The problem of apricot tree dieback

The problem of apricot tree dieback

For many years, we have annually observed a phenomenon affecting apricot trees. The main and fruit-bearing branches wither and die back. Numerous experts have been investigating this problem and we can today say that the causes of this apricot tree dieback are many and varied. Phytoplasmas, bacterial canker and changing weather conditions causing winter or summer necrosis are all possible causes for the death of apricot trees. Particularly during the summer of last year, it became increasingly evident that during periods of extreme heat a particularly large number of apricot tree branches were affected. The symptoms appeared particularly frequently during the August heatwave. Upon inspection of a number of trees, I observed that both pruned and unpruned trees were affected in equal measure.

Sudden death - apoplexy
Experts refer to this phenomenon as "apoplexy". In humans, the term describes a sudden clot in the brain that causes a stroke. A similar principle is at work in plants, particularly in the case of apricot trees.
Phytoplasmas are bacteria without cell walls
This disease is also known as European stone fruit yellows. Phytoplasmas are bacteria. They do not have cell walls and live in the sieve tubes. In these tubes, sugars, particularly sucrose and amino acids, are transported. These sieve tubes can become infested with phytoplasmas causing the flow to be halted. As a further consequence, entire branch sections can die back or the entire tree can be affected. The fruits are wrinkled. They often ripen too early, do not taste of apricot and drop prematurely. The flesh of some varieties turns brown and is generally rather spongy. The leaves roll up. A typical symptom for phytoplasmas is also that the leaf buds open before the blossoms in the spring.

Bacterial canker
Bacteria are the cause of bacterial canker, which attacks the shoots in the spring and can penetrate into the wood through cuts and frost wounds in the autumn. The disease is accompanied by a gummy brown ooze which appears in places on the twigs and bark. The next spring, some branches will no longer form shoots and die back. Only preventive measures are helpful in this case.
Apricot trees need porous soils which are not waterlogged. Particular care must be taken during replanting and the soil should previously be excavated. If an apricot tree is planted on top of an old apricot tree, this hugely promotes pathogens, including Pseudomonas syringae, the cause of bacterial canker.

Summer and winter necrosis
Summer and winter necrosis damage the vessels and the tree can no longer be supplied with water and assimilates. In western Austria, for example, apricot trees are traditionally planted against the houses using trellises as a support. This provides extra protection against rainfall and the trees are less susceptible to shotholes, monilia and other fungal diseases. South-facing trees in particular are subject to intense solar radiation. In the winter, stresses build up in the trunk and branches owing to the alternating cold nights and sunny days. This often leads to damage to the bark and wood, whereby the vessels are particularly affected. As a result, the branches and trees die off during the course of the vegetation period. However, the summer should not be underestimated either. If the trunk reaches temperatures exceeding 45 degrees Celsius, it is highly probable that the tree will suffer damage.
In order to counteract these effects to some extent, the trunks and thicker branches should be painted white, particularly during the first five to ten years of developmental growth. With good products, painting only needs to be carried out every five to seven years. Over this period, the tree can adapt well to the prevailing conditions. If the trees are planted facing south, I recommend always painting the trunks white. This is an effective way to prevent vessel damage and ice glaze damage to the trees.

The dieback has certainly also been influenced by the fact that the old, very resistant substrates and varieties have become less prevalent. We are only beginning to gather experiences regarding the new substrates and varieties. The less and more susceptible types will no doubt become distinguishable to a greater extent over time.
Regardless, we will have to deal with the phenomenon of apricot apoplexy (European stone fruit yellows) for a long time to come and, in my opinion, it will never be fully resolved. New substrates and varieties will, however, help to alleviate the problem to some extent in the future.

Manfred Putz
VIKING garden expert

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