Sclerotium rolfsii Causing Stem Rot of Impatiens walleriana in Brazil.

Abstract

Impatiens walleriana, busy lizzy or balsam (local names in Brazil maria-sem-vergonha or beijo-de-frade), is an African member of the Balsaminaceae that has long ago been introduced and established in Brazil. It is now widely cultivated commercially as a potted plant and a popular garden plant (3). It also is a common weed along the coast and is particularly troublesome in some banana plantation areas. There are only two records of fungal pathogens attacking this plant in Brazil: Cercospora fukushiana (leaf spot) and Oidiopsis haplophylli (powdery mildew). In January 2009, a population of diseased plants of I. walleriana was found in a private garden in the city of Rio de Janeiro, Brazil. Plants had rotted and girdled stem bases, leading to a collapse of stems Necrotic areas were covered with fans of white mycelium as well as abundant spherical sclerotia. The fungus was isolated in pure culture by direct aseptic transfer of mycelial fragments and sclerotia to potato dextrose agar (PDA) plates. Colonies were white, cottony, often forming fans, primary hyphae 3.0 to 6.0 μm in diameter, and bearing clamp connections; sclerotia formed after 7 days, initially white becoming dark brown with age, and 0.8 to 1.85 mm in diameter. These are typical features of Sclerotium rolfsii. A specimen was deposited in the local herbarium (Herbarium VIC) under Accession No. VIC 30732. Koch's postulates were performed by inoculating three healthy potted I. walleriana plants (10 × 40 cm high) with approximately 100 freshly collected sclerotia placed in close vicinity with the stem bases. Noninoculated plants kept in a separate pot served as controls. Plants were incubated in a dew chamber for 48 h at 25 ± 2°C. All inoculated plants showed symptoms of stem rot 72 h after inoculation, whereas controls remained healthy. S. rolfsii is a highly polyphagous species that has been recorded to be causing rots (also known as Southern blights) in Brazil on numerous hosts but there are no records of it attacking any members of the Balsaminaceae in Brazil. The only other published records of S. rolfsii on Impatiens spp. are from the United States (Hawaii and Illinois) (2) and the Philippines (1). In South America, there is a single report from Argentina (4) where the disease is regarded as a major threat to the potted plant industry because I. walleriana is one of the most popular potted plants in that country. The potential for losses is also significant for Brazil. To our knowledge, this is the first report of S. rolfsii as a pathogen of I. walleriana in Brazil. Although very damaging to I. walleriana, it is unlikely that this fungus can be used as a natural enemy of this plant species in weed situations because of its wide host range.