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Two diseases, many pathogens
Mildew is one of the most common fungal diseases in the garden. Behind the German name hide a variety of fungal genera that infect very different plants and some are very specific to the host. On the basis of their symptoms, experts divide mildew fungi into two groups - the so-called powdery mildew and downy mildew. Often the plant lover is difficult to distinguish - but it is important, because the two fungal groups have different demands. The downy mildew loves moisture, while the powdery mildew is a so-called fair-weather mushroom. Also in terms of their wintering, there are significant differences.
Powdery mildew manifests itself as a large white coating on the leaf tops. Here is a cucumber infested
Powdery mildew occurs mainly on roses and other ornamental plants such as autumn aster, larkspur, phlox, Indian nettle and sage, as well as vegetables such as cucumbers, zucchini, carrots and salsify. On fruit trees, apple powdery mildew and American gooseberry powdery mildew are the most important powdery mildew fungi. Like the powdery mildew, most other species not only infest the leaves, but also all other green parts of the plant.
The life cycle of the rose mildew: The fungus overwinters on the buds and forms here on asexual way on so-called conidiophores spores, which infect the new leaves from about the middle of April. In the summer mildew dew of the leaves the sexual reproduction takes place in the so-called secondary cycle. It produces fruiting bodies, which release their spores in late summer and autumn
The powdery mildew always sits as a wipeable coating on the leaf tops. He is white at first, later mostly dirty brownish to grayish. As the infection progresses, the leaves turn brown and eventually dry from the edge. The various pathogens that cause powdery mildew belong to the group of ascomycetes. Their spores usually reach the leaves of the endangered plants through the wind or through splashing water. They germinate there and penetrate into the upper cell layer of the leaves, where they form a mycelium. For germination, they need at least ten to twelve degrees air temperature and 70 percent humidity. Typical for most powdery mildew fungi are asexual and a sexual reproduction cycle (see drawing above). The first spores in spring, the so-called conidia, arise through asexual reproduction. Only in summer do fruiting bodies with so-called ascospores, which arise through sexual reproduction, form on the powdery mildew.
Downy mildew, here on a grapevine, can be recognized by yellow spots on the leaves
Downy mildew usually forms gray or gray-violet mushroom grass on the underside of the leaves. On the upper side of the leaf, only lucencies or yellowish spots are visible and the leaf dies over time. The fungi infest the plant by their spores swimming in the water film on the wet leaves and penetrate depending on the plant below or above through the stomata. Downy mildew fungi infest radishes, radish, horseradish, salsify, lettuce, peas, lamb's lettuce, cabbage, spinach, onions and grapevines. The downy mildew winters in contrast to the powdery mildew not on the plants, but in fall foliage or in harvest residues. The spores formed here in spring infect the leaves again when the leaf moisture is sufficient.
In terms of resistance to powdery mildew and downy mildew, plant breeding has made great progress in recent years, especially in the case of roses and crops. For example, the so-called ADR roses are now offering a steadily growing range of quite mildew-proof varieties with a wide variety of growth forms and flower colors. Among the apple varieties are the so-called re-breeds such as 'Regine' and 'Rewena' as resistant, also the variety 'Topaz'. From spinach and lamb's lettuce, there are also many varieties that are hardly affected by downy mildew.
The most susceptible perennials also depend on the variety selection. For example, asters that are less susceptible to powdery mildew are now being offered by asters, indians and phlox. Really resistant breeds, however, do not exist.
Since the pathogens of both types of mildew are omnipresent, one should take precautionary measures in plants that are particularly susceptible or already infected in the previous year. Maintain sufficient planting distance between the crops, especially in the greenhouse, and ventilate extensively. To protect against downy mildew, the leaves should not get wet. Always water the plants from below and if possible in the morning or in the afternoon, so that they come dry in the night.Very important is also a rather moderate nitrogen fertilization. If the nutrient is oversupplied, the plant tissue becomes very soft and the pollen tubes of the fungi are easier to penetrate during germination.
A mulberry bed can also effectively prevent infestation. Experiments with mulch material at the Horticulture Institute in Hanover have found that plants in mulberry flower beds are less prone to disease. For example, aster, goldenrod and solidaster were spared from powdery mildew. The mulch cover provides more balanced soil temperatures and lower humidity in the area of the plants, making them more resistant.
With preventive spraying of biological preparations, roses and other plants can be effectively protected against mildew
To prevent mildew and other leaf fungi, many hobby gardeners swear by cow's milk. At the health food store, buy untreated fresh whole milk (no homogenized or pasteurized milk) and mix with water at a ratio of 1: 8. This solution is sprayed during the season from the beginning of April to the end of July, first in the weekly, then biweekly on the endangered plants. The bacteria in the milk multiply strongly and form on the leaf surface a dense covering, which avoids pest exciters.
Herb broths, for example from horsetail (horsetail), strengthen the body's defenses and activate the soil life. Soak one kilogram of fresh or 150 grams of dried horsetail in ten liters of water for 24 hours. The next day, the broth is boiled for half an hour, strained and diluted with five times the amount of water to pour.
For direct control of mildew, there are not only various chemical preparations but also environmentally compatible products based on net sulfur or copper, which may even be used in organic farming.
Moldy parts affected by mildew should be cut and squeezed early. In heavy infestation, the whole plant must be removed from the bed and composted. You can easily cut off susceptible perennials such as asters, Indian forays or phlox in late summer or early fall. They'll be up and running again next year. The fungi die in the compost because they can only live on living plant tissue. Since the spores occur everywhere anyway, you do not have to give up composting of infected plants or plant parts.
Ornamental shrubs susceptible to powdery mildew include hedge plants such as hornbeam and field maple (large picture at the top). In these species, control measures are not required because the plants can live well with an infestation. In addition, this is reduced anyway by the normal annual shape cut. If the white leaves disturb you for visual reasons, you can simply reshape the plants at the end of August.
The containment of an acute powdery mildew infestation is possible after removal of infested plant parts only with fungicides. In addition to the network sulfur already mentioned, there are a whole series of other preparations in the garden center. Let us advise you on site, which are approved for your plants.