Department of Crop Sciences
USDA, ARS, and Department of Crop Sciences
Department of Crop Sciences, University of Illinois, 1101 W. Peabody Dr., Urbana 61801
Reddish brown lesions were observed on the lower stem and upper root area of velvetleaf (Abutilon theophrasti Medik.) plants growing in an Illinois soybean field in June 2000. The lesions were similar in appearance to those caused by Rhizoctonia root rot of soybean. Stems and roots with lesions were cut into ≈5-mm pieces, surface-disinfested, and placed on 2% water agar at pH 4.5. The cultural morphology of the two isolates fit the description of Phomopsis longicolla Hobbs (1). Colonies on potato dextrose agar (PDA) were floccose, dense, and white. The undersides of the cultures were colorless. Stromata were large, black, and spreading. The pattern of stromata in one isolate was effuse, and most of the stromata were immersed or semiimmersed in the medium, whereas the stromata from the other isolate were massive and prominent. Neither isolate turned green on PDA. Alpha conidia were hyaline, ellipsoidal to fusiform, and guttulate. DAPI (4′,6-diamidino-2-phenylindole)-stained alpha conidia were uninucleate. Beta conidia and perithecia did not occur on either PDA or oat flakes on water agar from 1 to 10 weeks at 25°C under a 12-h photoperiod. The DNA sequences of the mitochondrial small subunit rRNA genes of the two isolates were identical and shared 100% sequence identity with two P. longicolla soybean isolates that we had identified previously. Pathogenicity tests were conducted in a greenhouse by cutting the stems of 3-week-old soybean and velvetleaf plants at the second internode. Mycelial plugs (4 mm in diameter) from the margin of 1-week-old cultures of the two isolates from velvetleaf and one from soybean were individually placed mycelial side down directly on the top of cut stems of 10 to 15 plants per isolate. Controls included noninoculated plants with and without PDA plugs. Plants were kept in a mist chamber in the dark at 25°C for 4 days and were then transferred to a greenhouse with a 16-h photoperiod at 24 ± 3°C. Stem lesions were measured 7 days after inoculation. The experiment was repeated once. Mean stem lesion lengths caused by the velvetleaf and soybean isolates were 23 and 20 mm, respectively, on soybean stems, while negative controls produced no lesions. Mean stem lesion lengths caused by the velvetleaf and soybean isolates were 23.5 and 12 mm, respectively, on velvetleaf stems. P. longicolla was reisolated from the stem lesions of five randomly collected plants. This is the first report of P. longicolla being isolated from velvetleaf and causing stem lesions on inoculated soybean and velvetleaf plants.
Reference: (1) T. W. Hobbs et al. Mycologia 77: 535, 1985.