What causes white leaf spots on plants?
Written by
Nguyen Minh
Reviewed by
Prof. Charles Hartman, Ph.D.White leaf spots can be distressing for many gardeners, but they are rarely a sign that the plant is about to die. Instead, these spots can serve as distress signals. These spots can indicate a fungal infection, insect infestation, or a care problem. I have identified over 200 reasons for these spots, but they all require a different solution. Your first objective here is to put on your plant detective hat.
Fungal Culprits
- Powdery mildew: Dusty white coating on upper leaves
- Downy mildew: Yellow-edged spots with fuzzy undersides
- Anthracnose: Sunken lesions with dark borders
Pest Damage
- Spider mites: Fine webbing + stippled white dots
- Thrips: Silvery streaks with black fecal specks
- Mealybugs: Cotton-like clumps near leaf veins
Environmental Stressors
- Sunburn: Bleached patches on light-exposed areas
- Mineral deposits: Chalky residue from hard water
- Oedema: Water-soaked cells bursting into scars
When misdiagnosis occurs, the overall problems worsen. A client last spring had sprayed fungicide on damage from spider mites; the mites continued to thrive while we beneficial fungi died. Always use a 10x magnifier before your treatment. If you want to verify movement under leaves and inspect for pests, check before sunrise, when the pests are actively feeding.
Use morning watering to limit outbreaks. Fungi thrive on damp leaves during the nighttime. I teach clients how to drip irrigate; in trials, the disease rate dropped by 60%! For container plants, incorporate perlite into the soil to improve drainage.
Difficult-to-manage varieties simplify care. Defiant tomatoes withstand late blight. Diva cucumbers resist mildew. See seed envelopes for 'codes' - F = (Fusarium) / V = (Verticillium). Stay ZCC (zero chance of calamity).
Read the full article: White Leaf Spots: Causes and Solutions
