Gardening glossary
Blight
Blight refers to fast-moving plant diseases that kill foliage, stems, or flowers in sweeping patches rather than as small isolated lesions. The term covers several distinct pathogens and host combinations, and gardeners should learn the three most common.
Early blight (Alternaria solani) affects tomatoes and potatoes and starts as dark brown spots with concentric rings, often called target spots, on the oldest leaves. It spreads upward through the canopy in warm humid weather. Late blight (Phytophthora infestans), the oomycete responsible for the Irish potato famine, produces irregular grey-green water-soaked patches that turn brown and may show white fuzzy growth on leaf undersides in cool wet weather. Late blight can destroy a tomato or potato crop within days. Fire blight (Erwinia amylovora) is a bacterial disease of apples, pears, and related rosaceous plants that causes shoots to wilt suddenly into a characteristic shepherd's crook and turn black as if scorched by fire.
Other blights include botrytis blight (grey mold) on soft tissue in cool damp conditions, southern blight (Sclerotium rolfsii) on stem bases in warm climates, and bacterial halo blight on beans.
Prevention is more effective than treatment. Choose resistant cultivars, rotate vegetable crops on a three-year cycle, mulch to prevent soil splash, space plants for airflow, water at the soil line in the morning, and remove and destroy infected debris each autumn. For early and late blight on tomatoes and potatoes, copper-based fungicides and chlorothalonil applied preventively at 7 to 10 day intervals slow the spread but cannot reverse established infection. Fire blight requires pruning affected wood at least 30 cm below the visible canker, disinfecting tools between cuts with 70 percent alcohol, and applying streptomycin or copper at bloom in high-pressure orchards.