While you're learning how to distinguish the symptoms of downy mildew from those of spider mite damage, there are ways to minimize the chances that trouble starts at all. Roses need at least five hours of sunlight a day. Providing them with less (sitting them in shade) invites problems. And dead and damaged canes are like open doors to bugs and diseases, so remove them with sharp shears.
Foliage that is wet, especially overnight, encourages the spread of disease. Use a drip irrigation system, and don't water roses in the evening. You can foster good air circulation by thinning out crowded stems.
Excessive growth brought on by fast-acting chemical fertilizers attracts insects. Opt for an organic fertilizer and top-dress the garden annually with aged manure and garden compost.
These conditions tend to crop up in damp or humid weather.
Symptoms: white fuzz on young leaves, stems, and buds; distorted young leaves
Solutions: remove infected parts; rinse leaves; spray with a mix of 1 tablespoon baking soda, 2 tablespoons horticultural oil, and 1 gallon water every seven to 10 days; spray with sulfur-based fungicide during infestation
Symptoms: older leaves turn yellow; orange pustules on underside of foliage; defoliation. Note: common in foggy and seaside gardens
Solutions: remove infected foliage; spray with sulfur-based fungicide
Symptoms: purple-red spots on foliage and stems; downy fuzz on underside of foliage; defoliation. Note: prolific in cool, moist regions
Solutions: remove infected parts; clear garden debris; spray with horticultural oil during dormant season; spray with copper-based fungicide during infestation
Symptoms: black spots on foliage; leaves turn yellow; defoliation
Solutions: remove infected foliage; prune to encourage new growth; remove lower foliage (3 to 6 inches from the ground) to improve air circulation; spray with horticultural oil before spring, when black spot becomes active; spray with sulfur-based fungicide during infestation
Symptoms: petals turn brown; flowers fail to open; blooms fall apart. Note: common in damp climates
Solution: remove infected blooms
They can appear in any season, so check for bugs regularly.
Symptoms: Brown stains on flowers; blooms fall apart; tracks on petals and foliage where insects feed; leaves turn dusty gray-green
Solutions: spread diatomaceous earth around plant to control soilborne larvae; spray neem oil, pyrethrum, or garlic juice into open flowers; sticky-tape traps. Beneficial insects: ladybugs, lacewings. Companion plant: alliums
Symptoms: tiny, paprika-like specks on underside of foliage; older leaves turn dusty yellow; defoliation. Note: common in hot, dry conditions
Solutions: remove infected parts; prune to remove clutter; remove fallen leaves; keep beds free of weeds; spray with horticultural oil in late winter or early spring; during infestation, spray the undersides of the leaves with cold water. Beneficial insect: predatory mites
Symptoms: skeletonized foliage at start of season. Note: rose slugs are larvae of sawflies
Solutions: blast with cold water; spray with neem oil
Symptoms: small, soft-bodied insects cluster on young foliage, stems of newest growth, and buds. Note: common in cool conditions
Solutions: blast with cold water; spray with horticultural oil, neem oil, or insecticidal soap. Beneficial insects: praying mantis, aphid lions, hoverflies, parasitic wasps, ladybugs. Beneficial animals: birds. Companion plants: bronze fennel, yarrow, valerian, alliums
Some attack leaves; others go after flowers, stems, or roots.
Symptoms: larvae feeding on pith of cane; round hole visible at end of cut cane; dieback. Note: cane borers are larvae of small carpenter bees, leaf-cutter bees, and sawflies
Solutions: prune below hole; seal fresh cuts with nontoxic glue, wax, or lipstick
Symptom: semicircular cuts in foliage. Note: small carpenter bee makes similar cuts
Solution: damage is cosmetic; does not warrant treatment
Symptoms: tiny grubs feeding on new budsand leaves; new growth is blackened; no flower buds; severely crooked flower stems
Solutions: apply beneficial nematodes to soil; prune infected plant parts; spray with pyrethrum
Symptoms: holes in flower buds; ragged flowers riddled with holes; distorted new shoots
Solutions: treat turf with milky spore to kill beetle grubs; spray buds with neem oil; prune infected plant parts
Symptom: holes appear in foliage and flowers where beetles feed. Note: Pests that do similar damage include spotted cucumber beetles, Colorado potato beetles, rose chafers, and curculio beetles
Solutions: treat turf and rose beds with milky spore to kill beetle grubs; apply beneficial nematodes to soil; shake affected plant parts over a bucket of soapy water to knock off beetles; spray neem oil onto foliage, and spray pyrethrum onto beetles. Companion plants: four-o'clocks, larkspur, garlic, catmint, geraniums
Viral diseases can decrease plant vigor and diminish flower production.
Symptoms: attacks early in season; yellow patterns and crinkled edges appear on foliage; distorted growth. Note: spreads via propagation (cuttings and budding)
Solution: Dispose of infected plant if disease is rampant
Improper cultural practices can affect plant health.
Symptoms: leaf burn: mottled, discolored foliage; defoliation. Note: can be caused by spraying too much or at the wrong times
Solutions: follow package directions on pesticides and fungicides carefully; don't spray when temperature is 90 degrees or above
Symptoms: wilting foliage and flowers; leaf tips and margins turn brown; petals fall off prematurely
Solutions: ensure that plant gets at least an inch of water a week from irrigation or rain; apply mulch to preserve moisture in soil
Recommended reading: Stephen Scanniello, "A Year of Roses" (Cool Springs Press, revised edition; 2006)