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Virulence testing of South African Venturia inaequalis inoculum using qPCR

Trevor Koopman: ARC Infruitec-Nietvoorbij

<div><em>Venturia inaequalis</em>, the causal agent of apple scab, is one of the most important diseases of apple (<em>Malus pumila</em>), locally and internationally. There is eighteen known races of <em>V. inaequalis</em> and differences in single spore isolate virulence have been observed. Isolates had co-evolved with the cultivar and can only infect the cultivar it originates from. The objective was to determine virulence of <em>V. inaequalis</em> from South Africa using single spore isolates from different cultivars. Samples were collected from several cultivars in four apple growing regions. Lines were generated from single spores and conidia produced on cellophane media. Virulence testing of 17 single spore isolates was performed on ‘Golden Delicious’/’Royal Gala’ ('GD'/'RG'), ‘Fuji’ and the rootstock ‘M793’, with visual symptoms evaluated after 14 days. A qPCR technique was used to quantify the amount of fungal DNA in the leaf tissue after 7 and 14 days post- inoculation. Results showed differential infection ability and not all isolates could infect and sporulate on all the cultivars tested. Some single spore isolates caused chlorotic spots without any sporulation and might have presented a delayed sporulation if incubated for 21 days. The fungal DNA concentration was the lowest in inoculations of ‘GD’/‘RG’ and highest in ‘M793’. The single spore isolate LOSL 206-1, from ‘Cripps Red’, was the most virulent isolate on ‘RG’ and isolate OBL 210-1, from ‘Braeburn’, the most virulent on ‘M793’. Isolates that caused chlorosis also had high amounts of fungal DNA present in leaf tissue. Single spore isolates differed among themselves with respect to their ability to infect and sporulate on different cultivars. The results of the visual scoring and qPCR were comparable and the qPCR method showed clear advantages for the detection and quantification of the pathogen in asymptomatic apple leaves.</div>