Zinnias are susceptible to an array of issues like different leaf infections (some fungal, some bacterial), pest feeding (such as spider mites and aphids), and even plant viruses. Some varieties of zinnia have better inherent resistance to disease than others, but none are immune and the weather can play a large role in how severe a disease outbreak becomes any given year.

The photos are too small for us to see much detail, but we think at least some of the damage pictured could have been caused by either a plant virus or exposure to an herbicide (such as drifting evaporated chemical, rather than a direct spray or drips onto the plant). Certain viruses and herbicides can cause very similar symptoms and can be difficult to tell apart. Plants sometimes outgrow herbicide damage if the exposure was not too great, but virus infections are incurable and do not resolve themselves. Plant viruses are transmitted in various ways, but insects that suck the sap of infected plants can transmit it to other plants when they feed. Sometimes those infected plants came from the garden, but they can also be weeds.

If you want to keep the plants for awhile longer to see if they worsen or not before deciding whether to remove them, just keep monitoring them for watering by feeling the soil several inches deep near their roots and watering when it is becoming somewhat dry to the touch at that depth. If you clip these flowers for bouquets, clean your snips using either rubbing alcohol or a sanitizing wipe to reduce the risk of spreading a virus to other plants from contaminated sap clinging to the tool blades. (Don't use bleach because it can damage the metal.)

Miri