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.
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.
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.
Several different fungi can cause dieback. Each pathogen has its own set of symptoms and host plants:
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.
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.
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:
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.
You can grow a surprising amount of food in your own yard. Ask me how!