Higher Temperatures Increase Risk Of Apple Scab Infection

The dramatic increase in South Africa’s average winter temperature over the past 40 years has altered the susceptibility of apples to fungal disease, with fungi showing a much faster adaptation rate to changing temperatures than apples, writes Dr Julia Meitz-Hopkins, a researcher in the Department of Plant Pathology at Stellenbosch University.

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Apple orchards in the coastal regions of the Western Cape, such as Elgin, are affected to a greater extent by rising winter temperatures than those in the colder inland regions such as the Koue Bokkeveld. The question we have been trying to answer through our research is whether this change in temperature has increased the risk of apples acquiring fungal diseases.

Apple scab, one such disease, is caused by the fungus Venturia inaequalis, commonly known as Fusi. It is responsible for high economic losses to the apple export industry due to the blemishes and deformities it causes on the fruit.

The fungus overwinters in fallen leaf debris, where it reproduces sexually to form round, spore-bearing structures, called pseudothecia, within the leaf tissue. When conditions are favourable for scab infection in spring (16°C to 20°C, during rainy periods), the spores are discharged, causing infection in young leaves and developing flowers and fruit.

Regional Variation

In 2017, Dr Trevor Koopman of the Agricultural Research Council made an intriguing discovery using genetic fingerprinting of apple scab fungi during his studies in the Fruit and Postharvest Pathology Research Programme led by Dr Cheryl Lennox at Stellenbosch University.