Key Features
- Leaf spots and/or yellowing
- Black streaking on stem
- Wilt, leaf drop & plant death
Symptoms
Bacterial leaf spots are generally small, angular lesions that are limited by leaf veins. New spots are "wet" looking and semitransparent. Older spots can be yellow, green, or brown and may be surrounded by a yellow or reddish-purple ring. Mature spots turn brown or black and may grow together giving the appearance of large or spreading lesions. It may be impossible to distinguish between bacterial leaf spots and immature fungal leaf spots (those without fruiting bodies). Samples of these plants can be sent to a plant disease diagnostic lab for analysis. Bacterial blights begin as bacterial leaf or stem spots. Infected leaves turn reddish-brown or black and either drop or remain on the branch after they die. Dark, sunken stem spots (cankers) work their way back toward the crown. Leaves beyond the canker begin to wilt while the tip of the affected branch curls and droops like a shepherd's crook. Cloudy droplets of sticky fluid may accumulate on leaf tips, leaf surfaces, stems, and even infected flowers. Infected stems, cut just below the canker, can be suspended in a glass of water. If bacteria are present in high numbers, they will stream from the stem in a white cloud. Cutting through an infected stem, above or below the canker, may reveal dark staining of the tissue just beneath the outer portion and/or toward the center of the stem.
Biology
The bacteria overwinters in infected stems, plant debris, perennial weeds, and soil, as well as on the surface of healthy plants. Insects, pruning tools, rain, and wind spread bacteria that gain entry through wounds, stomates, or lenticels (natural and artificial breaks in the leaf or stem surface). Natural wounds are created when petals drop and pollinators can transmit ooze from infected to healthy flowers or the newest growth in the spring during hot, rainy weather. Wounded leaves, stems, or fruit can be infected at almost any time throughout the growing season. Once inside the plant, bacterial colonization is slowed by leaf veins, but the bacteria can travel through leaf veins if provided an entry point. Bacterial populations continue to grow, even when space has run out. Excess bacterial cells and waste are forced out of the plant as ooze. Bacteria contained in ooze droplets can be splashed or stuck to insects which transport the bacteria to new potential infection sites.
Management Recommendations
Avoid stress and injury resulting from frost damage, wounding, and improper fertilizing. These can increase plants' susceptibility to bacterial infection. Ensure that each plant receives adequate, but not excessive light and water. Shape or pinch during dry weather to minimize the chance of spreading the pathogen. Prune out and destroy diseased or injured shoots. Where possible, cut 10 to 12 inches below the visible infection. Be sure to sterilize pruning equipment between cuts with 10% bleach or 70% rubbing alcohol to prevent spreading the bacteria. Avoid overhead irrigation to minimize splashing the bacteria to healthy leaves and other plants. Thin the canopy and clear a path around the canopy to increase air circulation to decrease the bacteria's ability to reproduce and spread. Some bacterial and other diseases are spread by feeding insects, like aphids. Controlling insect pests may reduce disease pressure. Copper-based fungicides can prevent new bacterial infections. Copper-based pesticides will not cure or stop the spread of existing infections, especially cankers. Early detection and careful pruning or even removal of the entire plant is the best defense for existing blight infections.
Effective Pesticides
Active Ingredients include: Copper salts
Resources
