Etiology and management of black rot on 'Forelle' pears in the Western Cape

Thesis (MScAgric)--Stellenbosch University, 2023.

ENGLISH ABSTRACT: A recent increase in anecdotal black rot symptoms in stored pear fruit at Western Cape packhouses resulted in the initiation of the current study. An orchard survey for black rot decay confirmed the causal pathogen, Diplodia seriata, to be present in all orchards from all major pear production regions in the Western Cape. Black rot was better managed postharvest using a controlled atmosphere storage regime compared to regular atmosphere storage conditions. From isolations from black rot symptoms, Diplodia seriata was confirmed as the main causal organism for black rot with 86.21% of isolates representing these fungal species. Other Botryosphaeriaceae detected include Diplodia malorum, Neofusicoccum australe and Dothiorella sarmentorum. ‘Forelle’ pears are highly susceptible to black rot infection and develop severe lesions between 40.52 - 60.20 mm 6 days after wound infection. Fludioxonil and pyrimethanil are the currently relied-on postharvest fungicides. Diplodia seriata inoculated fruit were tested through a postharvest drench setup to determine the efficacy of a fungicide drench with these two products. The drench results showed fludioxonil is the most effective fungicide for black rot management and that D. seriata is non-sensitive towards pyrimethanil in vivo. In vitro trials were also performed to determine if the in vivo results persist, and this was in the form of a baseline sensitivity trial and a screening trial. For the baseline sensitivity, an EC50 value of 0.0055 mg/L and a discriminatory dose of 0.0500 mg/L were determined for fludioxonil. All D. seriata isolates obtained in this study were screened and killed at the discriminatory dose. For the screening with pyrimethanil, no isolates were reduced to 50% relative growth using a published EC50 value for D. seriata. From this study, there are several factors highlighted that could contribute to the increased incidence of black rot in the Western Cape. From management practices, the non-sensitivity of D. seriata towards pyrimethanil in vivo and in vitro experiments could prove a shift of resistance of these pathogens towards pyrimethanil. Other factors could be the increased stress on pome fruit trees after the 5-year drought that the Western Cape experienced. Drought stress weakens pome fruit trees which leads to increase canker infection and development. Another possible reason is the miss identification of black rot that could have occurred over the years. Black rot symptoms can easily be confused with symptoms of grey-mould (Botrytis cinerea) or Alternaria rot (Alternaria spp.) infections. The work done in this study is an initial step towards understanding black rot in the Western Cape. Key future work should investigate alternative management strategies for the control of black rot postharvest since industry currently relies on one fungicide active (fludioxonil). Aerial inoculum investigations should also be conducted to determine when a high inoculum load is present to optimise the timing of cultural and chemical canker management within the pome fruit orchards.

AFRIKAANSE OPSOMMING: Die onlangse toename in swartvrot waarnemings het gelei tot die motivering van die huidige projek. In ons boordopname was swartvrot teenwoordig in alle boorde van alle produksiestreke. Swartvrot was beter beheer na-oes deur gebruik te maak van beheerde atmosfeer teenoor relatiewe atmosfeer toestande. Diplodia seriata is bevestig as die hoofoorsaaklike organisme vir swartvrot. Nuwe verslae op 'Forelle'-pere word tydens hierdie studie gemaak met die belangrikste ander Botryosphaeriaceae wat gevind is Diplodia malorum, Neofusicoccum australe en Dothiorella sarmentorum. ‘Forelle’ is hoogs vatbaar vir swartvrot infeksie en hoë siekte ontwikkel tydens gunstige toestande word waargeneem. Noudat swartvrot bevestig is, is 'n ondersoek na die doeltreffendheid van tans geregistreerde bestuurspraktyke uitgevoer. Fludioxonil en pyrimethanil word tans as hoof na-oes swamdoders gebruik. Diplodia seriata geïnokuleerde vrugte is getoets deur 'n na-oes drenk opstelling om die doeltreffendheid van die swamdoders te bepaal. Die drenk resultate het duidelik getoon dat fludioxonil die mees effektiewe swamdoder vir swartvrot bestuur is en dat die swartvrot patogeen nie-sensitief is vir pyrimethanil in vivo. In vitro proewe is ook uitgevoer om te bepaal of die in vivo resultate voortduur, en dit was die geval. Toetse in die vorm van ‘n basislyn sensitiwiteitsproef en ‘n siftingsproef was gedoen. Vir die basislyn sensitiwiteit is 'n EC50 waarde van 0.0055 mg/L en 'n diskriminerende dosis van 0.05 mg/L vir fludioxonil bepaal. Alle D. seriata isolate wat in hierdie studie verkry is, is ten volle onderdruk teen die diskriminerende dosis. Vir die sifting met pyrimethanil is geen isolate verminder tot 50% relatiewe groei deur gebruik te maak van 'n gepubliseerde EC50 waarde vir D. seriata nie. Uit hierdie studie is daar verskeie faktore uitgelig wat kan bydra tot die verhoogde voorkoms van swartvrot in die Wes-Kaap. Uit bestuurspraktyke kan die nie-sensitiwiteit van D. seriata teenoor pyrimethanil in vivo en in vitro eksperimente die oorsaak wees van die verhoogde swartvrot voorkoms. Toekomstige werk behoort die moontlike ontwikkeling van weerstand in die D.seriata kulture wat in hierdie studie verkry is, te ondersoek. Ander faktore kan die verhoogde stres op kernvrugtebome wees na die 5 jaar lange droogte wat die Wes- Kaap ervaar het. Droogtestremming verswak kernvrugtebome wat lei tot verhoogde kankerinfeksie en -ontwikkeling. Dit lei tot verhoogde inokulum teenwoordig aangesien spore van D. seriata versprei word in die vorm van konidia spore vanaf ge-invekteerde kernvrugtebome. Nog 'n moontlike rede is die mis identifikasie van swartvrot wat oor die jare kon voorgekom het. Swartvrot simptoom uitdrukking soos gesien uit hierdie studie is soortgelyk aan grysskimmel (Botrytis cinerea) en Alternaria vrot (Alternaria spp.) infeksies. Produsente en pakhuise kon hierdie twee ander bekende na-oes vrot as swartvrot geïdentifiseer het aangesien swartvrot nie 'n bekende na-oessiekte was nie. Die werk wat in hierdie studie gedoen is, het 'n grondslag gelê vir toekomstige werk wat nodig is vir die voortdurende ontwikkeling van begrip van swartvrot in die Wes-Kaap. Toekomstige werk moet ondersoek instel na alternatiewe bestuurstrategieë vir die beheer van swartvrot na-oes as gevolg van die oneffektiwiteit van pyrimethanil. Luginentondersoeke moet ook uitgevoer word om te bepaal wanneer hoë inokulumlading teenwoordig is om die tydsberekening van kulturele en chemiese kankerbestuur binne die kernvrugteboorde te optimaliseer.

Masters