Could willow invigorate your apples?
26th May 2021
Like slugs, snails, bindweed and sometimes finding your summer line-dried laundry has acquired a hint of barbecue, apple scab is one of those inconvenient facts of life in the garden. Caused by a fungal infection that spreads on the wind, the best option for domestic growers is to plant resistant varieties. A recent trial aimed at commercial growers, though, has hinted there might be an environmentally friendly remedy that should be explored - and it could use a natural, renewable resource.
In 2019, a network called Innovative Farmers tested the hypothesis that willow could be used to reduce the incidence of fruit scab. Willow contains salicylic acid ('salicylic' comes from the Latin name, Salix) and there is some evidence that this chemical can help trees fight off fungal infection.
Mulches made from willow woodchip were applied around the base of trees on trial sites in January-February and incidence of scab in mid August was recorded. It was theorised that salicylic acid should take 4-6 weeks to leach into soil and could then be taken up by trees as they come into leaf, before scab has had a chance to affect the tree.
Treated and untreated trees were monitored in August for severity of scab infection. The analysed results showed that there wasn't a statistically significant reduction in scab, but there was a trend towards lower severity, as well as high leaf nutrient content in trees that had received the treatment.
The weak effects might be explained in that most sites used a mulch based on white willow and crack willow (Salix alba and S. fragilis), which have relatively lower levels of salicylic acid, and some sites did not apply sufficient mulch as might be required to see an effect on large orchard trees.
One site using mulch made from Salix daphnoides, which has higher levels of salicylic acid, did have significantly lower levels of scab in treated trees, suggesting the hypothesis could be correct if the right type of willow is used. Also, the key ingredient is concentrated in willow bark, so mulch made from this could have the best effect.
The experiment needs to be repeated for more robust results, but this was certainly an interesting investigation. If you wanted to try the treatment at home and have willow cuttings to hand, you would need to shred them to make them into a mulch. Dancing the classic barn dance number Strip the Willow after your apple tree produces beautiful scab-free fruits is optional, but a bountiful crop of home-grown apples is something that makes us feel like dancing!