Spinach leaves with black patches and mold are not appealing to eat. Sometimes the inner leaves of infected plants escape damage.
Choose resistant varieties, which are widely available. However, new races of this fungus are able to overcome resistance, so be sure to plan ahead for fast drying of spinach foliage after rains. Make sure plants get good air circulation and plenty of sun, and keep weeds controlled to promote prompt drying. Rotate spinach to a fresh site to prevent disease buildup, and use mulch to keep soil from splashing onto plants during heavy rains. Avoid using sprinklers or other overhead irrigation methods after spinach plants have grown large and leafy.
Clip off and compost affected leaves, and then spray plants with 1 teaspoon baking soda mixed with one quart of water. Spinach downy mildew usually disappears if the weather become warm and dry.