Epidemiology and etiology of Neofabraea species causing lenticel decay of pome fruit in the Western Cape of South Africa

[摘要] ENGLISH ABSTRACT: the fungal genus Neofabraea, four species are known to affect apple and pear worldwide, namely: N. alba, N. perennans, N. malicorticis and N. kienholzii. These species are a disease complex sharing similar symptoms, morphology, hosts and can occur in the same pome fruit growing region. This disease complex can cause bark cankers on the trunk, limbs and pruning wounds of the tree and postharvest fruit rot from latent lenticel infections. This type of postharvest rot is also colloquially referred to as 'bull's eye rot' from the target-like appearance of alternating brown and dark brown concentric decay rings.In South Africa, only N. alba has been identified to cause bull's eye rot of apple and is responsible for major losses in the late maturing 'Cripps Pink' apple variety in the Western Cape. As very little information is available on N. alba and its epidemiology in South African orchards, producers and packhouses struggle to effectively control this pathogen. This study therefore aimed to add knowledge of N. alba on 'Cripps Pink' apple in order to contribute to, and improve the current management strategy of this pathogen.A rapid molecular detection method was developed and 'Cripps Pink' apples were screened in 2011, 2012 and 2013 from two orchards, one in the Witzenberg Valley and the other in Grabouw. Neofabraea alba was detected sporadically on the surfaces of 'Cripps Pink' apple in each season in both orchards, with increased frequency at harvest in autumn. Cankers were not observed in 'Cripps Pink' orchards and N. alba was not detected on plant materials from the orchard floor. Surprisingly, conidia of N. kienholzii and N. perennans were detected on the surface of 'Cripps Pink' apples in spring of 2013. Neofabraea kienholzii was detected in November at Grabouw orchard and N. perennans in December at the Witzenberg Valley and Grabouw orchards. Postharvest evaluations of 'Cripps Pink' apples from these two orchards in 2013 found N. alba to be the causal species of bull's eye rot, not N. perennans or N. kienholzii. Further postharvest investigations of 'Cripps Pink' apple collected at harvest for three consecutive seasons from producers in the Witzenberg Valley, Ceres, Grabouw, Elgin and the Hemel-en-Aarde Valley found that N. alba was the causal agent of bull's eye rot. This pathogen was present in the five growing regions with decay incidences ranging between 0 to 73% in 2010, 0 to 6% in 2011 and 0 to 30% in 2012.Currently, there are no fungicides registered for specific use against N. alba of apple in South Africa. In vitro mycelial sensitivity assays of the phenyl pyrrole fludioxonil and anilinopyrimidine pyrimethanil on a N. alba population from 'Cripps Pink' apple indicated fludioxonil to have no effect on the growth of the pathogen whereas the majority of isolates were sensitive to pyrimethanil. For pyrimethanil, a mean EC50-value of 6.69 mg/ L was calculated and ranged between 0.93 mg/ L and 56.6 mg/ L. Several N. alba isolates (N=8) were capable of growing on concentrations above 50 mg/ L pyrimethanil, indicating a possible shift in sensitivity.A preliminary study on the presence of bull's eye rot on pear found the species N. alba, N. perennans and N. kienholzii to be present on two pear varieties in the Western Cape. Neofabraea alba and N. perennans were detected on 'Packham's Triumph' pears in the Witzenberg Valley and Grabouw, respectively. Neofabraea kienholzii was identified from decay on 'Packham's Triumph' and 'Golden Russet Bosc' pears in the Witzenberg Valley. Interestingly, N. alba was detected on the surface of the 'Golden Russet Bosc' in the same fruit lot affected by N. kienholzii. This is the first report of bull's eye rot on pear in the Western Cape and the first report of N. perennans and N. kienholzii in South Africa. The source, incidence and control of bull's eye rot of pears needs to be investigated as these findings have a phytosanitary implication for the management of pears and its export from South Africa.

[发布机构] Stellenbosch University