Thesis (PhD(Agric))--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Postharvest decay of stone fruit in the Western Cape province of South Africa is
caused primarily by Botrytis cinerea (grey mould) and Monilinia laxa (brown rot). Little is
known about the relative importance and seasonal occurrence of the two pathogens in
nectarine and plum orchards, the mode of penetration of fruits by M laxa, latency and
subsequent disease expression by the latter pathogen. These aspects were investigated in this
study.
By sampling from the Unifruco Quality Evaluation Scheme and from 11 stone fruit
orchards, observations were made over a 3-year period of the occurrence of grey mould and
brown rot in the major stone fruit regions. Botrytis cinerea was found to be the most
important pathogen causing blossom blight and postharvest decay on stone fruit. The
pathogen was most prominent on early- and mid-season culti~ars. Brown rot was exclusively
caused by M laxa and no evidence was found that M fructicoZa had been introduced into the
region. Monilina laxa was most prominent on the later maturing cultivars. Botrytis cinerea
blossom infection did not contribute directly to postharvest decay. Both surface inoculum
and latent infection consistently occurred on fruit in each orchard, although at fluctuating
levels. Disease expression on developing fruit was not governed by the amount of B. cinerea
occurring on fruit surfaces, but by the ability of fruit to resist disease expression. The amount
of B. cinerea on fruits was generally higher during spring than during summer. Monilinia
laxa occurred sporadically on the blossoms of late-maturing cultivars. Immature fruit were
generally pathogen-free and disease expression occurred on maturing fruit only. These
findings suggest that conidia of M laxa are generally produced in orchards when fruits are
approaching maturity and can penetrate and infect maturing fruit only.
The behaviour of airborne M laxa conidia was subsequently studied on nectarine
(cultivar Flamekist) and plum (cultivar Laetitia) fruit. For these studies, an inoculation
method that simulates natural infection by airborne conidia was used. Fruit at pit hardening, 2 wk before harvest, harvest stage and after cold storage (nectarines 4 wk at -o.soC followed
by 1 wk at 23°C at ±56% RH; plums 10 days at .....().5°C,18 days at 7.5°C followed by 1 wk at
23°C at ±56% RH) were dusted with dry conidia of M laxa in a settling tower. The fruits
were incubated for periods ranging from 3 to 48 h at high relative humidity (2':93%, humid
fruit) or covered with a film of water (wet fruit). Behaviour of the solitary conidia was
examined with an epifluorescence microscope on skin segments stained in a differential stain
containing fluorescein diacetate, aniline blue and blankophor. The ability of solitary conidia
to colonise the fruit surface, penetrate fruit skins and to induce disease expression was
determined by using a differential set of tests. For these tests, fruit were surface-sterilised
(30 s in 70% ethanol) or left Unsterile. From each group, fruit were selected for isolation
(skin segment test), immersed in a 3% paraquat solution (paraquat-treated fruit test) or left
untreated (sound fruit test). 1be findings demonstrated that solitary conidia of M laxa
behaved consistently on plum and nectarine fruit surfaces: appressorium formation and direct
penetration was not observed on any of the fruit surfaces and germ tubes penetrated fruit
predominantly through stomata, lenticels and microfissures in the fruit skin. The monitoring
of airborne conidia revealed subtle effects of the fruits on the behaviour of solitary germlings,
which could not be seen when using conidial suspensions. On both fruit types, no deleterious
effect was seen on conidial and germling survival when fruit were kept humid at pit
hardening, 2 wk before harvest and harvest. However, conidial and germling survival were
drastically reduced by prolonged wet incubation of fruits. The findings on disease expression
in the skin segment, paraquat-treated fruit and sound fruit tests clearly showed that the skin of
both nectarine and plum fruits were not penetrated at the pit hardening stage, latent infections
were not established and fruitsreacted resistant to disease expression. These facets on both
fruit types were furthermore unaffected by wetness. The barrier capacity of the fruit skin of
the two stone fruit types however differed drastically later in the season. On nectarine, fruit
skins were more readily penetrated and disease expression became more pronounced when
fruit approached maturity. Penetration and disease expression on ripening nectarine fruit
were furthermore greatly influenced by wetness. Maturing plum fruit, on the other hand, did
not display the drastic change in the barrier capacity of fruit skins as observed on nectarine.
The influence of wetness on infection and disease expression was also less pronounced than
on nectarine. In fact, plum fruit remained asymptomatic in the sound fruit test after
inoculation and humid incubation at the 2 wk before harvest stage, harvest stage and after
cold storage. Plum fruit at these stages only developed disease after a prolonged period (~12 h) of wet incubation. The paraquat fruit test revealed that these fruits became more
susceptible to latent infection, but they were not as susceptible as nectarine. Collectively,
these findings indicate that M. laxa fruit rot epidemics on plum and nectarine are driven by
inoculum levels on fruit approaching maturity and by weather conditions prevailing during
the preharvest and harvest period. However, the barrier capacity of plum skins is
considerably more effective than that of nectarine fruit. Wounds would therefore play an
important role in the epidemiology of M. laxa on plum fruit.
Infection of fresh wounds by airborne M. laxa conidia, and by conidia and germlings
that have established on fruits, was therefore investigated. Plum fruit (cultivar Laetitia) at pit
hardening, 2 wk before harvest, harvest stage and after cold storage were dusted with dry
conidia of M. laxa in a settling tower.- Infection of rionwounded fruit and of fresh wounds by
\
the airborne conidia on dry, humid and wet plum fruit surfaces, and by conidia and germlings
that have been established on fruits under the wetness regimes was then investigated.
Nonwounded immature and mature fruit remained mostly asymptomatic, whereas
nonwounded cold stored fruit decayed readily. Wounding drastically increased infection by
airborne conidia. Immature fruits were less susceptible to wound infection by the airborne
conidia than mature fruits. Conidia dispersed freshly were more successful in infecting fresh
wounds than conidia that were deposited, or germlings that established, on fruit surfaces
4 days prior to wounding. This decrease in infectivity was especially pronounced on humid
and even more on wet incubated fruit. This study clearly showed that in order to reduce. the
incidence of brown rot, inoculum levels on fruit approaching maturity should be reduced by
sanitation practices and fungicide applications. Furthermore, it is essential to protect fruits,
especially. near-mature fruits, from being wounded. / AFRIKAANSE OPSOMMING: EPIDEMIOLOGIE VAN MONILINIA LAXA OP NEKTARIEN EN
PRUIM: INFEKSIE VAN VRUGTE DEUR KONIDIA
OPSOMMING
Naoesverrotting van steenvrugte in die Wes-Kaap provinsie van Suid-Afrika word
hoofsaaklik veroorsaak deur Botrytis cinerea (vaalvrot) en Monilinia laxa (bruinvrot). Min is
bekend oor die relatiewe belang en seisoenale voorkoms van hierdie patogene in nektarienen
pruimboorde, asook oor die infeksieweg, latensie en daaropvolgende siekte-uitdrukking
van M laxa. Hierdie aspekte is in dié studie nagevors.
\ \
Monsters IS oor 'n 3-jaar periode van die Unifruco Kwaliteitsevalueringskema, en ook
van 11 steenvrugboorde verkry. Die voorkoms van vaalvrot en bruinvrot in die hoof
steenvrugareas is so bepaal. Botrytis cinerea was die belangrikste patogeen wat betref
bloeiselversenging en naoesverrotting. Verder was hierdie patogeen ook meer prominent op
die vroeë- en middel-seisoen kultivars. Bruinvrot is uitsluitlik deur M Iaxa veroorsaak en
geen aanduiding omtrent die moontlike voorkoms van M fructicola in Suid-Afrika is
waargeneem nie. Monilinia laxa was meer prominent op die laat-seisoen kultivars. Botrytis
cinerea bloeiselinfeksie het nie direk bygedra tot naoesverrotting nie. Beide oppervlakkige
inokulum en latente infeksie het deurgaans, maar wel teen wisselende hoeveelhede, op vrugte
in die onderskeie boorde voorgekom. Siekte-uitdrukking op ontwikkelende vrugte is egter
nie beinvloed deur die hoeveelheid B. cinerea op die vrug nie, maar eerder deur die vermoë
van die vrug om siekte-uitdrukking te onderdruk. Die hoeveelheid B. cinerea op vrugte was
verder hoër gedurende lente as gedurende somer. Monilinia laxa het slegs sporadies op die
bloeisels van laat-seisoen kultivars voorgekom. Groen vrugte was in die algemeen vry van
die patogeen en siekte-uitdrukking het slegs op ryp vrugte plaasgevind. Hierdie bevindinge
dui daarop dat M laxa in boorde hoofsaaklik op ryper vrugte geproduseer word. Hierdie
swam infekteer ook net ryp vrugte.
Die gedrag van luggedraagde M laxa conidia is bestudeer op nektarien- (kultivar
Flamekist) en pruimvrugte (kultivar Laetitia). 'n Inokulasie-metode wat natuurlike infeksie
deur luggedraagde konidia simuleer, is vir hierdie studies gebruik. Vrugte van die pitverharding-, twee weke voor oes-, oesstadium, asook koud-opgebergde vrugte (nektariene,
4 weke by -o.soe gevolg met 1 week by 23°C en ±56% RH; pruime, 10 dae by -O.5°e, 18
dae by 7.Soe gevolg deur 1 week by 23°C en ±56% RH), is met droë konidia in 'n inokulasietoring
geïnokuleer. Die vrugte is vir periodes wat gewissel het van 3 tot 48 h geïnkubeer by
hoë relatiewe humiditeit (~93% RH, vogtige vrugte), of dit is bedek met'n film water (nat
vrugte). Die gedrag van die enkelspore (konidia) op die vrugoppervlak is met 'n
epifluorisensiemikroskoop bestudeer. Skilsegmente is gekleur in 'n kleurstof, bevattende
fluorisein diasetaat, analien-blou en blankofor. Die vermoë van die enkelspore om die
vrugoppervlak te koloniseer, te penetreer en om siekte-uitdrukking te induseer, is met 'n
differensiële stel toetse bepaal. Vir hierdie toetse is die vrugte oppervlakkig gesteriliseer
(30 s in 70% etanol), of nie-steriel gelaat. In elke groep is vrugte geneem vir isolasie
(skilsegment-to\~ts), of gedoop in "n 3% parakwat-oplossing (parakwat vrugtoets), of\,
onbehandeld gelaat (onbehandelde vrugtoets ). Die. bevindinge het op die soortgelyke gedrag
van M laxa enkelspore op die verskillende vrugsoorte gedui: appressoria en direkte
penetrasie is nie waargeneem nie, en kiembuise het die vrugte hoofsaaklik deur
huidmondjies, lentiselle en mikro-krakies .in die vrugskil gepenetreer. Deur luggedraagde
spore te bestudeer, is sekere subtiele effekte van die vrug op die gedrag van enkelspore op die
vrugoppervlak waargeneem. Op beide vrugtipes is geen nadelige effek op konidiurn- en
kiembuisoorlewing opgemerk wanneer die vrugte onder hoë vogtoestande geïnkubeer is.
Konidiurn- en kiembuisoorlewing is egter drasties verlaag hoe langer die vrugte onder nat
toestande geïnkubeer is. Die bevindinge van die skilsegment-, parakwat en onbehandelde
vrugtoetse het duidelik daarop gewys dat die vrugskil van nektarien en pruim nie gepenetreer
is tydens die pitverhardingstadium nie, latente infeksies is nie gevorm nie, en die vrugte was
bestand teen siekte-uitdrukking. Hierdie fasette op beide vrugtipes is ook nie beinvloed deur
inkubasie-natheid nie. Die beskermingskapasiteit van die vrugskil van hierdie steenvrugtipes
het egter drasties verskil later in die seisoen. Nektarien-vrugskille is meer geredelik
gepenetreer en siekte-uitdrukking het toegeneem met rypwording. Penetrasie en siekteuitdrukking
is verder in 'n groot mate deur inkubasie-natheid bevoordeel. Rypwordende
pruime het egter nie so In drasties verandering in die beskermingskapasiteit van die vrugskil
getoon nie. Die invloed van inkubasie-natheid op infeksie en siekte-uitdrukking was ook
minder opsigtelik as op nektarien. Pruimvrugte van die twee weke voor oes-, oesstadium, en ,
koud-opgebergde pruime, wat onder hoë vog geïnkubeer is, het simptoomloos in die
onbehandelde vrugtoets gebly. Vrugte van hierdie stadia het slegs simptome ontwikkel na periodes van langer as 12 h onder nat toestande. Die parakwat-behandelde vrugtoets het
egter gewys dat die pruimvrugte meer vatbaar vir latente infeksies raak, maar steeds nie so
vatbaar soos die nektarienvrugte nie. Gesamentlik dui hierdie bevindinge daarop <41tM laxa
bruinvrot epidemies op pruim en nektarien afhanklik is van inokulumvlakke op rypwordende
vrugte, asook die weerstoestande gedurende die vooroes- en oesstadia. Die
beskermingskapasiteit van pruim vrugskille was egter aansienlik meer effektief as dié van
nektarien vrugte. Wonde op vrugte sal dus 'n groter rol speel in die epidemiologic van M
laxa op pruim.
Infeksie van vars wonde deur luggedraagde M laxa konidia, en deur konidia en
kiembuise wat reeds op die vrugoppervlak gevestig is, is gevolglik bestudeer. Pruimvrugte
(kultivar Laetitia) van die pitverharding-, twee weke voor oes-, oesstadium, asook koud-
\ \
opgebergde vrugte is in 'n inokulasie-toring geïnokuleer met droë M laxa konidia. .Infeksie , ,
van nie-gewonde vrugte en van vars wonde deur luggedraagde konidia op droë, vogtige en
nat pruim vrugoppervlaktes, asook deur konidia en kiembuise wat reeds op die vrugoppervlak
onder hierdie toestande gevestig is, is bepaal. Nie-gewonde groen tot ryp vrugte het meestal
simptoomloos gebly, terwyl koud-opgebergde ryp vrugte wel verrot het. Wonde .het die
hoeveelheid infeksie deur luggedraagde spore drasties vermeerder. Konidia wat geïnokuleer
is op vrugte met vars wonde, was meer in staat om hierdie wonde te infekteer as konidia en
kiembuise wat 4 dae voor wonding gevestig is. Hierdie afname in infektiwiteit was meer
sigbaar op die vogtige, maar veral die nat vrugte. Hierdie studie het duidelik gewys dat
inokulumvlakke op rypwordende vrugte verlaag moet word deur sanitasie-praktyke en
fungisiedtoedienings. Dit is verder belangrik om vrugte, veral rypwordende vrugte, teen
wonding te beskerm.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52259 |
| Date | 03 1900 |
| Creators | Fourie, Paul H. (Paul Hendrik) |
| Contributors | Holz, G., Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 181 p. : ill. |
| Rights | Stellenbosch University |
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