Copper..There are many copper compounds that are used as fungicides. The most common is Bordeaux mix which is a combination of copper sulphate and hydrated lime. It is accepted in organic farming provided that the number of applications is strictly followed and a proper soil amendment is observed to prevent copper accumulation in the soil.
Bordeaux mix sprayMaterials needed to make a gallon mixture
3 1/2 tbsp of copper sulphate
10 tbsp of hydrated lime
1 gallon of water (4 liters of water)
Wooden stick
Plastic bucket
How to prepare?
1.Add copper sulphate and hydrated lime in water. Make sure to use plastic container.
2.Stir well using a wooden sick or ladle.
3.Protect self from direct contact with the solution.
How to use?
1.Spray plants thoroughly preferably early in the morning, in a dry and sunny day. In this way, the plants have the time to dry and the solution can not penetrate into the leaves' tissues
2.Constantly shake the sprayer while in the process of application to prevent the solution from clogging
Pest controlled
1.Flea beetles on tomatoes and potatoes
2.Anthracnose
3.Bacterial blight
4.Bacterial wilt
5.Black spot
6.Downy mildew
7.Late blight on solanaceous crops
8.Powdery mildew
9.Rust
10.and many other disease causing pathogens
Standard procedures for the preparation and application of homemade extracts
1.Read and follow the label instructions carefully. Ask for assistance from your local agriculturist office when using copper for the first time.
2.Monitor plants regularly and spray only when necessary as copper can accumulate into the soil.
3.Spray in the early morning or late afternoon.
4.Use utensils for the extract preparation that are not use for your food preparation and for drinking and cooking water containers. Clean properly all the utensils every time after using them.
5.Do not have a direct contact with the crude extract while in the process of the preparation and during the application.
6.Make sure that you place the extract out of reach of children and house pets while leaving it overnight.
7.Harvest all the mature and ripe fruits before extract application.
8.Always test the extract formulation on a few infected plants first before going into large scale spraying. When adding soap as an emulsifier, use a potash-based one.
9.Wear protective clothing while applying the extract.
10.Wash your hands after handling the extract.

There is no soil additive to prevent diseases except for a couple of systemic ones. Then you have it in the food as well as in the plant.

There are a number of different disease control sprays that are quite effective, depending on the disease in question, but most are synthetic chemicals, not organic.

If the disease in question is caused by a fungus then as Terry said any of the various copper fungicides are the most organic control you can get. But they have limited effect and have the problem of copper build-up in the soil.

If the disease is bacterial in origin then other controls will be necessary.

So basically it boils down to what is the disease in question? Different diseases have different treatments.

But good garden hygiene and good garden practices such as good air circulation, weed elimination, avoiding over-crowding, and adequate sun exposure play as much of a role in disease prevention as sprays.

Dave

Looking up on web looks like on my Pole Beans was Bacterial Bean Blight : Bright yellow or brown spots on the leaves.
And my Cucumbers looks like powder mildew spots - then the leaves die & fall off & it spread thru the whole vines in no time & I pulled them all up.
My squash had Aphids I couldn't get rid of & also had the powder spots on them - I pulled them up weeks ago.
The only plants that weren't effected & still producing are the Okra, bell peppers & eggplant - I live in Zone 9 LA.