Insect tunnels in leaves
Spinach Leafminer (Pegomya hyoscyami), the larva of a small gray fly. Laval, or maggot, stage is a white or yellow legless insect about 1/5 of an inch in length.
Several generations of this pest can occur in a single season. Since the insect is safely encased within the leaf surfaces, contact insecticides will not work. Proper management includes elimination of weed hosts like lamb's quarters, chickweed, and nightshade near spinach fields. In addition, other crops such as beets and Swiss chard can host this pest and should not be planted near spinach.
Mottled or wrinkled leaves that turn yellow, then die
Spinach Blight (Cucumber Mosaic Virus).
Symptoms are often worse during warm weather. Control weeds near spinach fields. Do not plant spinach in a location where cucurbits were grown in the previous season.
White, blister-like pustules on the underside of leaves
White rust (Albugo occidentalis), a fungus.
Use approved copper fungicides and rotate crops annually. Also control weeds and volunteer spinach near production fields.
Gray or purple mold on the underside of leaves
Downy mildew (Peronospora effusa), a fungal disease.
Disease outbreaks are often worst during cool, wet weather and severe outbreaks can destroy entire fields within several days. Disease is also worst in poorly drained fields with standing water, or when leaves remain wet. For this reason overhead irrigation should be avoided. Approved copper fungicides are available, as are resistant varieties. Different disease "races" appear periodically and new resistant spinach varieties are in constant development.