CSIRO PUBLISHING www.publish.csiro.au/journals/app Australasian Plant Pathology, 2007, 36, 87–93 Failure of Phyllosticta citricarpa pycnidiospores to infect Eureka lemon leaf litter M. Truter A,C , P. M. Labuschagne A , J. M. Kotz´ e B , L. Meyer A and L. Korsten A A Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria 0002, South Africa. B PO Box 1567, Kokanje, 0515, South Africa. C Corresponding author. Email: mariette.truter@up.ac.za Abstract. Pycnidiospores of Phyllosticta citricarpa from pure cultures, symptomatic citrus black spot Valencia orange fruit and peelings were evaluated for their potential to infect and colonise citrus black spot-free Eureka lemon leaf litter in a controlled environment and in the field in different production regions of South Africa. Leaf litter, consisting of freshly detached green and old brown leaves that were exposed to viable pycnidiospores under controlled conditions or in the field underneath citrus trees, were not infected and colonised by P. citricarpa. Ascospores, conforming to Guignardia citricarpa, the pathogen, or G. mangiferae, a cosmopolitan endophyte, were collected with a Kotz´ e Inoculum Monitor from leaves placed in the field only at Tzaneen and Burgersfort. Distinguishing between these two species on ascospore morphology alone is not possible. A diagnostic polymerase chain reaction conducted on representative leaf material from all the treatments revealed the presence of only G. mangiferae on 12.5% of the treatments. This study demonstrated the failure of P. citricarpa pycnidiospores to infect citrus leaf litter under controlled and field conditions. Symptomatic citrus black spot fruit or peel lying on the ground underneath citrus trees, therefore, cannot lead to infection and colonisation of freshly detached leaves or natural leaf litter or represent a source of inoculum in citrus orchards for these leaves. Additional keywords: inoculum load, spore trap. Introduction Citrus black spot (CBS) is caused by Guignardia citricarpa (anamorph Phyllosticta citricarpa) and the superficial cosmetic fruit spots are unacceptable in the global fresh fruit trade. Symptoms can develop on more than 90% of the fruit produced from unsprayed orchards, ranging from one up to a thousand spots per fruit (Calavan 1960). Three kinds of symptoms are widely recognised: hard spot, freckle and virulent spot (Cobb 1897; Kiely 1948). Two other symptoms, speckled blotch and cracked spot, occur predominantly in South Africa (Kotz´ e 1963; McOnie 1963; Brodrick 1969) and Brazil (De Goes et al. 2000), respectively. Of these symptoms, hard spot and virulent spot may contain pycnidia within the lesions, although freckle spot may turn into virulent spot and speckled blotch may turn into hard spot as the season progresses (Kotz´ e 1981). Black spot is an economically important disease of citrus in summer rainfall regions of South Africa and various other subtropical countries. Although the disease has spread to most of the summer rainfall areas in South Africa since its first reported occurrence in 1929 (Doidge 1929), it has not been able to establish in predominantly winter rainfall areas. These areas have official CBS-free status and consist of the citrus production regions of Northern Cape and Western Cape (European Union 1998; Mabiletsa 2003). Confirmation of this distribution pattern in South Africa was recently illustrated by Paul et al. (2005) using global modelling of weather patterns to map CBS occurrence. The global distribution of CBS is restricted by specific climatic parameters and cold-stress with temperatures below 11 ◦ C indicated to be the main restrictive parameter (Paul et al. 2005). Environmental conditions required for successful infection of susceptible citrus material include the presence of adequate moisture and relatively high temperatures, ranging between 18 and 30 ◦ C for at least 15 h (Kotz´ e 1963; McOnie 1967). These conditions usually prevail in the summer rainfall areas of South Africa from late spring to autumn. The critical infection period is usually from October until January, as fruit susceptibility and the main ascospore release period coincide (Kotz´ e 1981, 1996). The critical infection period may start and end a month earlier or later depending on prevailing rainfall and mean temperature. Fruit remains susceptible to infection from fruit set up to 5 months later, whereas leaves remain susceptible from development up to 10 months of age (Kiely 1948, 1950; Kotz´ e 1963; McOnie 1964b; Truter et al. 2004b). Two types of spores produced by the pathogen can infect susceptible citrus material (Kiely 1948; McOnie 1965; Whiteside 1967; Kotz´ e 1996). The airborne ascospores from perithecia are only produced on leaf litter and are the main source of inoculum and dissemination of the disease (Kiely 1948; McOnie 1964b, 1965; Kotz´ e 1981; Korf 1998). Pycnidiospores of the anamorph are produced in pycnidia on symptomatic fruit, leaf litter and with the highly © Australasian Plant Pathology Society 2007 10.1071/AP06087 0815-3191/07/010087