The Shasta daisy has three major pest problems that can be treated fairly successfully.
Gray mold can survive over winter on any plant material. It attacks all parts of a plant, and will produce brown, dead looking areas
If there is a high level of humidity, the affected areas take on the gray and fuzzy appearance that gives this disease its name.
It is possible that fungicides can help prevent this problem, but you might have to keep repeating the treatment until you have destroyed the source of the infection.
Powdery mildew is another mold that causes white patches on leaves and buds. The patches take on a furry appearance and turn gray to light brown in color. The patches will eventually form a coat on the whole plant and the leaves will simply fall off while the buds can become deformed.
Like gray mold, powdery mildew survives over winter on plant material. When the damp spring weather arrives, the mildew releases its spores and they attach themselves to damp surfaces of healthy plants. As long as warm dry days are flowed by cool damp nights, the disease will continue to spread.
Often, the cause of the spread of powdery mildew is growth in areas to which the plant is not native or does not have enough growing room. The lack of ventilation in a crowded flower bed will cut off the breeze that could have blown the spores away.
Fungicides work as long as the treatment starts as soon as the first symptoms are spotted and identified.
The best protection is to grow healthy, resistant plants. If the infection cannot be cleared up, then it will have to be allowed to run its course and be cleaned out in the winter.
Aphids are attracted to new stems and succulent leaves. Their mouth parts are adapted for cutting into the leaf or stem surface and extracting the sap. The capillary action of the plant actually forces the sap into the aphids, which are not actually strong enough to suck the sap out of the plant.
The aphids excrete honeydew, which can cause a black mold to grow on the plant. If enough of this mold grows, it can seriously reduce the amount of photosynthesis the plant is capable of.
Aphids can be cleared by ladybugs and lacewings, but you need a lot of them to clean up even a mild infestation. Using insecticidal soap can clear a plant if used properly. You can also wash aphids off a plant with simple water from a hose.
There are some systemic pesticides that work from inside the plant that are safe to use on daisies since daisies are not edible. These give a very thorough clear-out of the aphid population and can remain active for several weeks to prevent a new infestation.
Fortunately Shasta daisies are very tough and seldom suffer from molds and insects. When they do, it is invariably because something is not quite right with the conditions in which they are growing.