Tamarix ramosissima wood canker and branch death in Argentina: assessment of the role of associated fungi and possible implications for classical biocontrol

Abstract

Saltcedars (Tamarix spp.), are among the invasive alien shrubs and trees present in Argentina causing major ecological and economic problems. As such, they have been selected as one of the pilot programmes of the National Strategy on Invasive Alien Species. A biological control (BC) project against Tamarix spp. was initiated in 2017, funded by the Ministry of Science, Technology and Productive Innovation with the main objective of assessing the feasibility of introducing the defoliating and stem damaging beetle Diorhabda sublineata, a BC agent successfully released against Tamarix spp. in the USA. It is common practice for these projects, to record the organisms associated with the target in its adventive range, to identify any that may either interfere with the BC agents intended for release, or that may themselves have potential as such. To this end, surveys were conducted to search for fungal pathogens in Tamarix populations in Argentina. T. ramosissima trees showing discoloration and death of branches were found at two sites located around 700 km apart. Several fungi were identified associated with these symptoms, among which Cytospora unilocularis and Neomicrophaeropsis tamaricicola, appeared as the most likely causal agents. Both were isolated on artificial culture media to provide inoculum for pathogenicity tests. Branches of six-month-old T. ramosissima potted plants were inoculated with C. unilocularis by either placing a drop of a conidial suspension in water into ad-hoc produced wounds or by spraying undamaged ones. Inoculation with N. tamaricicola was performed only on wounded branches. Control plants were treated with water alone. Wounded plants inoculated with C. unilocularis developed cankers, branch discoloration and death of branches after two months. Pycnidia in association with the symptoms were also observed on one of the plants. The fungus was re-isolated from the cankers allowing for the completion of Koch's postulates. To our knowledge, this is the first report of C. unilocularis causing disease on T. ramosissima. In this ongoing experiment, incipient symptoms have developed so far on the plants inoculated with N. tamaricicola. Both fungi have recently been described from the plant´s native range, suggesting they must have arrived in Argentina together with their host. Because C. unilocularis seems to infect only through wounds, the effect of this pathogen would be enhanced should D. sublineata eventually be introduced, thus contributing to a better control of populations where the pathogen is already present.