Epidemiology and control of Pseudocercospora angolensis fruit and leaf spot disease on citrus in Zimbabwe

[摘要] ENGLISH ABSTRACT: Fruit and Leaf Spot Disease (FLSD) of citrus, caused by Phaeoramularia angolensis, isfound only in 18 countries in Africa, the Comores Islands in the Indian Ocean and Yemen inthe Arabian peninsula. The major citrus export countries in Africa are Morocco, South Africa,Swaziland, and Zimbabwe. Zimbabwe is the only country affected by FLSD. FLSD is adisease of major phytosanitary and economic importance and its devastating effect on citrus ishighlighted by the fact that the damage is cosmetic, which renders the fruit unmarketable.Total crop losses are not uncommon in Kenya. The aims of the present study, therefore, waswas to determine the occurrence of P. angolensis in Zimbabwe and neighbouringMozambique, to compare these isolates with the Cercospora Fresen. isolates from Swazilandand South Africa, to determine the epidemiology of the pathogen and to implement aneffective control strategy to prevent the spread of FLSD.Leaf samples with citrus canker-like lesions collected in the early 1990's in Zimbabwewere found to be infected by the fungus, Phaeoramularia angolensis. Surveys wereundertaken to determine the spread and intensity of FLSD in Zimbabwe and Mozambique. InZimbabwe, P. angolensis was limited to an area above the 19° south latitude, predominantlythe moist areas and not the low-lying drier parts of the country. In Mozambique, no P.angolensis symptoms were found. Observations during the survey indicated that no propermanagement systems were implemented by Zimbabwean growers.A cercosporoid fungus causing a new Fruit and Leaf Spot Disease on Citrus in SouthAfrica was identified. From morphological and rDNA sequence data (ITS 1, 5.8S and ITS 2),it was concluded that the new disease was caused by Cercospora penzigii, belonging to theCercospora apii species complex. The genera Pseudophaeoramularia and Phaeoramulariaare regarded as synonyms of Pseudocercospora, and subsequently a new combination wasproposed in Pseudocercospora as P. angolensis. Cercospora gigantea was shown to notrepresent a species of Cercospora, while Mycosphaerella citri was found to bemorphologically variable, suggesting that it could represent more than one taxon.A control strategy for the control of FLSD was evaluated in the study. The data showedthat P. angolensis in Zimbabwe can be managed successfully by the removal of all old and neglected orchards, and on timely fungicide applications. Trifloxystrobin + mancozeb +mineral spray oil (20 g + 200 g + 500 ml/100 l water) applied in November, January andMarch was the most effective treatment. Three applications of benomyl + mancozeb +mineral spray oil (25 g + 200 g + 500 ml/100 l water) applied during the same period, wasthe second most effective treatment, and two applications (November and January) oftrifloxystrobin + mineral spray oil (20g + 500 ml/100 l water) and difenoconazole (40 g) per100 l/water applied twice in November and January, the third most effective treatment.The spore trap and weather data showed that P. angolensis needs high moisture andtemperatures in excess of 25°C for disease development. It is concluded that P. angolensis inZimbabwe can be managed successfully by implementing a holistic approach, which shouldbe supported by the authorities, organised agriculture and all technical personnel involved incitrus production.

[发布机构] Stellenbosch University