Blight

Blight is not a disease. It is a symptom of plant disease.
 
One day, your plant looks lush and healthy. The next day, several leaves are yellow and then turn brown. Stems, flowers, and branches also turn brown and then die. What can cause this sudden, severe damage? It might be blight.

Fire blight on apple (Kate Russell)

Blight strikes quickly

Blight is what several different plant diseases look like. The initial symptoms appear suddenly and spread very quickly. Leaves, fruit, stems, and flowers can all be affected. The first symptom is tiny lesions on leaf tissue where the bacteria or fungi first enter the plant, usually through small tears. The pathogens may also enter through vulnerable new growth. What starts out looking like leaf spot soon covers entire regions of a plant. Leaves and flowers suddenly lose all their green and turn pale yellow (chlorosis), followed by spotting, browning, withering, and dying.

Tomato stem infected with bacterial blight (Scot Nelson) Public Domain

Blight diseases

Blight is common in tomatoes and potatoes (nightshade family) and in apples and pears (pome fruits). Ornamental plants are not exempt. Here is a list of the most common blight diseases:

• Ascochyta blight --Ascochyta rabiei attacks garbanzo beans.
• Ashy stem blight (charcoal rot)--Macrophomina phaseoli attacks cucurbits.
• Bacterial blight--Pseudomonas attacks peach, nectarine, and kiwifruit.
• Bacterial leaf blight--Xanthomonas campestris pv. carotae attack carrots
• Botrytis blight (gray mold)--transforms fruit into gray fuzz.
• Cane blight--Leptosphaeria coniothyrium fungus attacks raspberries! [Evil fungi!]
• Chestnut blight--Cryphonectria heterostrophus destroyed chestnut trees across the USA.
• Common bacterial blight--Xanthomonas campestris pv. phaseoli attack legumes.
• Early blight—a fungal disease  of tomato and potato caused by Alternaria fungi.
• Fire blight--Erwinia amylovora bacteria attack apples, pears, and some cane fruits.
• Halo blight--Pseudomonas syringae pv. phaseolicola attacks legumes.
• Late blight (potato blight)--the Great Irish Famine disease of potatoes caused by Phytophthora infestans.
• Rhizoctonia blight--Rhizoctonia solani attacks escarole, legumes, and turnips.
• Sclerotinia blight--Sclerotinia sclerotiorum attacks cole crops, legumes, kiwifruit, sunflowers, sweet potatoes, and tomatoes.
• Southern blight (white mold)--Sclerotium rolfsii kills more southern plants than any other pathogen.
• Spur blight--Didymella applanata attack raspberries.
• Stem blight--several fungi attack the stems of fruit trees, blueberries, cucurbits, and soybeans.

Late blight on potato leaf (Howard F. Schwartz) CC BY 3.0

How to control blight

Since blight enters plants through wounds and tender new growth, there are three ways you can reduce the likelihood of
 occurring in your garden:

• Protect plants from physical damage.
• Avoid pruning too heavily.
• Use the least amount of fertilizer necessary.

These are especially important when temperatures of 75° to 85°F occur along with rain. Pruning and feeding stimulate new growth that cannot defend itself. Also, do not irrigate trees while they are in flower. Monitor plants closely after these conditions have occurred. Remove and destroy any infected tissue, cutting several inches below the infected tissue. Sanitize your pruners with a household cleaner between each cut to reduce the spread of the disease. Fixed copper sprays may provide some protection.