Calonectria pentaseptata Causes Severe Leaf Disease of Cultivated Eucalyptus on the Leizhou Peninsula of Southern China

Eucalyptus (Myrtaceae, Myrtales) trees are widely cultivated for commercial purposes worldwide. Calonectria leaf blight is one of the most prominent diseases associated with Eucalyptus trees grown in plantations in Asia and South America. Recently, symptoms of leaf blight, shoot blight, tree death, and seedling rot caused by Calonectria species have been observed in commercial Eucalyptus plantations and nurseries in Leizhou Peninsula, which is one of the most densely Eucalyptus-planted areas in southern China. Disease samples were collected from 10 Eucalyptus species and a number of Eucalyptus grandis, E. tereticornis, and E. urophylla hybrid genotypes that were planted on plantations at 13 sites and one experimental nursery. A total of 773 isolates of Calonectria were obtained from 683 plantation trees and nursery seedlings. Fifty-five representative isolates from all the surveyed sites and Eucalyptus species/genotypes were selected for molecular identification. These 55 isolates were identified by DNA sequence analyses based on the calmodulin (cmdA), histone H3 (his3), translation elongation factor 1-alpha (tefl), and beta-tubulin (tub2) gene regions, as well as a combination of morphological characteristics. The results indicated that these 55 isolates present one single species, Calonectria pentaseptata. Determined by sequences of cmdA, his3, tefl, and tub2 gene regions, only two genotypes were identified among the 55 representative isolates; 54 of these isolates share the same genotype, suggesting that the genetic diversity of Ca. pentaseptata collected during this study was relatively low. A growth study indicated that Ca. pentaseptata is a high -temperature species. The mating test results suggested that Ca. pentaseptata is heterothallic or lacks the ability to recombine to produce fertile progeny. Inoculation results showed that Ca. pentaseptata causes leaf blight and stem rot, resulting in tree death of the two widely planted Eucalyptus genotypes in southern China, and that the two genotypes differ significantly in their susceptibility to infection by Ca. pentaseptata. A selection program to develop Eucalyptus planting stocks with high levels of resistance to Calonectria leaf blight in China during the long-term should be urgently initiated.