Every gardening season brings new disease challenges. Here are the ten most common plant diseases you'll likely encounter, with symptoms and management strategies.
- Fire Blight (Ornamentals) – Blossoms and leaves turn brown/black but remain attached. Cankers on branches are dark brown/purplish; terminal branches bend into a shepherd's crook. Control: Prune infected branches immediately; sterilize tools with alcohol/bleach between cuts. Remove water sprouts. Choose resistant crabapple and pear varieties. Chemical control not recommended for homeowners.
- Nuisance Molds in Mulch – Slime molds and mushrooms appear overnight due to low oxygen in organic mulch. Control: Sprinkle high-nitrogen fertilizer (e.g., 24-0-0) and rake into affected areas.
- Damping Off – Bacterial disease killing seedlings in wet, cool, poorly drained soil. Control: Use sterile containers (10% bleach soak), allow soil surface to dry between waterings. Dust with cinnamon powder or spray weak chamomile tea (4 bags/quart water) weekly as natural fungicides.
- Rust – Fungal disease on ornamentals and fruit trees; dry, reddish-orange spores on leaf undersides. Severe infections can kill plants. Control: Remove and bag/burn infected leaves early. Never compost. Sulfur sprays and chamomile tea may help.
- Downy Mildew – Devastating on cucurbits (cucumber, squash, melon). Yellow spots between veins turn leaves yellow; plants die quickly. Control: Space plants for air circulation; avoid overhead watering. Apply preventive foliar fungicides (Daconil, Maneb) every 7 days. Choose resistant varieties.
- Dollar Spot (Turf Grass) – Fungal disease on bluegrass/fescue in cool weather. Tiny yellow spots expand to 2–3 inch dead circles. Control: Maintain healthy lawn with regular fertilization; avoid late-day watering; select resistant varieties. Chemical control difficult.
- Powdery Mildew – Whitish-gray powder on leaves; severe cases yellow foliage. Common on lilac, roses, zinnias, phlox, viburnum. Control: Buy healthy plants; space for air flow; avoid evening watering. Several fungicides available.
- Clematis Wilt – Sudden stem collapse as flowers open; stems discolor. Control: Cut affected stems below discoloration. Safer Garden Fungicide or Thiolux recommended.
- Black Spot (Roses) – Small dark circles on leaves; leaves yellow and drop. Untreated plants weaken and die. Control: Sanitation: remove diseased leaves to trash (not compost). Regular fungicide spray program (rotate brands every 4–6 weeks). Plant resistant varieties.
- Blossom End Rot (Tomatoes, Peppers, Eggplant) – Physiological disorder from calcium deficiency and irregular watering. Dark, leathery spot at blossom end of fruit. Control: Maintain even moisture; mulch; avoid over-fertilizing during fruiting; keep soil pH ~6.5; sprinkle ½ cup nonfat dry milk around base for calcium.
Monitor plants frequently and act at first sign of disease.
References: Ohio State University Plant Pathology Bulletin; Virginia Tech Cooperative Extension; Oregon State University Extension. Photos courtesy of those universities.
