Characterisation of mites and peniciccium species associated with apple core rot diseases

[摘要] ENGLISH ABSTRACT: Dry core rot (DCR) and wet core rot (WCR) are among some of the most important postharvestdiseases of apples in South Africa. Mouldy core (MC) is also a symptom associated withthe core region of apples, but it is not of economical importance since apple tissue surroundingthe core region is not affected as is the case with DCR and WCR. The incidence of core rots inharvested fruits can be as high as 12%, but in general ranges from 3 to 8%. Infections and lossescan also occur during fruit handling in pack houses and during storage. Additionally, yield lossesalso occur prior to harvest within orchards due to premature fruit drop of core rot affected fruits.The incidence of core rot diseases in apples differ among apple cultivars, with most RedDelicious varieties being susceptible to the development of core rots, whereas core rots haverarely been reported in other cultivars such as Granny Smith.The etiology and epidemiology of WCR and DCR are poorly understood. Although manyfungal genera have been associated with the diseases, small-spored Alternaria species are mainlyassociated with DCR, whereas Penicillium species including P. roquefortii, P. expansum and P.funiculosum have mainly been associated with WCR. Dry core rot infections have long beenknown to occur pre-harvest, whereas WCR is primarily known as a post-harvest disease whereinfections take place during fruit handling in pack houses. Recently, Tarsonemus mites have alsobeen indicated as being a potential role player in the etiology of core rot diseases. The miteshave been hypothesised to carry pathogen spores into the core region of apples, and they mayalso possibly cause small wounds that facilitate pathogen entry.In South Africa, apple growers have recently reported WCR as being present prior toharvest, which has not been reported previously. Therefore, the first aim of the study was toinvestigate the incidence, as well as the causal agent/s of pre-harvest WCR. The incidence ofWCR ranged from 0% to 1.7% in eleven orchards, and was in general lower than that of DCR(0.4% to 6%). Isolation studies from eight internal positions in WCR apples showed thatPenicillium was the predominant fungal genus in most of the positions, including the lesion area.Morphological and molecular characterisation of Penicillium isolates from WCR showed that P.2ramulosum prov. nom. was the main species isolated from lesions, as well as other isolationpositions. However, this species was also the main species isolated from DCR, MC andasymptomatic apples. Penicillium expansum was only isolated at low frequencies from WCRand DCR apples. Other Pencillium species that were occasionally isolated included P. glabrum,P. chloroloma, P. chermisinum and a putative new species with closest affinity to P. dendriticum(P. species aff. dendriticum) on a DNA nucleotide sequence basis. Pathogenicity and virulencestudies using three different inoculation methods showed that P. expansum was the most virulentspecies, followed by P. species aff. dendriticum. The P. ramulosum prov. nom. isolates varied intheir virulence, but were all considered to have low virulence.The role of Tarsonemus mites in the etiology and epidemiology of core rot diseases ispoorly understood, and therefore the second aim of the study was to investigate some of theseaspects. The specific aims of the study were to (1) investigate the ecology of Tarsonemus mitesin Red Delicious and Granny Smith orchards during different apple developmental stages, (2)determine if there is a significant association of Tarsonemus mites with diseased (WCR andDCR) fruits and (3) determine if potential core rot pathogenic fungi are associated with themites. Tarsonemus mites were found in all of the investigated apple developmental stages (buds,blossoms, 4cm diameter fruit, mature fruit and mummies), having the highest incidence inmummies and mature fruits from Red Delicious and Granny Smith orchards. In Red Deliciousfruits the Tarsonemus mites were found within the core and/or calyx tube, whereas in GrannySmith fruits the mites were restricted to the calyx tube. In Red Delicious fruits there was asignificant association between dry core rot as well as total core rot (wet- and dry-core rot) withthe presence of mites in the core, as well as total mites (mites in core and calyx tubes). Fungalisolation studies from the Tarsonemus mites showed that they carried potential core rot fungalpathogens within the genera Penicillium and Alternaria. The Penicillium species isolated fromthe mites included two of the most virulent WCR species, P. expansum and P. species aff.dendriticum.

[发布机构] Stellenbosch University