Crop loss, aetiology, and epidemiology of citrus black spot in Ghana

Abstract

Citrus Black Spot (CBS), caused by Guignardia citricarpa, was detected for the first time in Ghana and in West Africa. The disease was first observed in the Eastern Region in 1999 with typical disease symptoms including hard spot, virulent spot and false melanose were observed on several citrus species. A survey revealed that the disease has reached epidemic levels in the citrus-producing areas of the Eastern and Ashanti regions and is spreading rapidly within these areas and to other regions of the country. Currently, CBS is the most important fruit disease of citrus in Ghana, causing about 22% crop loss. Although the disease does not cause postharvest decay and the internal quality of the fruit is not affected, significant amounts of blemished fruit are discarded at the markets. Disease incidence and severity was found to be higher on mature than on young citrus trees. Pycnidia were found on fruit with hard spot symptoms, and pycnidia and pseudothecia typical of Guignardia spp. were found on decomposing leaves. Two species, G. citricarpa and G. mangiferae, were isolated from 15% of the samples collected and identified using the Oatmeal Agar test and by PCR with species-specific DNA primers. Isolates of G. citricarpa produced CBS symptoms after 80 to 233 days on 75% of the artificially inoculated young fruit of Valencia Late sweet orange. The fungus was reisolated from symptomatic, inoculated fruit completing Koch's postulates. Isolates of the endophyte G. mangiferae did not induce symptoms in the pathogenicity tests. In epidemiological studies, infections were detected from November to February for the minor cropping season and from May to November for the major season. Fruit of Valencia Late sweet orange Eur J Plant Pathol (2012) 133:657-670

Scientific Opinion on the risk of Phyllosticta citricarpa (Guignardia citricarpa) for the EU territory with identification and evaluation of risk reduction options

“…Degree-day models have been developed based on these data to predict the onset and duration of ascospore release in this region in South Africa (Fourie et al, 2013) and as a function of temperature and wetness in Misiones in Argentina (Dummel et al, 2012). Studies conducted in Brazil and Ghana indicated a susceptibility period of six and seven months after fruit set, respectively Baldassari et al, 2006;Brentu et al, 2012), although longer periods were not evaluated. Under artificial inoculation conditions, leaves of lemon (C. limon (L.) Osbeck) cv.…”

“…In artificial inoculations conducted under greenhouse conditions, the incubation period ranged from over 200 days for 3-cm-diameter sweet orange fruit to about 50 days for 7-cm-diameter fruit (Aguiar et al, 2012). Timmer (1999) © American Phytopathological Society and modified according to Aguiar et al (2012), Brentu et al (2012), Reis et al (2003) and Truter (2010)) However, they are rare and present in only lemons or trees in poor condition (Kotzé, 1981(Kotzé, , 2000.…”

“…Based on these data, an EU map of NUTS3 citrusgrowing regions have been developed to be used for extraction of weather data for the simulations run for this opinion (Figure 27), to allow the comparison of simulation results with the citrus acreage for each NUTS3 region/province. Studies conducted in Brazil and Ghana, under non-limiting inoculum and weather conditions for infection, indicated a susceptibility period of six and seven months after fruit set, respectively Baldassari et al, 2006;Aguiar et al, 2012;Brentu et al, 2012), but longer periods were not evaluated. The lack of fungicide sprays during the final stages of fruit development can also be a consequence of low inoculum levels and unfavourable weather conditions at that time.…”

“…After infection, the duration of the incubation period until symptom appearance is approximately 2-12 months, depending on environmental factors, tree age and condition (Brodrick and Rabie, 1970;Kotzé, 1963Kotzé, , 1981Ninin et al, 2012). Studies conducted in Brazil and Ghana under non-limiting inoculum and weather conditions for infection indicated a susceptibility period of six and seven months after fruit set, respectively Baldassari et al, 2006;Brentu et al, 2012), but longer periods were not evaluated. However, ontogenic resistance has not been confirmed experimentally, and the lack of fungicide sprays may also be driven by low inoculum levels and unfavourable weather conditions during the later stages of fruit development.…”

Genetic variation among Phyllosticta strains isolated from citrus in Florida that are pathogenic or nonpathogenic to citrus

“…Around the 1920s, it was found in South Africa, first along the coastal region, then spreading inland to the main citrus production areas, causing major damage (Kotzé, 1981). CBS is now present in South East Asia, many countries in Africa (Brentu et al, 2012), several countries in South America Paul et al, 2005), and North America (Florida only thus far; Er et al, 2013b). The disease is not yet present in Europe (Paul et al, 2005).…”