Perennials, taken as a class, under fairly favourable conditions are not particularly susceptible to disease. In general, when diseases attack perennial plants, the safest thing to do is to throw them away. Aphis and minor insect troubles may be easily controlled by spraying.
The following plants, if attacked by leaf spots or mildews, may be saved by fungicides (See Page 77):
Anemone (root decay and rust). Destroy affected roots.
Aquilegia (mildew and black spot).
Campanula (red and brown rust). Keep away from pinus rigida.
Chrysanthemum (leaf spot or powdery mildew). If rusted, plants should be destroyed. Convallaria (stem rot). Burn affected plants. Coreopsis (mildew).
Peony (stem rot, leaf spot, botyrides). Phlox (leaf spot and powdery mildew, and stem canker). Sedum (leaf spot). Thalictrum (red spot). Veronica (leaf disease), (leaf spot). Violets (leaf spot and leaf blight).
Root rot of violets and nematodes on the roots require soil treatment and sterilization.
Bulb spot of irises should be treated by soaking the bulbs for one hour in formalin solution, consisting of one ounce in two gallons of water. Root rot which is found usually only in the midst of an old matted clump is overcome by breaking the rhizomes from the clump, cutting off the decay, and planting in new soil with a watering of potassium permanganate, only enough to make the water of light pink colour.