Mexican petunia (Ruellia brittoniana) is an herbaceous flowering perennial with strikingly colored flowers, widely cultivated commercially as a potted plant and a popular garden plant. In July of 2010, root and stem rot that caused death was observed on Mexican petunia at the flower nursery of the Council of Agriculture & Chiayi County in Taiwan. Plants had rotted and girdled stem bases. Necrotic areas were covered with fans of white mycelium as well as abundant spherical sclerotia. A fungus was isolated from infected tissue and sclerotia, and maintained on potato dextrose agar (PDA) plates incubated at 25°C without light. Colonies were white, cottony, often forming fans; pure cultures were prepared by transferring hyphal tips to PDA. Sclerotia formed after 10 days, initially white becoming dark brown with age, and 0.5 to 0.6 mm in diameter. To confirm identity of the causal fungus, the complete internal transcribed spacer (ITS) rDNA region of the causal fungus was amplified using the primers ITS4 and ITS5 (2) and sequenced. The resulting sequence of 687 bp was uploaded in NCBI. The sequence was 98% similar to sequences of Athelia rolfsii (Sclerotium rolfsii) in NCBI (Accession No. JN543691.1). Koch's postulates were performed using two inoculation techniques. The soil near the base of healthy Mexican petunia plants (four plants per pot) were exposed to recently matured sclerotia (10 sclerotia per plant) developed from pure fungal cultures or 10-mm-diameter agar plugs of mycelium (one plug per plant). Noninoculated plants, in a separate pot, were used as a control. All plants were incubated in a growth chamber at 28 to 33°C. Disease symptoms occurred on all inoculated plants by 5 to 7 days and included yellowing of leaves, basal stem rot, and wilt. Ten days after inoculation, inoculated plants were dead whereas control plants remained healthy. The pathogenicity test was repeated twice with similar results and S. rolfsii was reisolated from infected plants in each test. The pathogen has been reported to cause substantial loss of Mexican petunia in Louisiana (1). The disease is becoming more common in Taiwan and could cause losses in Mexican petunia production. To our knowledge, this is the first report of disease on Mexican petunia caused by S. rolfsii in Taiwan.
References: (1) G. E. Holcomb. Plant Dis. 88:770, 2004. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, 1990.