First report of Coniothyrium canker of Eucalyptus in Mexico

Coniothyrium canker is an extremely damaging disease of Eucalyptus species caused by the pycnidial ascomycete Coniothyrium zuluense. The disease was first recognised in South Africa in 1989 although the causal organism was not characterised and described until later (Wingfield et al., 1997). Infection by C. zuluense initially results in measle-like necrotic spots on branches and stems (Fig. 1). These develop into large girdling cankers that reduce wood quality and may lead to the death of trees. The stem infections also restrict bark peeling prior to pulping resulting in increased labour and other costs (Wingfield et al., 1997, Van Zyl et al., 1997; Van Zyl, 1999).

During a recent survey of Eucalyptus diseases in the Tabasco state of Mexico, canker symptoms identical to coniothyrium canker were observed on approximately five year-old E. grandis trees near Los Choapas. Stem lesions contained pycnidia and conidia similar to those of C. zuluense. However, C. zuluense lacks robust morphological characteristics that clearly distinguish it from similar species. To confirm our field and morphological identification we examined the Mexican isolates by sequencing the internal transcribed spacer region (ITS) of the ribosomal RNA operon and analysed the data using PAUP* 4.0. This generated a single most parsimonious tree with a consistency and retention index of 0.9218 and 0.9161 respectively.

Sequence analysis confirmed our preliminary morphological identification of C. zuluense. A comparison between Mexican isolates (Genebank Accension nrs. AF385610, AF385611) and published C. zuluense sequence data (Van Zyl, 1999), show that the Mexican isolates group closer to those from South Africa than to isolates from Thailand, the only other known countries from which C. zuluense has been recorded. The Mexican isolates have been deposited in the culture collection of FABI, Pretoria.

The appearance of C. zuluense in Mexico is of considerable concern given its known impact in countries such as South Africa. Field evidence in Mexico suggests that C. zuluense isolates act in a similar manner to those in South Africa. However, inoculation experiments, currently not possible due to quarantine and other limitations, are needed to compare pathogenicity of strains from the two regions. Further work is needed to compare the pathogenic behaviour of the Mexican isolates.

Participants in emerging Eucalyptus plantation programmes in Mexico will need to seriously consider the possible impact of Coniothyrium canker. Breeding and selection of disease tolerant clones has been a useful strategy to reduce the impact of the disease in South Africa and it should be equally effective in Mexico.