With orchards starting to bloom soon, the backyard arborist has to start thinking of something to protect his apple blossoms. Apple scab is caused by the fungus Venturia inaequalis. This disease is a major problem in both organic and conventional apple growing. In many commercial orchards, apples are sprayed 10 to 20 times per year. The fungus winters in fallen leaves on the orchard floor, and in the spring it produces primary spores, ascospores, which are discharged after rain and dispersed to new leaves on the tree. The spores germinate on the leaf surface and attempt to penetrate the outer leaf layer, the cuticle, and grow between this layer and the outermost cell layer of the leaf.
The successful use of a water extract from ivy (Hedera helix) to control apple scab was reported in Switzerland some years ago. Ivy contains compounds, which showed fungicidal effect against spores of V. inaequalis. The StopScab project has included testing of ivy extracts in the orchard at DIAS Aarslev this summer (2004), and the trials will be evaluated at fruit harvest.
Many other plants contain similar compounds, e.g. soapwort (Saponaria officinalis), and extracts from roots of this plant has been reported to be effective against apple scab on apple seedling in greenhouse experiments carried out in Germany.
Other very promising natural products are extracts of Citrus spp. However, it has recently been found that some commercialized products based on extracts from grapefruit kernels had been preserved with synthetic chemicals thus making them non-organic alternatives.