In 2014, a new and serious leaf and shoot disease of unknown aetiology appeared in Eucalyptus plantations of North Sumatra, Indonesia. The disease is characterized by black necrotic spots that initially appear on young leaves and petioles, which become scab‐like as the lesions age. Infected trees respond to infection by producing shoots with small leaves that commonly appear feathered. Fruiting structures typical of most foliar pathogens are not seen, but using scanning electron microscopy (SEM), fungal spores are evident and associated with the scab‐like structures. Using culture‐dependent methods, cultures resembling a species of Elsinoe were isolated from the lesions. DNA sequence comparisons for four gene regions, as well as morphological observations, showed that the fungus is an undescribed species in Elsinoe, for which the name Elsinoe necatrix sp. nov. is provided. Pathogenicity trials on a Eucalyptus clone with the Elsinoe species resulted in scab‐like structures similar to those observed under field conditions and the fungus was easily reisolated from the resulting lesions. This study includes a description of the pathogen and characterization of the disease, for which the name Eucalyptus scab and shoot malformation is suggested.
Leaf blight caused by Teratosphaeria destructans is one of the most important diseases of Eucalyptus planted in the sub-tropics and tropics. In contrast, the better-known Teratosphaeria epicoccoides, while also a primary pathogen of Eucalyptus, causes less damage to trees in these areas. Although T. destructans is an aggressive pathogen, nothing is known about its infection biology. In this study, the conditions for infection and disease development caused by T. destructans and T. epicoccoides were evaluated and compared on a Eucalyptus grandis x Eucalyptus urophylla hybrid clone. The optimal temperature for the germination of T. destructans ranged from 25 to 30 oC and 15 to 20 oC for T. epicoccoides. The germination of these pathogens was favored under conditions of light and high levels of relative humidity. Penetration by T. destructans and T. epicoccoides occurred via stomata and the hyphae colonized the intercellular spaces of infected leaves. Symptoms were clearly visible three weeks after inoculation by both pathogens and reproductive structures started to develop in substomatal cavities at four weeks after inoculation. The results of this study will facilitate the establishment of rapid screening trials based on artificial inoculations aimed at reducing the impact of disease caused by T. destructans.
Shoot and leaf blight caused by Teratosphaeria destructans is one of the most devastating foliar diseases of Eucalyptus. Therefore, breeding for resistance to this disease is considered urgent. Differences in susceptibility to T. destructans have been observed in the field, but a robust inoculation protocol has, until recently, been unavailable and a disease scoring method for precise phenotyping has not been established. A first objective of this study was to determine the optimal conidial concentration for T. destructans inoculations on a susceptible Eucalyptus host. This concentration was then used to determine differences in susceptibility of six genotypes of Eucalyptus grandis x Eucalyptus urophylla to the pathogen by assessing the percentage of infected stomata using electron microscopy and the percentage of leaf area covered by lesions (PLACL) using image processing. In addition, we developed a disease susceptibility index (SI) of six categories ranging from highly resistant (SI= 0), to highly susceptible (SI= 1.5-2). The more resistant genotypes were moderately resistant with an SI value of 0.49 - 0.54 and a PLACL of 6.5 – 9%. In contrast, the more susceptible genotype scored an SI of 1.52 and PLACL of 48%. Host susceptibility was also assessed relative to the sporulation of the pathogen. This showed that the percentage of sporulation did significantly correlate with host resistance. Overall, the results provide the basis for rigorous screening and selection of resistant genotypes to the disease caused by T. destructans using artificial inoculation.
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