Rust and mildew are common problems for Zinnia elegans, especially in wet environments.

Here are some organic solutions you can try to prevent these problems:

1. Improve air circulation: 
Adequate airflow around plants can help prevent the development of rust and mold. You can achieve this by providing enough space between plants, trimming back bare leaves and using fans to improve air circulation.

2. Water the soil, not the leaves: 
Since you are already using drip irrigation to avoid water back on the leaves, make sure the water reaches the roots and does not wet the leaves. This can be achieved by using soaker hoses or watering directly at the base of the plants.

3. Apply organic fungicides:
Organic fungicides such as neem oil, baking soda and garlic oil can help prevent rust and mildew on your zinnia. These natural compounds interrupt the growth of fungal spores and are safe to use on humans, pets and beneficial insects.

4. Covering:
Covering around the base of zinnias with organic materials, such as straw or sawdust, can help prevent rust and mold growth by maintaining soil moisture and temperature.

5. Choose resistant varieties:
Some varieties of zinnias are naturally resistant to rust and mildew. When choosing your plants, look for varieties that are labeled as resistant to these diseases.

By applying this set of tips, you can prevent the development of rust and mildew on your Zinnia elegans while encouraging the development of a healthy and vibrant garden.
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