Dieback

It is normal and healthy for the pruned tip of a twig to dry up and seal itself off from pests and disease. When the tip of a twig dies and that death keeps moving inward, there’s a problem. This creeping death is called dieback. Dieback can be caused by environmental conditions, insect feeding, or disease.

Plum twig dieback caused by Botrytis (UCANR)

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Physical dieback

It is not understood why, but delayed leafing out seems to be associated with dieback. This may simply be because dieback may be caused by the same environmental conditions that cause delayed leafing out. We really don’t know. These conditions include winter drought, extreme cold, or insufficient chilling hours. This form of dieback is common in blackberries and raspberries. Other causes of dieback include poor irrigation and hot, dry winds, potassium or zinc deficiencies, phosphorus toxicity, and insect feeding by shot hole borers, black scale, wooly aphids, and mealybugs. Also, when the raspberry horntail, a tiny wasp, lays its eggs in a raspberry or blackberry cane, dieback can occur.

Hot wind and poor irrigation can cause twig dieback (UCANR)

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Dieback by nematodes

Nematodes are microscopic, eel-like roundworms that live in the soil. Some nematodes are beneficial predators, and some are plant-eating, disease-carrying parasites. Nematode feeding can cause reduced plant vigor, wilting, smaller fruits and leaves, and twig dieback.


Fungal dieback

Several different fungi can cause dieback. Each pathogen has its own set of symptoms and host plants:

• Anthracnose - spotted leaves, stem and twig cankers, stunting and chlorosis; berries, tomatoes, avocados, almonds, lettuce, citrus, cucurbits, and spinach
• Botryosphaeria (aka ‘Bot canker’) - sudden wilting and death to new leaves, dead spurs, cordons, canes; circular to oblong cankers; grapes
• Eutypa - sudden wilting and twig death in late spring or summer; leaves remain attached; discolored bark with amber-colored gumming
• Fusarium - tiny beetle entry holes show frass and a sugary discharge; apricot and other stone fruits
• Leptosphaeria (aka ‘cane blight’) - purplish-black lesions girdle canes; raspberries, blackberries, strawberries, stone fruits, currants, and blueberries
• Phomopsis - wedge-shaped wood cankers, dead spurs; grapes, avocado
• Phytophthora  (var. ‘Branch canker and dieback’) - white, oozing patch; avocado

Branch canker and dieback of avocado (UCANR)

Viral dieback

Lettuce is susceptible to a viral dieback caused by the lettuce necrotic stunt virus. Formally called ‘tomato bushy stunt virus’, this pathogen causes stunting, leathery, dark inner leaves, and rotted areas on outer leaves.


Bacterial dieback

Apple, citrus, stone fruits, and pear are susceptible to bacterial blast, blight, and cankers, all of which are caused by Pseudomonas syringae. This pathogen kills flower clusters and nearby leaves, along with twig tips. Fireblight is another bacterial infection that causes twig dieback. This disease is easy to spot because the dead twigs curl themselves into a shepherd’s crook shape. Watch for fireblight in June. Huanglongbing, a deadly disease of citrus, includes twig dieback as one of its early symptoms.

Fireblight of apple (Kate Russell)

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How to prevent dieback

Plants that are healthy can often protect themselves from dieback. These tips can help reduce the risk of dieback in your garden and landscape:

• Ensure that plants receive enough irrigation in late fall and winter, to help them survive colder temperatures.
• Allow infected areas to go fallow.
• Use crop rotation to break the disease triangle.
• Prevent introducing these pathogens to your garden by quarantining new plants.
• Prevent water from traveling from infected plants to healthy plants.

Clearly, there are many causes of dieback. Taking the time to determine the reason for twigs and stems dying back can help you find an effective treatment.