Thesis (MSc (Plant Pathology))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Dry core rot (DCR) and wet core rot (WCR) are among some of the most important postharvest
diseases of apples in South Africa. Mouldy core (MC) is also a symptom associated with
the core region of apples, but it is not of economical importance since apple tissue surrounding
the core region is not affected as is the case with DCR and WCR. The incidence of core rots in
harvested fruits can be as high as 12%, but in general ranges from 3 to 8%. Infections and losses
can also occur during fruit handling in pack houses and during storage. Additionally, yield losses
also 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 Red
Delicious varieties being susceptible to the development of core rots, whereas core rots have
rarely been reported in other cultivars such as Granny Smith.
The etiology and epidemiology of WCR and DCR are poorly understood. Although many
fungal genera have been associated with the diseases, small-spored Alternaria species are mainly
associated 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 been
known to occur pre-harvest, whereas WCR is primarily known as a post-harvest disease where
infections take place during fruit handling in pack houses. Recently, Tarsonemus mites have also
been indicated as being a potential role player in the etiology of core rot diseases. The mites
have been hypothesised to carry pathogen spores into the core region of apples, and they may
also possibly cause small wounds that facilitate pathogen entry.
In South Africa, apple growers have recently reported WCR as being present prior to
harvest, which has not been reported previously. Therefore, the first aim of the study was to
investigate the incidence, as well as the causal agent/s of pre-harvest WCR. The incidence of
WCR 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 that
Penicillium 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.
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ramulosum prov. nom. was the main species isolated from lesions, as well as other isolation
positions. However, this species was also the main species isolated from DCR, MC and
asymptomatic apples. Penicillium expansum was only isolated at low frequencies from WCR
and 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 virulence
studies using three different inoculation methods showed that P. expansum was the most virulent
species, followed by P. species aff. dendriticum. The P. ramulosum prov. nom. isolates varied in
their virulence, but were all considered to have low virulence.
The role of Tarsonemus mites in the etiology and epidemiology of core rot diseases is
poorly understood, and therefore the second aim of the study was to investigate some of these
aspects. The specific aims of the study were to (1) investigate the ecology of Tarsonemus mites
in 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 and
DCR) fruits and (3) determine if potential core rot pathogenic fungi are associated with the
mites. 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 in
mummies and mature fruits from Red Delicious and Granny Smith orchards. In Red Delicious
fruits the Tarsonemus mites were found within the core and/or calyx tube, whereas in Granny
Smith fruits the mites were restricted to the calyx tube. In Red Delicious fruits there was a
significant association between dry core rot as well as total core rot (wet- and dry-core rot) with
the presence of mites in the core, as well as total mites (mites in core and calyx tubes). Fungal
isolation studies from the Tarsonemus mites showed that they carried potential core rot fungal
pathogens within the genera Penicillium and Alternaria. The Penicillium species isolated from
the mites included two of the most virulent WCR species, P. expansum and P. species aff.
dendriticum. / AFRIKAANSE OPSOMMING: Droë kernvrot and nat kernvrot is van die belangrikste na-oes siektes van appels in Suid-
Afrika. Beskimmelde kern word ook met die kern van appels geassosieer, maar hierdie toestand
is egter nie van ekonomiese belang nie, aangesien die weefsel rondom die kern nie geaffekteer
word soos in die geval van nat- en droë kernvrot nie. Die voorkoms van kernvrot in vrugte na
oes, kan vlakke van tot 12% bereik, maar oor die algemeen is die voorkoms tussen 3 en 8%.
Infeksie en verliese kan ook voorkom gedurende die hantering en verpakking van vrugte in
pakhuise en gedurende storing. Addisionele verliese in opbrengs kan ook voor-oes voorkom in
boorde. Dit is te wyte aan voortydige vrugval van appels wat besmet is met kernvrot. Die
voorkoms van kernvrot by appels verskil tussen kultivars. Meeste van die “Red Delicious”
variëteite is vatbaar vir die ontwikkeling van kernvrot. Die toestand is egter skaars by ander
kultivars soos Granny Smith.
Die etiologie en epidemiologie van nat- en droë kernvrot word nie goed verstaan nie. ‘n
Groot aantal swamgenera is al met kernvrot geassosieer. Klein-spoor Alternaria spesies word
hoofsaaklik met droë kernvrot geassosieer en Penicillium spesies, insluitende P. roquefortii, P.
expansum en P. funiculosum, word meestal met nat kernvrot geassosieer. Dit is lank reeds
bekend dat droë kernvrot as voor-oes siekte kan voorkom, maar nat kernvrot is algemeen bekend
as na-oes siekte waar infeksie tydens vrughantering en verpakking plaasvind. Daar is onlangs
aangedui dat Tarsonemus myte potensiële rolspelers in die etiologie van kernvrot is. Hipoteties
is die myte in staat om spore van die patogene in die kern van die appels in te dra, asook om
klein wonde te veroorsaak wat infeksie deur patogene vergemaklik.
In Suid-Afrika is nat kernvrot wat voor-oes in die boorde ontstaan onlangs deur boere
aangemeld; hierdie toestand is nog nie op ‘n vorige geleentheid aangemeld nie. Die eerste
doelwit van hierdie studie was dus om die voorkoms en veroorsakende organisme/s van voor-oes
nat kernvrot te ondersoek. Die voorkoms van nat kernvrot was tussen 0 en 1.7% in elf boorde en
was oor die algemeen laer as die voorkoms van droë kernvrot (0.4 tot 6%). Isolasiestudies uit agt
interne posisies van nat kernvrot appels het getoon dat Penicillium die dominante swamgenus in
die meeste posisies was, insluitend die letsels. Morfologiese en molekulêre karakterisering van
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Penicillium isolate uit nat kernvrot letsels het aangedui dat P. ramulosum prov. nom. die spesie is
wat die meeste geïsoleer is vanuit die letsels, asook ander isolasie posisies. Dié spesie was egter
ook die mees algemene spesie wat uit nat- en droë kernvrot, asimptomatiese appels en appels wat
slegs swamgroei in die kern gehad het, geïsoleer is. Penicillium expansum was ook in lae getalle
uit nat- en droë kernvrotletsels geïsoleer. Ander Penicillium spesies wat ook soms geïsoleer is,
sluit P. glabrum, P. chloroloma, P. chermisinum, asook ‘n moontlik nuwe spesie wat op DNA
volgorde basis die naaste aan P. dendriticum (P. spesie aff. dendriticum) is. Studies wat
patogenesiteit en virulensie van die isolate ondersoek het, is ook uitgevoer deur gebruik te maak
van drie verskillende inokulasie metodes. Die studies het aangedui dat P. expansum die mees
virulente spesie is, gevolg deur P. spesie aff. dendriticum. Die P. ramulosum prov. nom. isolate
het variasie in virulensie getoon maar is oor die algemeen aanvaar om minder virulent te wees.
Die rol van Tarsonemus myte in die etiologie en epidemiologie van kernvrot word nie
goed verstaan nie en dus was die tweede doelwit van die studie om sommige van dié aspekte te
ondersoek. Die spesifieke doelwitte was (1) om die ekologie van die Tarsonemus myte in “Red
Delicious” en Granny Smith boorde tydens verskillende ontwikkelingstadiums van die appels te
ondersoek, (2) om te bepaal of daar ‘n betekenisvolle assosiasie van Tarsonemus myte met siek
(nat- en droë kernvrot) vrugte bestaan en (3) om te bepaal of potensiële kernvrot patogeniese
swamme geassosieer is met die myte. Tarsonemus myte is gevind in al die ontwikkelingstadiums
(knoppies, bloeisels, 4 sentimeter deursnee vrugte, volwasse vrugte en mummies) van appels wat
ondersoek is. Die hoogste voorkoms van myte was in die mummies en volwasse vrugte van “Red
Delicious”, asook Granny Smith kultivars gevind. In “Red Delicious” vrugte is myte in die kern
en/of kaliksbuis gevind, maar in die Granny Smith vrugte was die myte tot die kaliksbuis beperk.
In “Red Delicious” vrugte was daar ‘n betekenisvolle assosiasie tussen droë kernvrot, asook
totale kernvrot (nat en droë kernvrot) met die teenwoordigheid van myte in die kern, asook totale
myte (myte in die kern en kaliksbuis). Swam isolasiestudies vanaf die Tarsonemus myte het
aangetoon dat potensiële kernvrot swampatogene in die genera Penicillium en Alternaria wel by
die myte teenwoordig was. Die Penicillium spesies wat vanaf die myte geïsoleer is het twee van
die mees virulente nat kernvrot spesies ingesluit, nl. P. expansum en P. spesie aff. dendriticum.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4056 |
| Date | 03 1900 |
| Creators | Van der Walt, Lene |
| Contributors | McLeod, Adele, Spotts, R.A., University of Stellenbosch. Faculty of Agrisciences. Dept. of Plant Pathology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 121 p. : ill. |
| Rights | University of Stellenbosch |
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