Apple powdery mildew
| Apple powdery mildew | ||||||||||||
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Powdery mildew infection on the leaves of a young apple shoot |
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| Podosphaera leucotricha | ||||||||||||
| ( Ellis & Everh. ) Salmon |
The apple powdery mildew is one of the filamentous fungus Podosphaera leucotricha caused disease of plants of the genus Malus . In apple production, the disease causes considerable economic losses due to the weakening of the infected trees, damage to the flowers and thus the fruit set and the deterioration in fruit quality. In addition to the cultivated apple ( Malus x domestica ), other plants of the genus Malus are also attacked by powdery mildew.
Infection cycle
The pathogen Podosphaera leucotricha is an obligatory parasitic Ascomycete from the Erysiphaceae family.
The fungus overwinters as mycelium in infected, dormant flower and leaf buds. When the buds break in spring, the fungus begins to grow and colonizes the developing flowers, young leaves and shoots. Starting from this primary infection, it forms asexual spores ( conidia ) on short spore carriers . The spores are spread by the wind and germinate as soon as they hit susceptible plant tissue, where they then initiate a secondary infection and form mycelial colonies. The young tissue of the leaves that develop between mid-May and mid-June is most susceptible to infection; their susceptibility decreases with increasing age and maturation of the leaf.
For germination, the spores need a relative humidity of more than 70% and temperatures between 10 ° C and 25 ° C, with the optimum being 19 ° C and 22 ° C. Even on dry days, the required higher air humidity can temporarily be reached in the morning or evening, which is sufficient for the infection. In contrast to most other phytopathogenic fungi, they do not require free liquid to germinate; direct moistening of the leaves actually inhibits development.
Since the conditions required for the germination of the spores are best found in the microclimate on the underside of the leaves, the first powdery mildew colonies usually appear there as a white felt-like coating. Thread-like outgrowths develop that spread over the leaf surface, swell and form appressorias . These structures release enzymes that allow the fungus to enter the epidermal cells , where they form haustoria . Haustoria are specialized organs that are formed within living plant cells and serve to absorb nutrients and anchor the fungus.
Throughout the summer, spores and secondary infections occur on the leaves of the infected tree and on other trees that the spores reach with the wind. Above all, the just developing and still immature tissue of just unfolding leaves is susceptible to powdery mildew infections. Increased plant growth in the advanced vegetation period can thus lead to a sudden increase in powdery mildew activity, as the fungus thus has more and more susceptible young plant tissue available for secondary infections.
The buds that form in the course of the year and sleep until next spring are also infected via secondary infections, from which the new primary infections start in the following season and the infection cycle begins again. Since infected buds are less resistant to the effects of frost than healthy ones, low temperatures during winter lead to a reduction in primary infections in spring.
In late summer and early autumn, wintering fruiting bodies (Ascomata) also form within the mushroom mycelium on leaves and shoots . The sexual ascospores contained therein are mostly incapable of development and are of no great importance for primary infections in the coming spring.
Symptoms
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On infected plants, symptoms of powdery mildew appear on leaves and shoots as well as flowers and fruits. The severity and severity of the symptoms vary depending on the apple variety affected, the time and severity of the infection and the weather conditions.
Leaves and shoots
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The fungus overwinters on dormant flower and leaf buds. When the leaf buds sprout in spring, the fungus begins to colonize the young, green tissue. Both primary infections and secondary infections caused by wind-transmitted spores occur on the leaves.
Fungal colonies, which are made up of mycelium and spores, appear as white, felt-like coatings. Towards late summer, when the fungus begins to develop numerous fruiting bodies, the mycelium turns dark. Secondary infections are first visible on the underside of the leaf and appear on the upper side as lightened, chlorotic spots. Infected leaves, curl up or fold the leaf halves upwards and appear wrinkled, later they die and fall off.
Shoot infections arise from a hibernated infection of the terminal bud. They can die off immediately in spring or show significantly reduced growth during the growing season, appear compressed and have a silver-grayish color. Often later there is a total loss of leaves and the shoots die off.
Heavily infested trees are weakened by the infection through the loss of leaf mass and are susceptible to infections with other diseases.
Flowers and fruits
The overwintered spores in the flower buds, which trigger the primary infection, can be activated at a very early stage of flowering when the buds are still closed. The infected flowers open five to eight days after the healthy ones, which ensures that receptive plant tissue, such as the leaves that are just unfolding, the healthy flowers that have already opened, and young fruit sets are available for the fungus.
Flowers infected with powdery mildew appear silver-greyish. The petals are twisted and pale yellow or light green in color. Heavily infected flowers do not open at all or cripple completely and do not set any fruit.
Secondary infections of the flower recipe appear three weeks before to three weeks after flowering. Infected prescription acula shrink and usually do not bear fruit. Some infected flowers also produce fruits, which, however, show discoloration, reticulated russeting and dwarf or crippled growth. The tissue of young fruits is also susceptible to infection.
Economical meaning
The pathogen causing apple powdery mildew has been detected worldwide in all areas where apples are grown. Although other powdery mildew pathogens have also been detected sporadically in Malus species, unlike the pathogen causing apple powdery mildew, these have no economic significance for apple cultivation.
The powdery mildew infestation can lead to significant yield losses. On the one hand, the infection leads to the death of flowers and thus reduced fruit set. Originating from infected flowers or infected fruits in the early stages of youth show a reduced fruit quality due to their crippled and dwarf stature or the reticulate russeting.
As the primary infection of the following year emanates from infected flowers, severe infestation can also negatively affect the harvest of the following season. Without regulating control of the infection, the powdery mildew infestation can build up from year to year.
In addition to the acute loss of yield, powdery mildew infestation leads to a chronic weakening of the plants, which leads to a shortened useful life and thus to a reduced profitability of orchards.
In the past, apple powdery mildew was primarily a disease that primarily appeared on young plants in tree nurseries, while it was of little importance in commercial apple cultivation. Since young trees that do not yet show fruit set complete their shoot growth much later, they also have longer young tissue available for infections. From the 1940s, however, the disease increasingly became a problem in commercial fruit orchards, which is at least partially due to the replacement of sulfur preparations as fungicides with organic fungicides, since the focus of the use of fungicides is primarily on combating apple scab and preventing russeting lay.
When breeding new apple varieties, the main focus was on resistance to damp weather, while powdery mildew can spread well, especially in dry weather.
Today powdery mildew is a permanent problem in almost all growing regions. Severe outbreaks can be caused by inadequate early treatment, inconsistent spraying of fungicides during dry weather periods and resistance to some of the fungicides used.
Combat
Powdery mildew is a chronically recurring problem. A high level of infestation at the end of the season leads to a high proportion of infected buds, which lead to a high primary infection rate in the following spring and at the same time lead to a decrease in yield in the following season due to the damage to the flowers and the fruit set. For this reason, control management for this disease must aim to reduce the primary infection rate and protect the trees from secondary infection.
Variety choice
Since the individual apple varieties have different degrees of susceptibility to powdery mildew, the choice of suitable apple varieties is a very effective preventive measure. There are extremely resistant apple varieties for which chemical control is only necessary if the infection pressure is very high.
Jonafree, Prima and Enterprise, for example, of the varieties grown commercially are powdery mildew-resistant varieties. Although the use of fungicides and thus also the cultivation costs could be reduced considerably by choosing disease-resistant varieties, these varieties have not yet found widespread use, as the choice of variety is primarily determined by the demand from trade and consumers and other apple varieties have better sales opportunities.
The apple varieties Braeburn , Gala , Golden Delicious , Idared , Jonagold , Jonathan and Granny Smith , which are widely grown in commercial fruit growing , are particularly susceptible to powdery mildew, which is why their cultivation requires a high level of use of pesticides.
Cultural measures
The primary infections can be controlled by removing their sources (removing the flower and leaf buds infected in the previous year). These can be recognized by their whitish color and a noticeable spreading of the bud scales and should be removed during winter or early spring.
However, these measures can only be implemented with a great deal of work, especially in larger cultural facilities, so that they are usually not carried out for economic reasons. Such use can only make sense in freshly planted commercial fruit orchards, in which the development of young trees would be greatly weakened by a strong powdery mildew infestation and in which there are few primary infections per tree. Especially in amateur fruit growing, in which usually only a few individual trees are cultivated and chemical treatment with fungicides is not desired, the removal of primary sources of infection, e.g. B. in the context of winter pruning, but be an effective and environmentally friendly measure to control powdery mildew.
Chemical control methods
Secondary infections and the infection of the fruit can be controlled with the application of fungicides. Fungicides are regularly used to control powdery mildew in commercial fruit growing. The fungicides are usually used at 7 to 10 day intervals, beginning before the first flowers have fully developed (so-called tight cluster stage) until the end of the top shoot growth (around the end of June). This ensures that phases of strong leaf development and the post-flowering phase are covered and that fresh leaf growth that is particularly susceptible to infection is not unprotected over a longer period of time. For highly susceptible apple varieties, this can mean that they receive up to 18 fungicide sprays in one season.
Several substances from different groups of substances are permitted for powdery mildew treatment , for example copper and sulfur preparations, sterol inhibitors (e.g. myclobutanil ) and strobilurins (e.g. trifloxystrobin , kresoxime methyl ). All approved substances enable effective powdery mildew control, whereby it is recommended to use several substances of different substance classes for powdery mildew control. If possible, it should be switched or rotated between different fungicides and broad-spectrum fungicides should be used in times of low risk and for less susceptible apple varieties.
In organic farming, besides copper and sulfur preparations, fungicides based on rapeseed oil and sodium bicarbonate (baking powder) are used to control powdery mildew.
The early pre-bloom treatment is important for the success of the entire treatment, as this can prevent the growth of the fungus, which prevents uncontrolled secondary infections later on. Bringing infection under control within the growing season is difficult if injections were missed early in the development cycle.
Longer periods of drought in particular can cause fruit growers to stop spraying fungicides, as other phytopathogenic fungi require a humid climate to develop. However, since the development of powdery mildew is even promoted by dry weather conditions, the fungicide spraying must be continued during periods of good weather for effective powdery mildew control.
Studies have shown that it is more effective to shorten the spraying intervals than to increase the fungicide concentration.
Web links
Individual evidence
- ↑ a b c d e f g h i j k l m n o p q r s t u v w x S. C. Marine, KS Yoder, A. Baudoin: Powdery mildew of apple. In: The Plant Health Instructor. 2010, doi : 10.1094 / PHI-I-2010-1021-01
- ^ A b c d e f William W. Turechek, Juliet E. Caroll, David A Rosenberger: Powdery Mildew of Apple. In: Tree Fruit - Integrated Pest Management, 2004
- ↑ a b c apple powdery mildew. on the homepage of the Kompetenzzentrumd Obstbau-Bodensee, accessed on January 9, 2016
- ↑ Organic cultivation in pome fruit - what is feasible? ( Memento of the original from January 9, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. on the homepage of the Rhineland-Palatinate Rural Area Service Centers, accessed on January 9, 2016
