Pathogenicity evaluation of Cytospora eucalypticola isolated from Eucalyptus spp. cankers in Uruguay

Cytospora eucalypticola has been frequently associated with twig and stem cankers and as endophyte of Eucalyptus globulus and E. grandis in Uruguay. Mycelium discs of two C. eucalypticola isolates obtained from actively growing colonies were inoculated, both superficially and on experimentally wounded stems of E. globulus and E. grandis. No inoculated and control plants have shown any discoloration, gumosis or necrosis nor did they display lesions ten months after inoculation. Callus tissue was formed, partially or wholly occluding the wounds. The ability to penetrate healthy tissues and the inability to produce lesions evidenced that the presence of C. eucalypticola in twig and stem cankers could result from saprotrophic expansion of the endophytic mycelium in dying tissues, cankers probably being produced by different environmental stress conditions. Additional keywords: endophyte, stem wound, inouculation, saprotrophic expansion, environmental stress. RESUMO Avaliação da patogenicidade de Cytospora eucalypticola isolado de Eucalyptus spp. no Uruguai Cytospora eucalypticola tem sido, freqüentemente, associado a lesões de pequenos ramos e cancros de caules e como endofíto de Eucalyptus globulus e E. grandis no Uruguai. Discos de micélio de dois isolados do C. eucalypticola obtidos de colônias crescendo ativamente foram inoculados superficialmente e em caules de E. globulus e E. grandis feridos experimentalmente. Não se observou exsudação de goma, descoloraçao, necrose, nem lesões dez meses depois nos controles. O tecido de calo foi formado, parcialmente ou totalmente fechando as feridas. A capacidade de penetrar tecidos saudáveis e a incapacidade de produzir lesões evidenciou que a presença de C. eucalypticola em cancro do caule pode resultar em uma expansão saprofítica do micélio em tecidos enfraquecidos. O cancro provavelmente foi produzido por algum tipo de estresse abiotico Palavras-chave adicionais: endofíto, ferida do caule, inoculação, expansão saprofítica, estresse abiótico. 289 Fitopatol. bras. 30(3), maio jun 2005 Fungal diseases of Eucalyptus spp. could become a significant problem in Uruguay considering that c.a. 370,000 ha have been planted during the last ten years mainly with Eucalyptus spp. A species of the genus Cytospora has been associated with seedling and tree cankers in several countries (Keane et al., 2000) and also as endophytes on leaves and twigs of Eucalyptus nitens (Deane et Maiden) Maiden (Fisher et al., 1993). In Uruguay it has been found in sprouting stumps of E. globulus Labillardière (Bettucci & Saravay, 1993) and in healthy and symptomatic twigs of E. globulus and E. grandis W. Hill ex Maiden (Bettucci & Alonso, 1997; Bettucci et al., 1997; Bettucci et al., 1998). Several isolates obtained from these materials were identified by means of micromorphological and molecular characteristics as Cytospora eucalypticola Van der Westh. (Alonso et al., 2002). It had been considered a non pathogenic species by Old et al. (1986). This fact led the authors to evaluate if the isolates of this fungus obtained from cankers could be pathogenic to E. globulus and E. grandis. Among several isolates of C. eucalypticola, it was possible to distinguish two groups that presented some different cultural characteristics. One group was characterized as having white and abundant aerial mycelium while the other one had olive green and scarce aerial mycelium. One isolate of each group was selected to evaluate its pathogenicity to Eucalyptus spp., MVFI 51 was obtained from cankers on twigs of E. globulus, and belonging to the first group, while MVHC 7115, isolated from cankers on twigs of E. grandis belonged to the second one. Five, six-month old plants of E. globulus and E. grandis were inoculated. They were actively growing in an experimental field located in Montevideo, southern Uruguay, and were without apparent environmental stresses. The stems of the five Eucalyptus spp. plants were disinfected with 70% ethanol, and 5 mm-diameter disks of bark were taken with a cork borer and replaced with 4 mm diameter disks of mycelium obtained from the margins of a colony and then wrapped in a single layer of Milipore tape to prevent desiccation and cross contamination. Another five non-wounded Eucalyptus spp. plants were inoculated to