 
Published: May 2012

Cylindrocladium pseudonaviculatum Crous, J.Z., Groenew. & C.F. Hill 2002 was recently reported infecting common boxwood, Buxus sempervirens L., in Connecticut (2). We isolated the pathogen from leaf and stem lesions of B. sempervirens and obtained single-spored cultures on half-strength potato dextrose agar (½PDA). The pathogen was identified as C. pseudonaviculatum by morphological characteristics (1). Colony size reached 71 mm in diameter after 14 days at room temperature on ½PDA, and was fluffy with white aerial hyphae, mars brown, and reverse color chestnut brown at the center fading to pale brown forming concentric bands. Macroconidiophores were solitary or in a group of up to three, comprised a stipe, a sterile elongation, and one to three penicillate fertile branches. The stipe was up to nine septate, 90 to 250 μm long, colorless, smooth, terminating in a naviculate or broadly ellipsoidal vesicle with a pointed or papillate apex, and 27 to 50 × 6.5 to 9 μm. Primary branches were zero- to one-septate, 20 to 36 × 4 to 5 μm; secondary branches were aseptate and 11 to 20 × 3 to 4.5 μm; tertiary branches were rare, each terminal branch producing two to five phialides; phialides were doliiform or reniform, colorless, 12 to 18 μm. Conidia were cylindrical, rounded at both ends, straight, smooth, colorless, two-celled, 48 to 55 × 4.5 to 5.5 μm, and in colorless slimy cylindrical clusters. Microconidiophores were not observed. Chlamydospores were golden to dark brown, thick-walled, and smooth or rough. Microsclerotia were present on ½PDA. Primers T1 and T22 (3) were used to amplify a portion of the β-tubulin gene from isolates Cps-CT-L1 and Cps-CT-S1. Amplified sequences were used in a BLAST search against the GenBank database to demonstrate 100% sequence identity only with other C. pseudonaviculatum strains. Both sequences were deposited in GenBank (Accession Nos. JQ866628 and JQ866629), using corresponding gene data from C. pseudonaviculatum strain STE-U 3399 (GenBank Accession No. AF449455) to distinguish coding from noncoding regions. Healthy plants of Japanese spurge, Pachysandra terminalis, with three plants per 10 cm diameter pot, were inoculated with water alone or a conidial suspension of C. pseudonaviculatum isolate Cps-CT-L1 (ATCC MYA-4891) (1.0 × 106 conidia/plant) with a handheld sprayer until runoff. Plants were kept moist in a plastic bag for 48 h at laboratory temperature and then transferred to the greenhouse. Circular lesions (1- to 4-mm diameter) were evident on leaves after 10 days. All 12 inoculated plants developed lesions, and no lesions were observed on noninoculated plants. Leaves with lesions were surface sterilized in 0.5% NaOCl for 30 s, rinsed twice in sterile water, and lesion margins plated onto water agar or ½PDA. The pathogen was reisolated from at least one leaf per plant. Koch's postulates were performed again with isolate Cps-CT-S1 (ATCC MYA-4890). After 3 weeks, many of the leaves with lesions yellowed and dropped to the soil surface and heavy sporulation of C. pseudonaviculatum and microsclerotia were observed. To our knowledge, this is the first report of C. pseudonaviculatum causing a leaf spot disease on P. terminalis. Pachysandra is a widely grown ground cover suitable for shady, humid environmental conditions that may be conducive for the development of disease.

References: (1) P. Crous, et al. Sydowia 54:23, 2002. (2) K. Ivors et al. Plant Disease. 96:X, 2012. (3) K. O'Donnell and E. Cigelnik Mol. Phylogenet. Evol. 7:103, 1997.