Fruit tree cancer
| Fruit tree cancer | ||||||||||||
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Tree cancer on the branch of an apple tree |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Neonectria ditissima | ||||||||||||
| ( Bres. ) Rossman & Samuels |
The Canker is a by infection with the Pustelpilz Neonectria ditissima caused plant disease. The infection leads to the death of bark and wood tissue, the infected tree tries to overwhelm the resulting wound by the formation of wound tissue, which can lead to voluminous callus growths. Although the resulting tumors are not cancer in the medical sense, they are called tree cancer . The fruit tree cancer is of particular economic importance in the cultivation of apples , but the host range also includes a large number of other deciduous trees.
Pathogen
Taxonomy
The fruit tree cancer is caused by infections with the fungus Neonectria ditissima from the genus of the pustular fungi . The anamorphic of Neonectria ditissima is called Cylindrocarpon heteronemum ( Berk. & Br.).
The species was taxonomically revised several times. When it was first described in 1865, the fungus was taxonomically classified as Nectria ditissima ( Tul. & C. Tul. ) And was initially described as a causative agent of cancer in forest trees, especially beech. In 1901 the species was separated from Nectria ditissima and led independently under the name Nectria galligena ( Bres. ). On the basis of the host spectrum and microscopic examinations, the pathogen causing the fruit tree cancer was named Neonectria galligena ((Bres.) Rossman & Samuels ) from 1995 . guided. From the current point of view, Neonectria ditissima and Neonectria galligena are considered to be a species that goes under the name Neonectria ditissima .
Host spectrum
Fruit tree cancer is a widespread disease , especially in cultivated apples ( Malus x domestica ), and an economically important disease in commercial fruit growing. In addition to the cultivated apple, other species of the Malus genus are also attacked. The cultivated pear ( Pyrus communis ) can also develop fruit tree cancer, although the disease occurs here rather rarely and is only of subordinate economic relevance. In addition to apple and pear trees, numerous other deciduous trees of various genera are also susceptible to infection with the fungus. These include alder (Alnus), birch (Betula), hawthorn (Crataegus), beech (Fagus), ash (Fraxinus), holly (Ilex), walnuts (Juglans), poplar (Populus), oak (Quercus), currant (Ribes) ), Willow (Salix), linden (Tilia) and elm (Ulmus). Because of this wide range of hosts, the disease is often referred to as tree cancer.
distribution
The disease occurs worldwide and is an economically significant problem when growing apples, especially in regions with high rainfall. In order to cause serious symptoms of disease in trees, the fungus needs certain climatic conditions. For example, temperatures between 11 ° C and 16 ° C must prevail for more than 8 hours a day for at least five months per year and there must be precipitation on at least 30% of the days. In the northern hemisphere, favorable climatic conditions for the infection can be found almost everywhere during the summer months, with the areas north of the 52nd parallel north being particularly affected.
Infection cycle
Fruit tree cancer is a wound parasite that depends on damage to the bark as a portal of entry into the tissue of its host plants for infection. These injuries can occur naturally (e.g. frost cracks, hailstorms, injuries from sucking or eating insects) or through mechanical intervention by humans (e.g. from pruning a fruit tree, friction between the connection). Above all, the not yet completely scarred fruit and leaf attachment points that arise in autumn after harvest or leaf fall are important entry points for the infection.
New infections are possible both through conidia and ascospores . As soon as a spore hits a susceptible wound, it germinates there. To do this, it needs damp weather. Fungal threads then form that grow through the bark and wood.
Several weeks after the initial infection, the first superficial layers of conidia appear on the dead bark. They represent the anamorphic Cylindrocarpon heteronemum . Via the conidia formed and released in them in summer, the infection spreads passively through splashes of rain within the tree. Two types of conidia are formed: macro and micro conidia. The Macrokonidien are straight or only slightly curved with rounded ends. Macroconidia of natural origin are usually five-fold septated, while artificial cultivation on agar media, on the other hand, mainly develops simple and three-fold septate forms. The microconidia are short and cylindrical or elliptical in shape. They are undivided or simply septate. The macroconidia are highly infectious, but the role of the microconidia in the spread of the fungus is unclear.
Scattered, spherical, reddish colored fruiting bodies ( perithecia ), which represent the sexual stage of the fungus, usually do not form until the year after the infection . The formation of the fruiting bodies takes place mainly in late summer and autumn in periods of damp weather and cool temperatures. Under favorable, mild climatic conditions, it can last all winter until spring. The elliptical, simply septate and slightly drawn in ascospores on the septum are released from them. The ascospores are ejected explosively from the asci , which are then sometimes spread several hundred meters wide with the wind, causing the fungus to spread within the tree population.
The main distribution path of the fungus depends strongly on the climate of the respective location. In arid climates , such as parts of the USA and Chile, the fungus rarely forms fruiting bodies, so that the infection spreads mainly via the conidia. In the maritime climate , on the other hand, the sexual ascospores play an important role in the spread of the fungus, which can be transported here by air currents over several hundred meters up to a few kilometers. If the fruiting bodies ripen early enough in autumn that large amounts of ascospores are released at the time of the leaf fall, the disease can spread epidemically in fruit-growing areas under humid and mild climate conditions that are favorable for the infection. In order to be able to trigger severe epidemics, such as those observed in the Altes Land in the years 1930–1932, 1956–1958 and 1973–1975, these favorable infection conditions must occur in at least two consecutive years.
Conidia, on the other hand, are released all year round as long as the climate is humid enough and the temperature is above freezing point. In Central Europe, the time of the strongest conidia production begins with the fruit blossom and lasts until November.
Clinical picture
Symptoms on the plant
Both young and older wooden parts are damaged by Neonectria ditissima. In young trees in particular, an infestation with fruit tree cancer can be dangerous if the lesions on the main branches prevent a healthy crown from building up. Heavy infestation can make it necessary to clear the entire tree. Older trees are more likely to be damaged by branches.
After the infection, a small, pale brown discolored, sunken spot can be seen on the affected shoots, usually in close proximity to one eye. While the first symptoms appear after an infection in spring and summer after two to three weeks, with infections in autumn and winter these usually only appear at the time of the next flowering. The plant tissue below the lesion becomes necrotic, dries out, and turns brown in color. In the section of the infested site, a sharply delineated transition from healthy to diseased tissue can be clearly seen. The focus of the infection increases rapidly until the bark at the infected area bursts. These symptoms of a fresh infection are more pronounced in the spring after the fruit tree blossoms, but can be observed all year round as long as temperatures are above freezing point.
A few weeks after the infection, whitish or pale yellow, superficial conidia beds appear in the area of the bark that has died from the cancer infection and are easily visible to the naked eye. At older cancer sites, scattered red, spherical fruiting bodies with a diameter of approx. 0.5 mm and which can be seen with the naked eye usually do not appear until the following year, in the case of infections that occurred in early spring also in late autumn of the same year.
Since the dead tissue interrupts the flow of nutrients and water, branches above the cancerous wound wither. On younger shoots or on young trees, the destruction by the fungus can completely encompass one of the leading branches or the trunk, causing it to die. Often this leads to the clearing of the tree because a stable scaffolding is no longer possible.
Larger areas of infestation can develop on thicker branches and on the trunk of older trees. The tree tries to close the resulting open wounds by forming new tissue, which creates bulge-like overburden. Because of this lumpy-looking collar, the disease was also called cancer . However, the overburden does not prevent the fungus from spreading within the tissue.
Thicker branches can often live with an infection for years without dying off. If the damage does not include the branch or twig completely, the branches and twigs above the cancer site can continue to be supplied with nutrients and water via the remaining bark sections so that they do not die. However, since the supply of nutrients is reduced, the fruits in these areas often remain underdeveloped.
Symptoms on the fruit
In addition to the wood, the fruits can also be attacked by the fungus. The flower can die off after infection or it can initially develop into a fruit set. The tissue in the area of the calyx dies on these fruits (pre-harvest calyx rot). The first symptoms typically appear from late June to mid-July, when the fruit is about half its final size. The mushroom mycelium slowly spreads over the surface of the fruit and into the interior of the fruit. Once the infection reaches the core, the fruit begins to show symptoms of precociousness. More rarely, the infection can start directly from the core; then the fruits only show the symptoms of precociousness without any external damage to the fruit being evident. The calyx rot caused by the Neonectria ditissima is similar in the initial stage to the symptoms of the disease caused by an infection with various Botrytis species or Gloeosporium , but can later be identified by the appearance of whitish spore masses.
If the fruit is infected later, symptoms do not appear until harvest or during storage (storage rot ). The infection usually occurs within the last two to four weeks before harvest. Since the infection is caused by injuries to the skin of the fruit (e.g. from sucking insects or a scab infection) or the lenticels, symptoms can appear anywhere on the surface of the fruit. Typically, the infected tissue is sharply separated from the healthy pulp and can be neatly separated from it using a spoon with little pressure. A short immersion bath in hot water, which activates the natural defense mechanisms of the fruit through a heat shock, can delay the onset of symptoms on the bed.
Combat
Preventive measures are particularly important in the fight against fruit tree cancer. The disease can at least be contained through well-thought-out management of commercial fruit orchards. If the disease has already broken out, it is important to trim the affected areas, otherwise the fungus will spread further.
Variety choice
The different apple varieties show a very different, variety-dependent susceptibility to the disease. The choice of variety is therefore of great importance for prophylaxis when replanting an orchard.
In general, the varietal differences in susceptibility to infection seem to be based on partial field resistance, the genetic basis of which is so far unknown. So far, no apple variety with a complete, monogenically anchored resistance to fruit tree cancer has been found. It is believed that the higher resistance of some apple varieties is based on strong defense reactions of the plant, which susceptible varieties do not show.
Susceptibility of various apple varieties common in commercial fruit growing to fruit tree cancer:
| very high | high | medium | low | very low |
|---|---|---|---|---|
| Kanzi | Braeburn | Bramley | Topaz | Santana |
| Rubens | Cameo | Golden Delicious | Nicer from Boskoop | Pinova |
| Cox orange | Holsteiner Cox | Elstar | ||
| Discovery | Ingrid Marie | Jonagold | ||
| Gala | ||||
| Gloster | ||||
| Gold parmane | ||||
| James Grieve | ||||
| Wavy |
Cultural measures
Since pruning the fruit tree causes injuries that can serve as a gateway for the fungus, the infection rate must be reduced through the correct planning of pruning measures. In orchards with a high infection pressure, cutting should only be carried out in dry weather so that the spores that have been brought onto the wounds cannot germinate.
In addition to the cutting measures, any further mechanical injuries (e.g. chafing due to connections, injuries during mowing measures) should be avoided. Slit branches should be completely removed at an early stage, as the roughened bark in the area of the fork and the local tree anatomy can easily lead to retention of moisture and thus to favorable infection conditions for the fungus. In addition, small growth and stress cracks often arise in the bark, which can serve as a gateway for the fungus.
Strong shoot growth promotes infestation. For example, young trees, which have a higher proportion of growing shoots, are more susceptible to the disease than old trees, in which the majority of the plant mass consists of older shoots without length growth. For this reason, trees should never be over-fertilized. Above all, an excessive introduction of nitrogen during the first 3 to 5 years of standing should be avoided. The shoot growth can also be regulated by a balanced pruning of the plants.
Since impermeable soil and waterlogging are also conducive to the disease, such locations should be avoided. The same applies to locations with high humidity, frequent fog and early frost.
Since the uncarried sores that occur when the fruits are harvested are important entry points for the infection, harvesting should only be carried out in dry weather.
Tree surgical measures
Cutting out the cancerous areas on an infected branch is an important measure to prevent the infection from spreading within the tree as well as within the stand. The cancer site should be removed as completely as possible, the cut should go into the surrounding healthy wood. In order not to transfer the infection from one tree to the next, it is important to carefully disinfect the cutting tools used before moving to the next tree.
The best time to prune is controversial among fruit growing scientists. While cancer sites in the wood are easy to identify in winter, the wounds that have developed remain susceptible to re-infection for longer due to the trees being hibernated. For this reason, the cancerous areas should only be cut out outside of the growing season during long periods of good weather. In addition, it should be done before the winter fruit tree pruning, as it also causes susceptible wounds.
Under the north-western European conditions, the removal of the cancerous sites should only start with the beginning of flowering. It is important to regularly check infested trees for new sources of infection throughout the vegetation period and to remove them immediately so that no conidia can mature.
Ascospores can be released from cut infected twigs and branches for up to two years, which are then spread with the wind. The tree material removed during the pruning measures should therefore be removed from the area of the orchard and destroyed.
With a preventive treatment of the wounds caused by the removal of the cancerous areas with a fungicide , an attempt can be made to prevent the infection from flaring up again if all of the infected tissue cannot be removed. In the past, it was recommended that the cut-out sites be treated with agents containing copper or mercury , but these are no longer approved. The benzimidazoles have lost their effectiveness due to the formation of resistance.
Applications of fungicides
The infection with Neonectria ditissima can be combated by treatment with fungicides . In commercial fruit growing, the fruit tree canker is also recorded by the fungicide sprayings carried out regularly during the vegetation period against scab and mildew . Since these measures are not continued until the leaf falls, the critical time for the spread of the fruit tree cancer infection, it makes sense to carry out another fungicide treatment at this point in time. Copper compounds show a good effectiveness against Neonectria ditissima and also have a certain depot effect, so that the trees are protected over a longer period of time.
The fruit tree cancer treatment recommended for Germany includes spraying with Captan at the end of the harvest, followed by a copper spray at the time of the 30% leaf fall and shortly after the leaf has fallen. Depending on the weather in winter, a third copper spray is recommended if there are longer periods of rain during winter or if the trees have been severely wounded by frost cracks.
Web links
Individual evidence
- ↑ David Brayford, Barry M. Honda, Feky R. Mantiri, Gary J. Samuels: Neonectria and Cylindrocarpon: the Nectria mammoidea group and species lacking microconidia . In: Mycologia . tape 96 , no. 3 , 2004, p. 572-597 ( PDF ).
- ↑ a b c d e f g h i j k l m n o p q r s t u v w R. WS Weber: Biology and control of the apple canker fungus Neonectria ditissima (syn. N. galligena) from a Northwestern European perspective . In: Commercial fruit growing . tape 56 , no. 3 , July 22, 2014, p. 95-107 , doi : 10.1007 / s10341-014-0210-x .
- ↑ a b c d e f g h i fruit tree shrimp. on the homepage of the Competence Center Obstbau-Bodensee, accessed on March 21, 2016.
- ↑ a b Cylindrocarpon rot of apples and pears caused by Nectria galligena Bresad. In: Anna L. Snowdon: Color Atlas of Post-Harvest Damage to Fruit and Vegetables. Volume 1: General Basics - Fruit. Ferdinand Enke Verlag, Stuttgart 1995, pp. 184-185