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The fruit tree cancer is a plant disease, which is caused by an infection with the fungus Neonectria ditissima. As the name suggests, the tree crab prefers fruit trees. Especially the cultivated apple (Malus x domestica), but also different ornamental apple varieties are affected. The pear (Pyrus communis) is also attacked by the fruit tree crayfish, but the disease is less common here and does not cause such great damage in pear orchards and orchards.
In addition to the apple and the pear, many other deciduous trees are susceptible to infection with the fungus. These include alders (Alnus), birch (Betula), white thorns (Crataegus) and beech (Fagus). Also ash (Fraxinus), holly (Ilex), walnut (Juglans) and poplars (Populus) are not spared. After the onset of the disease, the bark dies of the affected trees in several places and the wood tissue is damaged. Since the fungus has a very large host range, the disease is also commonly called tree cancer.

Occurrence of the fruit tree cancer

The tree crab occurs worldwide. However, the disease is a threat above all in rainy regions. In the commercial cultivation of apples, the infestation causes considerable economic damage. To spread, the fungus needs special climatic conditions: for at least half a year, temperatures should be between 11 and 16 degrees Celsius for more than eight hours daily, and it should rain on 30 percent of the days. Especially in the summer months, these conditions are in many regions of Germany - for example, in the rainy North German lowlands.

The wound parasite Neonectria ditissima

As wound parasite, the cause of the fruit tree cancer relies on injuries of the tissue of its host plants. These enable him to enter the plant tissue. Frozen cracks, hailstorms or even injuries from sucking or eating insects can therefore promote an infection. Even mechanical interventions by humans such as the fruit tree incision cause wounds. The leaves and leaves exposed in autumn after leaf fall or even after harvesting are also important entry ports for the fungus.
A new infection by fungal spores (conidia) is possible. Once a spore lands at a potential entry point, it sprouts. As a result, so-called fungal threads form, which grow through the bark and the wood of the tree. Infections are possible throughout the year, however, the main infection time is autumn, because of the leaf fall and the harvest especially many small injuries.

The damage picture in fruit tree cancer

In spring and summer, the first symptoms of the disease appear after two to three weeks, but in the case of an infection in autumn or winter only at the time of the next flowering. Infested shoots initially show small, pale brown, sunken patches on the surface of the bark. They are easily recognizable to the naked eye. The affected areas dry out and turn brownish. In the cross-section one sees a clear transition between healthy and diseased tissue at the affected areas. The infection spreads rapidly until the bark of the affected tree pops open. At older sites of infection usually appear in the following year about 0.5 millimeters large red fruiting bodies.
In severe infestation, the tree tries to close larger infection sites on thicker branches and on the trunk with new bark tissue: it comes to bead-like overflow. However, a spread of the fruit tree cancer can not be prevented. Both young and older branches and branches are affected by the fruit tree cancer. The infestation is particularly serious in young trees, as the disease can prevent the development of a healthy, evenly branched treetop. It may therefore be necessary to replace the entire tree. Older trees are more likely to affect individual branches. Because the affected tissue dies, the nutrient and water supply is interrupted at this point. As a result, the branches and branches overgrow the larger wounds. The low nutrient intake often leads to underdeveloped fruits.
By the way: If the wooden body is exposed, one speaks of the so-called open cancer - the wounds are completely overwhelmed, called the disease as a closed cancer.

The life cycle of the fruit tree cancer pathogen Neonectria ditissima

The life cycle of the fruit tree cancer pathogen Neonectria ditissima

Symptoms of the fruits

In addition to trunk and branches are often infested also the fruits of the fruit trees. Even if a fruit set initially forms, the flowers can die in case of an infection with the fruit tree cancer. From late June to mid-July, the first symptoms of the fruit occur. The mushroom initially spreads slowly over the fruit surface and finally inside the fruit. Once the fruit tree cancer pathogen has penetrated into the core, symptoms of precocious maturity appear.In rare cases, the infection can also emanate directly from the core. Then the fruits show only the symptom of precocity, without external damage. Because the infection is caused by injuries to the skin (such as sucking insects), the damage occurs throughout the fruit surface. In a later illness, the symptoms appear only at harvest or during storage (storage rot).

variety selection

Because the susceptibility of the fruit tree cancer varies according to the apple variety, you should prefer resistant varieties when buying a new apple tree. So far, however, no apple variety has been found with a complete resistance to plant disease. Resistant varieties are believed to have stronger defensive responses to fruit tree cancer. Vulnerable apple tree varieties are 'Cox Orange', 'Gala', 'Braeburn', 'Idared', 'Clark apple' and 'Oldenburg', as well as 'Topaz', 'Boskoop' and 'Pinova'.

Preventive measures

The correct execution of cutting work on fruit trees is particularly important because the resulting injuries are possible entry ports for the pathogen.
The best way to cut your fruit trees in dry weather. Then the likelihood that spores can germinate is very low. Avoid mechanical injuries to their trees such as barking through tree piles or intersecting branches and twigs. When mowing, make sure not to touch the tree bark. Slot branches should be removed as early as possible. Otherwise, moisture accumulates, which in turn benefits the pathogen of the fruit tree crayfish. Check their trees regularly for stress and frost cracks. A timely white finish in the fall prevents such damage.
Too strong shoot growth promotes the infestation with fruit tree cancer. Strongly growing young trees are therefore more susceptible to the fruit tree cancer than older trees. Appropriate plant-cutting and the selection of an apple tree with a weak-growth inlay can regulate shoot growth. In addition, do not over-fertilize your apple trees - especially an excess supply of nitrogen is critical in the first three to five years.
Other factors that favor tree crabs are impermeable soil and waterlogging. Prepare the soil thoroughly before planting by loosening it with compost and sand if necessary and removing deep compaction. Moist locations with high humidity, frequent fog and early frost should also be avoided. In very mild and humid climates, it can even lead to the epidemic-like spread of fruit tree crayfish - especially if two consecutive years favorable infection conditions occur. This problem occurred in the years 1930 to 1932, 1956 to 1958 and 1975 to 1978 in the fruit-growing region Altes Land near Hamburg.

Fighting of fruit tree crayfish

Regular monitoring of their apple trees for fruit tree crabs is very important in order to detect the infection in time and to prevent its spread. If an infestation is found, cut the affected branches immediately back into the healthy wood. Do not put the infected clippings on the compost, but dispose of them in organic waste. Otherwise, the diseased, cut branches pose a renewed danger, as the pathogens are spread by wind and rain, infecting new trees. After work has been done, disinfect all tools used before reusing them.
Which is the best time to cut out sick shoots, is still controversial among experts: While you can see the infected areas well in winter, the resulting wounds remain due to the hibernation of the trees for a long time prone to a new infection. For this reason, you should cut back affected shoots only in mild weather conditions. It is best to remove the diseased parts of the plant at the beginning of flowering.

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