When Does Apple Scab Occur?
Apple scab occurs when an apple tree falls victim to the ascomycete fungus called Venturia Inaequalis, a malignant growth which slowly eats away at the leaves and fruit. It is a widespread and serious disease affecting apple trees all over the world, but is more powerful in warm climates. Farmers control the fungus by spraying the trees with sulphur. That is one reason why apples should be washed before eating.
The disease manifests as dull black or grey-brown lesions on the surface of tree leaves, buds or fruits. Lesions may also appear less frequently on the woody tissues of the tree. Fruits and the undersides of leaves are especially susceptible. The disease rarely kills its host, but can significantly reduce fruit yields and fruit quality. Affected fruits are less marketable due to the presence of the black fungal lesions.
The infection cycle begins in the springtime, when suitable temperatures and moisture promote the release of V. inaequalis ascospores from leaf litter around the base of previously infected trees. These spores rise into the air and land on the surface of a susceptible tree, where they germinate and form a germ tube that can directly penetrate the plant’s waxy cuticle.
A fungal mycelium forms between the cuticle and underlying epidermal tissue, starting as a yellow spot that grows and ruptures to reveal a black lesion bearing asexually as the conidia are released and germinate on fresh areas of the host tree, which in turn produce another generation of conidial spores. This cycle of secondary infections continues throughout the summer, until the leaves and fruit fall from the tree at the onset of winter.
Over the winter, V. inaequalis undergoes sexual reproduction in the leaf litter around the base of the tree, producing a new generation of ascospores that are released the following spring. Scab lesions located on the woody tissues may also overwinter in place, but will not undergo a sexual reproduction cycle; these lesions can still produce infective conidial spores in the spring.
Remember that the disease is most easily spread if the conditions are cool and moist. What if the grower could figure out when to spray, based on the likelihood that the weather conditions would be conducive to the spread of the disease, either through conidia or ascospores?
This is called disease forecasting, and there are computer programs commercially available that can combine weather data with what is known about the disease cycle. The orchard owners would only have to spray fungicides when the weather data and the computer program tell them to do so, thus cutting down on the amount of fungicide in the environment and a lower economic cost to the orchard.
A more long term solution is breeding resistant varieties of apples. However, it may take several years to get the resistance plus all the characters of a particular variety of apples. The major problem after that is that, through their sexual reproduction, Venturia can relatively quickly overcome this resistance.