Overwintering of Venturia inaequalis conidiospores in South African apple orchards

Abstract: In milder climates in Europe and North America Venturia inaequalis, the causal agent of apple scab, has been reported to overwinter in both the asexual (conidial and mycelial) and sexual (pseudothecial and ascosporic) forms. Viable asexual inoculum is not currently controlled by standard chemical spray programmes, which focus on preventing infections by ascospores early in the season. Objectives of this study were to (1) determine the occurrence of overwintering of asexual inoculum of V. inaequalis in three apple production regions of the Western Cape in South Africa, and (2) clarify their role in inciting early spring infections. Over three seasons, thirty buds were randomly sampled in late winter (August) from six ‘Braeburn’ orchards in the Koue Bokkeveld, Witzenberg Valley and Elgin regions for detection and/or quantification and for conidial viability testing of V. inaequalis. Buds were dissected to separate inner and outer bud tissue, which were analysed separately for viable conidiospores using microscopy. Scabbed pygmy apples that remained on trees during winter were also collected and tested for conidial viability. Conidia per orchard in bud-washes ranged from 0 to 677. Viable conidia were found only in inner bud tissues of orchards with high scab levels the previous season. Viability ranged from 0 to 29% and averaged 4%. Results suggest that orchards with high scab levels in the previous season have a higher risk of developing pygmy apples, and of viable conidia overwintering on pygmies and in bud tissues. Results also suggest that scabbed pygmy apples harbour a higher percentage viability and number of conidia, and may be a more likely source of viable conidial inoculum than apple bud tissue that may cause infection before matured ascospores are available to cause infections. This is the first report of viable conidia of V. inaequalis overwintering on pygmy apples in South Africa.