Disease and their control in organic farming
Diseases in crops are of big concern when organic farmers have conventional farmers that use chemical control as neighbours. Not only do they have to cope with drift from insecticide and herbicide spraying but thay also have to contend with the problem of beneficial insects being killed by the sprays. Additionally blight and rust etc. in abandoned crops will have the affect that the spores may get blown onto your crops. All this is besides instances where fruit flies and other insects that are hosted in wild growing fruits and berries growing closeby. Unfortunately none of these problems have easy solutions. Ideally buffer zones should be put in place around your fields, or (in Nirvana) all the adjacent neighbours should convert to organic methods.
We overcame these problems by taking into account the prevailing winds for spores and co-ordinating with neighbours on what crops to grow when and where. After 3 years we found that as the soils improved the incidence of disease and pests decreased. The only way to control disease is to have all the relevant micro-organisms in the soil. In China farmers have been planting tomatoes on the same fields for 400 years without disease by spraying the necessary microbes on the crops. The biggest carrier of disease is water soluble nitrogen of any form. Large scale application of cattle, chicken or pig manure will cause disease in the same ways as chemical fertilisers do, as the nitrogen is instantly dissolved in water which makes the plant susceptible to disease. As earthworm castings are not water soluble, this problem does not occur when farming with vermicompost.
We have recipes for organic sprays, although these are now seldom put to use.