Fungicides can be used to treat a wide range of diseases on apple trees, since many such diseases are caused by fungal disease agents. For various reasons, timing a fungicide application incorrectly can cause more harm than good. The well-prepared apple tree grower will learn how to properly time fungicide treatments so as to protect apple trees from all the negative effects of fungal infection.
Cedar Apple Rust
Cedar apple rust is among the most common apple tree diseases caused by fungi. The most conspicuous symptoms of this disease are protruding orange fungal growths called rust galls. Fungicide treatments for cedar apple rust should be applied several times -- first when new buds are pink, then when 75 per cent of the tree's flower petals have fallen off and again 10 days later.
Sooty blotch does not cause any significant damage to apple trees or to the fruit that they bear, but it still should be treated with an approved fungicide if necessary. Lime sulphur is the preferred treatment in this case; while not exactly a fungicide, it is a fungicide in effect when used in this capacity. Spray the apple trees with lime sulphur during the spring when the tree is still dormant. Combine the lime-sulphur treatment with biological measures, such as pruning the apple tree, to improve air circulation and sunlight penetration.
Scab is a fungal disease that can cause significant damage to apple trees. Fungicide applications for scab control must be preventive in nature, preceding the appearance of disease symptoms. Apply a preventive fungicide whenever green tissue first appears in April and reapply every seven to 10 days until early June. From June to early August, scale back fungicide applications to once every two to three weeks.
Other Fungicide Considerations
The use of chemical fungicides carries with it certain inevitable risks to human health and the environment at large, so they should only be considered as a last resort in the failure of other control methods. Select disease-resistant cultivars of apple and cultivate them. Regular pruning, as well as effective watering and fertilisation schedules, go a long way in preventing infection from fungal disease agents and preventing your need to use any fungicides at all.