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Diseases

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Hybrid poplars are subject to attack by many diseases. Different clones vary widely in their resistance to damaging agents. Effective remedial action, even if possible, is often too expensive to consider. The key is therefore to prevent or minimize damage. A plantation's health and vigour are crucial factors in warding off infection.

There will be a number of occasions when you will be assessing your plantation. Different types of damages are most visible at different times of the year.

- Canker diseases are best assessed in the winter when there are no leaves on the trees
- Leaf rusts must be assessed when the trees are in full leaf and towards the end of the growing season.

Melampsora leaf rust

(Melampsora spp.) - The severity of infection depends on clonal susceptibility and the timing of the first infection. Infection becomes increasingly severe through the growing season and can result in decreased growth and vigour.

Several different species of Melampsora are responsible for the same type of damage. The consequences of the attack is early falling of the leaves, reduced photosynthesis and therefore reduced increment, physiological weakening of the plants, incomplete lignification of sprouts and the development of a predisposition to the attack of Dothichiza populea.

Those identified in Europe on black poplars:

- Melampsora allii-populina

- Melampsora larici-populina

Marssonina leaf spot

(Marssonina spp.) - Leaf spotting, yellowing and premature defoliation. The foliage, shoots and small branches are affected, the result being decreased photosynthesis. The effects are cumulative. Heavy or repeated attacks reduce carbohydrate and nutrient reserves causing eventual thinning and dieback of the lower crown. Repeated attacks can cause death.

A delay in hardening of green shoots makes trees susceptible to early fall frosts. Low vigour and frost damage also predispose trees to Cytospora canker.

Symptoms : leaves: very small, red-brown to black spots first appear on leaves in the lower crown. The spots have dark margins with lighter centres and are surrounded by yellowing leaf tissue. Many spots may appear on a single leaf and even join together.

Dothichiza populea 

 (Poplar Bark Canker)

Cortical tissue disease - Dothichiza presents one of the key problems in the process of poplar production. It is a phenomenon which has limited or completely arrested work on raising new plantations of poplars in certain periods. The attacks showed that Euramerican poplar do not possess any defence mechanisms capable of preventing or weakening the invasion of the pathogen in its progress.

 After the epiphytotic occurence the infection potential gradually decreases to reach its pessimum when à relatively quiet period ensues.

In estimating the infection time in the course of a year, the results of experiments with artificial contamination with pycnospores were taken into consideration. The results of these experiments indicate that the degree of the infections realized was subject to certain oscillations. It depended on many factors, which were often difficult to control.

Poplar infection by Dothichiza populea is possible all the year round, depending on the climatic and edaphic conditions, the vitality of the plants, etc. The degree of infection depends most of all on the quantity of inoculum in nature. Generally, the mass infections occur in the period April-June and in the period October-November. The first maximum is more intensive; it coincides with the period of the greatest fruit-bearing capacity of the pathogen and of th highest level of the infection potential.

Septoria leaf spot

(Septoria musiva) - Severe spotting and defoliation. Spring : infection first appears as leaf spots in the lower crown within 3 to 4 weeks of budbreak. The spot begin as pale yellow areas on the leaves. They can be small and numerous on some clones; large and few on others. The spots can be circular, but are usually irregular and angular. They turn grey or brown with dark borders and light, whitish to ash-grey centres. Small black bumps may be present in spot centres. Summer : the spotting spreads through the crown as the summer progresses. On some clones the spots join together to form large blotches. Premature leaf drop can result from a heavy infection.

Septoria canker

(Septoria musiva) - Cankering and death of shoots, branches and mainstems. Infection can result in decreased growth, vigour and wood quality. Perennial cankers cause severe deformation and wind breakage of mainstem.

Septoria canker do not appear on native poplars planted within their natural range. Hybrid poplars can be highly susceptible, although not all clones susceptible to the leaf spot are susceptible to canker.

The fungus overwinters on leaf litter. In the spring spores are released which infect leaves and stems. Stems can be infected through the branch nodes, stipules, lenticels and small wounds. Infected areas become blackened and slightly depressed or sunken, and sometimes pale white to yellow in the centre.

Cytospora canker

(Cystospora chrysosperma) - Dieback in crown, death of whole tree. Cytospora is a weak parasite and only attacks weakened or stressed trees. Small branches can be girdled, causing dieback. Cankers can cause girdling of large branches and trunks, with death occuring after several years. With an improvement in tree vigour, trees can recover by growing over the canker.

Cytospora is usually present on the bark and becomes parasitic if the tree is weakened. Spores can infect wounds or dead branch tips. Infection is strongly associated with frost-damaged shoots. Early symptoms are brown-black blisters on the bark. The underlying inner bark blackens and emits an odour. The sapwood becomes watery and red-brown. The hearthwood may be discoloured.

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