Basic organic disease prevention

Quite often, people think that there is little or nothing that they can do to control plant disease in organic gardens. With an organic garden, you can't simply apply synthetic fungicides to eliminate disease problems. However, there's plenty you can do to organically promote and maintain healthy, productive plants.
There is an old saying "an ounce of prevention is worth a pound of cure" and is popular in the world of disease control, in both the animal and the plant world. It takes on a real meaning in organic gardening, as many of the disease control methods focus on healthy plants and avoiding diseases.

(aphids on lettuce plant)

The main damage is from insects and what they do apart from eating your plants, is help the spread of viruses. Natural immunity is your plants' only defence. A weak or unhealthy plant is more likely to suffer disease. A strong, healthy plant can defend itself.
Your first line of defence against pests and diseases is to choose plants with the most genetic potential for self-defence. Always plant more seed than you need; select the strongest and cull the weakest. Do this at the seedling stage and again when planting out into your garden beds.
 If you have reason to expect a disease to appear, like when late blight is in the neighbourhood, or if inspections turn up the first signs of a disease such as powdery mildew, then it may be necessary to apply a protectant material to prevent any crop loss.

There are quite a few copper and sulfur-based fungicides which are approved for organic use, and they are relatively inexpensive and pose a minimal environmental risk. Copper fungicides have some activity against a wide range of fungi and bacteria, but their effectiveness is very limited, and under high disease pressure, probably will not provide first-rate control. Sulfur products also offer some control of many pathogens, but they usually are good for managing only some diseases such as powdery mildew. Both copper and sulfur can burn sensitive crops, so precaution is advised

(powdery mildew on a cabbage)

Organically-approved bicarbonate fungicides have now recently become available. Bicarbonates have demonstrated good protection against powdery mildew and a few other diseases. Peroxide-type materials are also available for general suppression of diseases on plant surfaces.

The other organic options for disease restraint are products that contain microbes or their by-products, including species of Trichoderma, Bacillus, and other beneficial organisms. For the best results possible with these materials, proper application and timing are essential. Most materials do not perform well if the disease is established, so applications should be made prior to infection. As mentioned above "an ounce of prevention is worth a pound of cure.”

Finally remember no genetic inheritance will reach its full potential without the right nutrition. Well cared for plants can fend off most diseases themselves. Try not to overindulge with water-soluble fertilizers. If your fertilizer is very Nitrogen-rich, plants can put on lots of new growth and appear to be healthy and doing well. However, without the other nutrients which are necessary to construct healthy new cells, this new growth can be made up of thin-walled, weak cells that are excessively vulnerable to pests (particularly sap-suckers like aphids) and diseases (particularly fungus diseases). Malnourished plants succumb to diseases that healthy plants hardly notice.

Micronutrients probably perform something like the equivalent function in a plant's diet that vitamins do in a humans diet. A great remedy for a number of plant ailments, especially aphids, is a good dose of seaweed brew. (Seaweed left to rot down naturally and the liquid collected and applied as a feed).
It doesn't kill the pest, but (probably because it is so rich in micronutrients) it increases resistance to the point where the pest is irrelevant.