Exploring the host range and infection process of Venturia paralias isolated from Euphorbia paralias in France

Abstract

In 2008, a fungal pathogen causing severe necrotic leaf and stem lesions on Euphorbia paralias (commonly referred to as sea spurge) was collected in France. The fungus has recently been described as Venturia paralias and offers potential for the classical biological control of sea spurge in Australia, where the plant is invasive in dune and foredune ecosystems. To explore the host range of V. paralias, a series of experiments comprising 45 species from the Euphorbiaceae, Picrodendraceae and Phyllanthaceae families were performed. In addition, the development of V. paralias on leaves of sea spurge and the other species inoculated was examined microscopically. Results revealed that V. paralias is highly specific, only causing disease symptoms on sea spurge and Euphorbia segetalis, its most phylogenetically related species tested that is present in Australia. Microscopic observations showed that the infection process of V. paralias on its two susceptible hosts is similar to that of other Venturia species, with characteristic expanded hyphal networks within the subcuticular space of leaves. In contrast, V. paralias did not penetrate leaves of other species inoculated, except for three (Euphorbia myrsinites, Euphorbia helioscopia and Mercurialis annua) on which it penetrated the leaf cuticle and developed an initial infection hypha that did not expand any further. Results from this study support the conjecture that V. paralias will not pose a threat to valued non-target Euphorbiaceae species should the fungus be approved for introduction to Australia for the biological control of sea spurge.