The key to keeping zinnias healthy all season is planting them in the right spot and choosing the right variety. Plant your zinnias where there?s full sun and well-drained soil. Shady conditions or wet soils encourage fungi to grow. Zinnias planted close together are more prone to fungal attacks. Avoid getting the foliage wet, especially during the evening and night; wet foliage encourages the growth of harmful fungi. Many of the newer varieties are more disease-resistant than the older types. Choosing disease-resistant varieties is an important factor in keeping your plants healthy. The Profusion Series and Pinwheel Series of zinnias provide large-flowered, mildew-resistant zinnias in a broad array of colors. Smaller-flowered species zinnias, such as narrowleaf zinnia (Zinnia angustifolia) and Mexican zinnia (Z. haageana), seldom are affected by powdery mildew.