Interaction of Calibrachoa and Selected Root and Foliar Pathogens in Greenhouse Settings

Calibrachoa (Calibrachoa  hybrida) is a popular annual ornamental that was introduced in the late 1990s by the greenhouse ornamental industry. Little is published about its interaction with pathogens commonly associated with greenhouse production. We report here for the first time the response of Calibrachoa to infection by pathogens that may be introduced in greenhouse production cycles through the use of infested soil, contaminated tools, infected cuttings, and contaminated irrigation water. Rooted cuttings of ‘Colorburst Violet’ were artificially inoculated with isolates from Phytophthora, Pythium, Verticillium and Botrytis. Symptoms expressed in response to infection included interveinal chlorosis of young leaves, wilting and necrotic root tips with fewer or no secondary or tertiary roots. Non-challenged plants had healthy root systems with an abundance of primary, secondary, and tertiary roots. We observed a 12 to >80% decrease in root fresh weight in symptomatic plants compared to plants that showed no disease symptoms. All isolates from infected plants were recovered and identities confirmed. Greenhouse managers and clinicians should be aware that Calibrachoa is susceptible to several important plant pathogens and should scout regularly for them in order to exclude them as much as possible from their production systems. INTRODUCTION Calibrachoa (Calibrachoa  hybrida) is a solanaceous plant with some similarities to petunia (Petunia spp.) (Becktell et al., 2006). Both Calibrachoa and petunia do well in USA hardiness zones 10-11, and so are considered to be annuals in temperate regions. However, the two are different in that Calibrachoa is predominantly woody while petunias are mostly herbaceous (Reis et al., 2002.). Although Calibrachoa has become a popular ornamental since its introduction in the late 1990s by the greenhouse industry, little has been reported about its response to infection by plant pathogens that can be introduced through the use of infested substrates, contaminated tools, or contaminated water supply. Many species of the water mold pathogens Phytophthora and Pythium that are pathogenic or can be pathogenic to nursery and greenhouse crops (Ali-Shtayeh et al., 1991; Hong and Moorman, 2005), were often detected in irrigation water from nursery and greenhouse effluents, and from dust and soil-mix particle samples from walkways, floors and beds within the greenhouse and also from seedling flats (Stephens et al., 1983). Species of Phytophthora commonly cause root and crown rot in plants and at times foliar dieback when inoculum from the substrate or contaminated irrigation water is splashed onto susceptible stems and leaf tissue (Benson and von Broembsen, 2002; Bush, et al., 2006). Pythium species, on the other hand, infect plants in seedbeds, and seedlings and cuttings in the greenhouse, resulting in damping-off, decay and rotting of the seed before germination, or rotting of seedlings below the soil line (Moorman, 2002). Moreover, Pythium propagules in partially rotted plants and cuttings may be carried to a [email protected] Proc. IS on High Technology for Greenhouse Systems GreenSys2009