Leaf pathogens of Eucalyptus in South Africa with particular reference toTeratosphaeria destructans

Abstract

Commercial plantations comprising of Eucalyptus species and their hybrids make up approximately 44 % of the South African forestry industry. The majority of Eucalyptus plantations in the country are planted in the KwaZulu-Natal and Mpumalanga provinces, with smaller areas in the Limpopo, Eastern and Western Cape Provinces. The constant increase of pests and pathogens from native and exotic environments threatens the productivity, health status and ultimately the economic importance of Eucalyptus species. Globally, species of Calonectria, Coniella, Quambalaria, Teratosphaeria, the rust fungus Austropuccinia psidii and the bacterial species, Pantoea, are considered to be important Eucalyptus leaf pathogens. The diseases caused by these leaf pathogens include leaf and shoot blight, leaf spot and leaf blotch, which may cause shoot death, tip die-back, defoliation and even plant death. Studies conducted for this dissertation updated and increased knowledge pertaining to the foliar pathogens of Eucalyptus genotypes in South Africa. Firstly a survey of commercial Eucalyptus plantations was conducted in the KwaZulu-Natal, Mpumalanga and Limpopo Provinces to identify possible important leaf diseases of these trees. Based on symptoms, spore and culture morphology as well as DNA sequence data, Calonectria pauciramosa, Quambalaria eucalypti and Teratosphaeria destructans were identified from diseased leaves. Importantly, this study expanded the geographic range of T. destructans in the country and also provides the first report of Calonectria leaf spot in a South African plantation. This disease was previously only known from nurseries in the country. Teratosphaeria destructans is one of the most important leaf pathogen of Eucalyptus in South Africa. The results obtained from chapter two confirm that the presence of T. destructans in South Africa has resulted in an expanded geographic range and even more destructive impact. This prompted the need to further study the epidemiology of this pathogen. The results obtained from chapter two confirm that the presence of T. destructans in South Africa has resulted in an expanded geographic range and even more destructive impact. This prompted the need to further study the epidemiology of this pathogen. Prior to the research conducted for this dissertation nothing was known about optimal conditions for spore germination, growth and infection of this pathogen. The growth of T. destructans cultures took place at an optimum temperature of 25 ˚C irrespective of light conditions. Conidial germination obtained from cultures took place at optimum temperatures of 20 and 25 ˚C, while those obtained from fresh leaves took place mostly at 20 ˚C, with or without 150 light. Greater conidial germination was observed from spores that were obtained from naturally infected leaves (88 %) as opposed to that of spores obtained from cultures (12 %). Teratosphaeria destructans was able to infect young and intermediate leaves of E. grandis x E. urophylla at temperatures of 20 and 25 ˚C and leaf wetness periods of two to eight days with symptoms developing approximately four weeks after inoculation. The production of fruiting bodies was observed at 20 and 25 ˚C and at leaf wetness periods of seven (spores obtained from fresh leaves) and eight (spores obtained from cultures) days four - eight weeks after inoculation. This information can be used for the development of artificial inoculation protocols for use in breeding programs to select more disease resistant planting material for commercial deployment.