alternatives to pesticides - plant disease - scab
Scab is a name given to several fungus diseases. One infects apple and other fruit trees, another infects cucumbers and yet another can infect potatoes. On fruit trees, scab generally appears first on the undersides of the leaves as pale yellow spots that gradually darken until they are nearly black. Leaves may have numerous scab spots and become distorted. The scab fungus over winters on the fallen infected leaves, producing spores in spring. The spores are wind borne and infect the young leaves and fruit during periods of rain. Warm rainy weather is ideal for scab.
For potatoes, scab is particularly severe in alkaline soils and usually worse in dry soil. Avoid using lime, fresh manure or wood ashes on your potato beds, as these will increase the alkalinity. If potato scab has been a problem, you should practice a three- to five-year crop rotation schedule; always plant resistant varieties of potatoes and apples.
Because the fruit tree fungus spends the winter on fallen infected leaves, rake them up carefully and dispose of them. Careful pruning for good air circulation creates a less favorable environment for the spread of scab.
Least-toxic chemical control
Dormant lime sulfur sprayed on the fruit trees during the winter will help control it. In the spring, if your plants are in serious trouble, continue using lime-sulfur spray or wettable sulfur