Incidence and distribution of Botrytis convoluta in a collection of Iris × barbata irises and options of control

In the years 2005-2007 in a collection of Iris × barbata irises in the Botanical Garden and Arboretum of MZLU in Brno a disease spread appearing in symptoms of poor budding and growth of the irises, or rotting and dying off of the rhizomes. As the causal agent we identified the fungus Botrytis convoluta. In the present study we describe the isolated pathogen. The disease appears in a number of species of the genus Iris and has been reported in the USA, Europe, Israel and Japan. Over a period of three years we evaluated the frequency of incidence of the pathogen in 527 iris cultivars and its distribution on five plots. While in the first year the incidence of the pathogen appeared in 4.4-28.7 % plants, in the last year of our investigations the pathogen spread to 17.7-66.6 % plants. In the course of three years seven cultivars out of the 527 planted out died as a result of B. convoluta attack (i.e. 1.33 %). The results were processed statistically. Spacing of the plants and gradient of the plot influenced the spreading of the pathogen. The weather in the winter played an important role, particularly mild and humid winters with temperatures above 0 °C. Even though B. convoluta was and remains to be a factor limiting the overwintering of irises, there are very few data on fungicide control of irises. Even fewer data are available about the control of irises against latently infected rhizomes. At the present time no fungicides protecting irises against this pathogen have been registered in the Czech Republic.

Keywords: Iris × barbata, Botrytis convoluta, botrytis rhizome rot