Disease prevention is more effective than controlling them. Once a plant becomes infected, there is little to do other than pruning out the diseased part or removing the entire plant in the case of root rots or virus infection.
A disease prevention program should use a combination of cultural and chemical treatments. This requires some understanding of the disease-causing organisms and the chemicals to be applied.
By following a few basic measures, many diseases of perennials can be avoided or minimized in the landscape.
Planting Practices for Improved Disease Resistance
Spacing: use the correct spacing since crowded plants do not do well and too-close spacing can promote disease by inhibiting drying and air circulation.
Planting Depth: plant at the depth recommended for each species; this is especially important for winter-hardiness and, in some cases, for flowering.
Cultural Care— maintenance steps to improve disease resistance
Fertilizing: appropriate applications to maximize plant growth and vigor and to avoid plant stress due to deficiencies or toxicities.
Mulching: properly applied mulch helps with weed control, soil temperature moderation, and soil moisture retention; summer mulches should not be applied too thick or too close to the stems or crowns of plants.
Watering: maintain adequate soil moisture for the plant species; this usually translates to approximately one inch of water per week; in the absence of natural rainfall, irrigation should be used and, depending on soil type, this is best delivered as a deep soaking; avoid overhead irrigation or water plants early in the day to allow foliage to dry before nighttime.
Winter Protection: winter mulches can be effective to protect plants from heaving during freeze-thaw cycles; mulches should be applied AFTER the ground has frozen and removed BEFORE or WHEN new growth starts in the spring.
Plant Selection— selecting plants specifically for your geographic area and ones that are resistance to common diseases in that area
Hardiness: often an overlooked aspect of disease prevention; select only plants designed for your specific location.
Plant Requirements vs. Site Characteristics: match these as closely as possible; special attention to soil type and pH, drainage, and light level.
Genetic Resistance: if resistant or tolerant cultivars or species are available, they should be selected for use.
Plant Health: purchase healthy, vigorous plants and carefully inspect the root system before planting.
Sanitation— clean up the garden each fall to help reduce over-wintering disease causing pests
Remove Infected Plants and Debris: symptomatic plants or infected plant parts should be removed promptly to minimize disease spread; after tops are killed by frost in autumn, all plant debris should be removed to reduce or minimize over wintering inoculum.
Groom Plants: remove spent flowers and leaf debris during the growing season to minimize inoculum buildup and spread.