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Title: Interaction of Calibrachoa and selected root and foliar pathogens in greenhouse settings

item Omer, Medani
item Locke, James
item Frantz, Jonathan
item Horst, Leona
item Krause, Charles

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2010
Publication Date: 5/19/2011
Citation: Omer, M.A., Locke, J.C., Frantz, J., Horst, L., Krause, C.R. 2011. Interaction of Calibrachoa and selected root and foliar pathogens in greenhouse settings. Acta Horticulturae. 893:173-180.

Interpretive Summary: Calibrachoa, a popular greenhouse ornamental, is a member of the night shade family with some similarities to petunias. Little is known about its response to infection by plant pathogens (e.g., water molds-species of Pythium and Phytophthora) that are commonly present in the greenhouse during the production process. Rooted cuttings of Calibrachoa were transplanted to 4 ½-inch pots containing a peat:perlite mix. All plants were treated once with an iron chelate (Fe-EDTA) solution to avoid iron deficiency that is commonly found in Calibrachoa production. Calibrachoa plants were treated with Pythium aphanidermatum, Pythium ultimum, Phytophthora cactorum, Phytophthora cinnamomi, Phytophthora citrophthora, Phytophthora nicotianae, Botrytis cinerea, and Verticillium dahliae. These pathogens may be introduced through the use of infested soil, contaminated tools, infected cuttings, and contaminated irrigation water. Visual inspection of disease symptoms showed that response of Calibrachoa plants to the pathogen treatments fell into three distinct categories: 1) Leaves showed severe disease symptoms and roots were dead. This was noticeable on plants treated with Phytophthora citrophthora and Phytophthora nicotianae. 2) Moderate disease severity on plants treated with Botrytis, Phytophthora cinnamomi, and Verticillium. 3) Very little or no symptom on leaves and well developed root system in plants challenged with Phytophthora cactorum, Pythium ultimum, and Pythium aphanidermatum. It appears that Calibrachoa is prone to infection by a diverse group of pathogens. Three of the Phytophthora species tested (P. nicotianae, P. citrophthora and P. cinnamomi) killed the plants within 2-5 weeks. Conversely, the two Pythium species (P. aphanidermatum and P. ultimum) had little effect on Calibrachoa. That could be due to the woody nature of Calibrachoa roots. Phytophthora species tested were known to infect woody plants. Growers and clinicians should be aware of Calibrachoa’s susceptibility to these plant pathogens. Steps should be taken, particularly by plant propagators, to exclude these pathogens as much as possible from their production systems.

Technical Abstract: Calibrachoa (Calibrachoa x 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 cycle through the use of infested soil, contaminated tools, infected cuttings, and contaminated irrigation water. Rooted cuttings of cv. 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.