organic pest treatments

• compost - Most store-bought fertilizers are inorganic and don't contain micronutrients that are also required by plants.  Add organic fertilizers (also seaweed, Turkey Trot, animal and mushroom manures) to feed the soil, not the plants.  Ultimately, plants will be so healthy that they won't be as susceptible to pests.
• diatomaceous earth - a sharp dust that cuts the soft bodies of slugs as they travel over it
• baking soda - a tsp to a litre of water, with a drop of dish detergent added, can be sprayed on leaves with blackspot (roses) or powdery mildew.  It won't cure them, but will keep the diseases from spreading.