Plant Diseases 101: What to Do When Your Plant Looks Sick

Recognize the Signs of Plant Disease

Blight. Rust. Fusarium wilt. Botrytis. Smut. Scab. The names sound ominous, and so are the consequences—they can appear suddenly and severely weaken or kill a plant within days. Diseases are caused by fungi, bacteria, viruses, or nematodes. Pathogens may be soil-borne, waterborne, airborne, or insect-transmitted. Wet, humid weather encourages infection and spread.

Common Symptoms

Leaf Spots

Fungal diseases often first appear as leaf spots. Shape, color, and margins can identify the pathogen, but general prevention/control measures apply for most fungal leaf diseases.

Vascular Diseases

Fusarium and verticillium wilts penetrate tissues and clog the vascular system, preventing water transport and causing wilting. If one section wilts dramatically with no external stem damage, suspect vascular disease.

Powdery Mildew

Looks like a dusty/powdery coating on leaves. One of few diseases that infect without moisture—plants most vulnerable in hot, dry weather. Strains are host-specific (mildew on beans won't spread to pumpkins). Choose resistant varieties; space plants for air circulation.

Diseases on Fruits

Leaf spot diseases often affect fruits (cucumbers, tomatoes). Symptoms: dark spots, corky areas, sunken patches, discoloration. Often cosmetic—cut away affected area, use rest. Blossom-end rot (black decay at blossom end of tomatoes, peppers, squash) looks like disease but is caused by soil moisture/nutrient fluctuations.

Root Problems

"Root rot" = diseases attacking roots. Overwatering inhibits oxygen, causing root decay. If entire plant wilts but soil moisture is adequate, suspect root rot.

First Line of Defense: Good Cultural Practices

• Build healthy soil; use compost.
• Keep plants appropriately watered.
• Rotate crops.
• Don't crowd plants.
• Keep garden tidy.

Because most diseases infect only when foliage is moist, keep foliage as dry as possible.

Nutritional Problems Can Mimic Disease

Improper pH, nutrient deficiencies/excesses, or temperature extremes cause disease-like symptoms. Address with lime, compost, fertilizer, or specific amendments. Foliar feeding with organics (seaweed, fish emulsion) boosts general health and disease resistance.

As you learn to keep plants healthy, you'll have fewer pest/disease problems, and control becomes easier without chemicals.