Bacterial Leaf Scorch

Bacterial leaf scorch is a collection of diseases that can affect a wide variety of plants.

Bacterial leaf scorch (BLS), mistakenly known as bacterial leaf spot, is a tricky disease, because it is actually several diseases caused by different strains of a single pathogen. That pathogen, Xylella fastidiosa, causes different diseases in different plants. And sometimes those bacteria strains overlap their feeding habits, making classification and control difficult.

Leaf scorch diseases

Collectively, these diseases caused by Xylella fastidiosa are known as scorch diseases. Blueberries, mulberries, pears, peppers, plums, tomatoes, and even coffee plants can become infected with bacterial leaf scorch. Scorch diseases that go by a different name include:

• alfalfa - alfalfa dwarf
• almond - almond leaf scorch
• citrus - citrus variegated chlorosis
• grapes - Pierce’s disease
• olive - olive quick decline syndrome
• peach - phony peach disease

Oak, sweetgum, oleander, sycamore, gingko, flowering dogwood, silk trees, jacaranda, maple, and elm can also become infected.

Blueberry leaf scorch (Jerzy Opioła) CC BY-SA 4.0

Lifecycle of Xylella fastidiosa

This particular bacterium is what’s called a fastidious mollicute, which means it must live within a plant’s xylem to be able to reproduce. The diseases they cause occur because they get so overcrowded that they block the flow of water and nutrients through the xylem.

Not all plants are negatively affected by this bacterium. Clover, blackberry, goldenrod, and many grasses can host this pathogen, acting as a way station without suffering any consequences. Unfortunately, when a sharpshooter feeds on one of these plants and then moves to your garden, trouble can start.

Symptoms of bacterial leaf scorch
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Scorch diseases are characterized by the same symptoms you would see as a result of environmental conditions, such as herbicide overspray or too much fertilizer, or other diseases, such as verticillium wilt. Initially, you will see wilting and/or chlorosis. Leaf edges look, well, they look scorched! Then leaves start dropping. Fast. Before you know what happened, the plant dies. Except when it doesn’t, because sometimes it won’t. Scientists are still trying to sort it all out.

Bacterial leaf scorch (Cornell University)

Disease vectors

The bacteria that cause scorch diseases are carried into your garden by leafhoppers and spittlebugs. Actually, it’s in their saliva. Sharpshooters are the biggest carriers of the disease, as far as we know. These insect pests have a wide host range of their own. As sapsuckers, every bite they take infects the plant on which they are feeding. Because their host range is so large, they are spreading diseases to plants that have never been exposed before, so they have no defenses in place.

There are no known chemical treatments for scorch diseases, so controlling the disease carriers is your best bet. If the disease appears, remove the infected plant completely and put it in the trash.
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Mulching and proper irrigation can help your plants protect themselves.