Canker on umbra tree caused by Fusarium sambucinum

Umbra tree (Phytolacca dioica L.) is an ornamental plant native to South America, that grows on the pampas plains and is cultivated in gardens and parks. It is a large-sized herbaceous plant which is cultivated to provide shade and for medicinal purposes. Since 1999 typical canker symptoms have been observed in plants from Buenos Aires province, Argentina. Small cankers appeared on young branches as elliptical greyish-brown areas, with dark brown and irregular margins. As the cankers enlarge up to 4-5 cm they girdle branches, defoliate and kill the apical portion. The surface of the central infected tissues appears slightly swollen and cracked. Perithecia were observed on older cankers.

Necrotic tissue fragments were surface-sterilised and plated on potato dextrose agar (PDA). A Fusarium sp. was the only fungus isolated. Single spore isolates were obtained and the micromorphology and culture features were examined in PDA and carnation leaf agar (Nelson et al., 1983). All isolates obtained were identified as Fusarium sambucinum, based on the characteristic macroconidia with beaked apical cell. The perithecia were identified as Gibberella pulicaris based on the morphology (Booth, 1971) and the culture of ascospores which have produced F. sambucinum-type cultures. For the pathogenicity test a spore suspension (3.5 x 10⁶ conidia per ml) was sprayed onto wounded branches of 12 healthy umbra plants and respective controls. The plants were placed in greenhouse and observed daily for symptom development over two months. Cankers developed slowly, reaching a length of 2-3 mm in 20-25 days post inoculation. As the necrosis progressed the young branches showed symptoms similar to those described above. F. sambucinum was successfully re-isolated from branches of the inoculated plants.

The disease could be spread throughout the growing season by conidia. Also, considering the occurrence of the sexual state, G. pulicaris on the aged cankers we speculate that wind and rain could disseminate ascospores.

There are no published reports of cankers on umbra tree caused by Fusarium species. Nevertheless F. sambucinum is known as a canker-causing organism on herbal and woody plants (Booth, 1971). In the USA, F. roseum is cited on Phytolacca americana L. (Farr et al., 1989). This is the first report of canker disease caused by Fusarium sambucinum in umbra tree.