The diagnostic decision tree
Step 1: Is the white coating powdery and wiping off?
Rub the white material between your fingers. If it comes off easily and leaves a slightly grayish smear — that's the fungal mycelium of powdery mildew. If the surface feels rough or the spots don't rub off — proceed to step 2.
Step 2: Are the white spots discrete spots or a diffuse coating?
Powdery mildew starts as small circular white patches that expand and merge into a general coating. Downy mildew produces angular yellow spots on the upper leaf surface with gray or purple-gray fuzzy growth on the lower leaf surface — not typically white on top. Herbicide damage produces irregular pale patches or bleached areas that don't follow the typical spreading pattern of a fungal disease. Sun scald produces bleached, papery patches on the sun-facing side of leaves only.
Step 3: Which side of the leaf?
Powdery mildew: primarily upper surface, eventually both sides. Downy mildew: yellow or light patches on upper, fuzzy growth on lower. Herbicide damage: any distribution, often following a spray drift pattern (one side of a plant, plants near a fence or road). Sun scald: always the sun-exposed surface.
Cause 1: Powdery mildew
How to confirm
White powdery coating on leaves, usually starting on upper surfaces. Begins as discrete circular white spots, then expands to coat much of the leaf surface. Young growth and buds may be distorted or killed. Per Penn State Extension, powdery mildew "is caused by several different fungal species" that are host-specific — the species causing powdery mildew on squash cannot infect your roses, and vice versa. Unlike most fungi, powdery mildew thrives in warm, dry conditions with relatively low humidity, though it needs a thin film of moisture on the leaf surface to germinate.
Most susceptible plants
Squash, cucumbers, and melons; roses; phlox; lilacs; bee balm (Monarda); zinnias; peonies; and many native trees and shrubs in late summer. Per UC IPM, "powdery mildew is most common in late summer and fall when days are warm, nights are cool, and humidity fluctuates."
How to treat
For light infections on ornamentals: improve air circulation around plants (space them farther apart, remove interior stems to open the canopy), avoid overhead watering in the evening. Per UC IPM, "removing heavily infected leaves and shoots reduces inoculum." Fungicide options include: neem oil (applied to all leaf surfaces), potassium bicarbonate (effective and low-toxicity), or conventional fungicides containing myclobutanil or tebuconazole for severe infections. Apply preventively on known susceptible plants in late summer before symptoms appear.
Recovery timeline
Existing infected tissue will not recover — the mycelium is already in the leaf. Treatment prevents spread to new growth. Plants rarely die from powdery mildew alone, though severely affected plants are weakened. By mid-fall, most powdery mildew infections slow as temperatures drop. Full plant recovery is expected by the following spring, assuming the rest of the plant is healthy.
Cause 2: Downy mildew
How to confirm
Look for angular yellow-green spots on the upper leaf surface (angular because they're bounded by leaf veins) with a gray to purple-gray fuzzy or water-soaked growth on the corresponding lower leaf surface. The upper-surface spotting may look whitish in some species but is typically more yellow or light green than the white coating of powdery mildew. Per Penn State Extension, downy mildew "thrives in cool, humid conditions with wet leaves — the opposite of powdery mildew's preferred conditions."
Most susceptible plants
Impatiens (downy mildew devastated commercial impatiens production in the 2010s), basil, cucumbers, lettuce, spinach, and many ornamental plants. Per UC IPM, each downy mildew species is highly host-specific — impatiens downy mildew affects only impatiens.
How to treat
Downy mildew is caused by oomycetes (water molds), not true fungi, and most conventional fungicides that work against powdery mildew are ineffective. Per Penn State Extension, effective materials include copper-based fungicides, mancozeb, and products containing mandipropamid or dimethomorph. Cultural practices — improving air circulation, avoiding overhead irrigation, removing infected plant material — are the foundation of management. For impatiens, switching to New Guinea impatiens or SunPatiens (resistant hybrids) or to an entirely different bedding plant is more practical than attempting chemical control in an infected bed.
Recovery timeline
Downy mildew can kill plants rapidly in cool, wet conditions — 2–3 weeks from first symptom to complete defoliation is possible in severe cases. Act quickly once confirmed. Infected annuals in beds are usually best removed entirely rather than treated, to prevent further spread.
Cause 3: Herbicide damage
How to confirm
Herbicide damage produces several distinctive patterns: chlorotic (pale yellow or white) patches that don't follow disease spread patterns; distorted, cupped, or twisted new growth (especially from broadleaf herbicides like dicamba or 2,4-D); bleached areas on only one side of a plant or along one row (suggesting drift from a neighboring spray application); and injury appearing after a neighbor, lawn care company, or farm has applied herbicides nearby. Per Penn State Extension, "herbicide drift can travel considerable distances, particularly in warm, windy conditions."
How to treat
There is no chemical cure for herbicide damage. Per Utah State University Extension, management is supportive: water the affected plant well to encourage new growth, remove severely damaged tissue, and wait. Most plants that have suffered non-lethal herbicide exposure will produce new, unaffected growth within 2–4 weeks. Fertilize lightly to support new growth. If the herbicide that caused the damage is systemic (such as glyphosate), and a significant portion of the root system was exposed, recovery may not be possible.
Recovery timeline
Light drift damage: full recovery in 4–6 weeks as new growth emerges. Severe exposure to systemic herbicides (glyphosate, imazapyr): may be fatal within 2–4 weeks of exposure. Document the damage with photos and dates — herbicide drift from neighboring properties or commercial applicators may be grounds for a claim.
Cause 4: Sun scald
How to confirm
Bleached, papery, tan or white patches on the surface of leaves that was most directly exposed to intense sunlight. Most common on: plants recently moved from shade to full sun without acclimation; shade-adapted plants placed in too much sun; and plants during heat waves when light intensity is extreme. Per UC IPM, "sun scald is not a disease and will not spread — it is physical injury from UV radiation and heat."
How to treat
Remove severely damaged leaves. If the plant is in the wrong location for its light requirements, transplant to a shadier site or provide temporary shade cloth during heat waves. Acclimate plants moved from indoor or greenhouse conditions to outdoor sun over 7–10 days by starting with 1–2 hours of direct sun and gradually increasing exposure.
Recovery timeline
Existing damaged tissue will not recover, but plants produce new growth from unaffected tissues. Once the stress source is removed or the plant is properly sited, new leaves will be healthy. Full recovery typically takes 3–6 weeks depending on the plant's growth rate.
Related diagnostic guides on this site
- Brown tips on leaves — causes and fixes
- Silver leaves — thrips damage vs. other causes
- Begonia problems — Botrytis and powdery mildew
Sources
- Penn State Extension: Powdery Mildew on Ornamentals
- UC IPM: Powdery Mildew on Ornamentals
- Penn State Extension: Downy Mildew of Vegetables
- UC IPM: Downy Mildew
- Penn State Extension: Herbicide Drift Damage to Plants
- UC IPM: Abiotic Plant Disorders