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Gardeners who have inherited a plot with fruit trees, for example, should be aware that most plants are susceptible to numerous diseases. The appearance of tubercles, dark spots or the death of the bark, rot on the fruit, all this can be caused not only by excessive watering or a hot climate. One of the most common diseases is powdery mildew of apple trees and other plants.
What is powdery mildew?
Powdery mildew of apple trees is a fungal disease caused by powdery mildew fungi (Erysiphales), dangerous parasites for many plants. There is the so-called downy mildew, a disease not only of trees and bushes, but also of other plants. Its causative agents are parasitic fungi of the class Phycomycetes. As a rule, this disease affects the green part of the plant, mainly the leaf.
The marsupial fungus forms a mycelium, which is attached by appressors directly to the feeding organs of the plant, from which the haustoria extend deep into the tissues. On such superficial myceliums, abundant conidial sporulation develops in spring and summer, after which the cleistothecia stage with bagospores and bags is formed.
In most pathogens, the mycelium is often annual; it dies off together with the infected part of the plant. However, it can be perennial, remaining under the bark, in the roots and other hibernating organs. The fungus quickly spreads very quickly between plants with the help of conidia, causing new infections. The incubation period for the appearance of a characteristic plaque with conidia depends on the ontogenetic state of the host plant and is 4-10 days. The development of sporulation lasts, on average, 50-90 days, in rare cases more. It is noted that in hotter weather, the trees are less resistant to apple powdery mildew.
Symptoms
Powdery mildew of the apple tree is found in early spring, when the buds are just beginning to bloom (bud separation phase) and at the very beginning of the formation of the first leaf. It almost simultaneously appears on leaves and shoots. A dense white coating appears on the inflorescences, they are severely deformed, wither without forming fruits, and some fall off. Affected shoots and fruits can be seen in the photo below.
The shoots are covered at first with a white, and later with a brown-gray bloom, which darkens with time, cleistothecia of the fungus are formed on it, which look like black dots. On the petioles and leaves, as a rule, a whitish-gray coating appears on their lower side, which subsequently acquires a reddish tint. Affected leaves stop their development, twisting into a boat along the main vein.
On the fruits, this disease manifests itself in the form of a whitish coating at the beginning of their formation. Such plaque quickly disappears and the so-called rusty mesh remains on the surface, resembling cork tissue in its structure, which occurs as a result of various mechanical damage.
It is noteworthy that at the beginning of the disease, mealy spots on leaves and inflorescences are easily erased mechanically, but then reappear, increasing in size and changing color to rich gray or purple. Over time, the mycelium thickens, becoming almost brown. Plant growth slows down or stops altogether, the leaves gradually wither, dry out and fall off, flowers and buds fall off. This photo illustrates a whitish coating on the leaves – a clear sign of the onset of the disease.
Distribution area
According to the materials of research specialists, this disease is widespread in the countries of the former CIS and the Baltic countries. Its high harmfulness (infection rate 60-90%) is observed in the Alma-Ata fruit growing zone of Central Asia and Kazakhstan, Transcaucasia, in the regions of the North Caucasus, Azerbaijan, Ukraine, Our Country, Belarus, Latvia, Moldova and Armenia. The average development of the disease (the percentage of damage is 40-60%) includes the Krasnodar Territory, certain regions of Georgia, Kyrgyzstan, Transcarpathia. In a weak degree, the disease (the percentage of damage is 20-40%) manifests itself in the central regions of the Baltic states, the Chernozem zone, and the Volga region.
What is harmful
With a strong development of powdery mildew, its harmfulness can be very large. Leaves that are affected are underdeveloped and fall off, while infected shoots continue to grow and their upper part dries out. The formed ovaries also crumble. This disease can reduce yield levels by 40-60%.
In addition, the winter hardiness of apple trees is significantly reduced. First of all, the affected buds and shoots die. However, the mycelium often dies along with them, which explains the significant decrease in infection after a harsh winter. This disease is especially dangerous in nurseries, when it can affect a large number of plants.
Prevention
For the prevention and protection against this disease, three times spraying with fungicides or Bordeaux mixture is carried out. Pollination with sulfur at least three times in the summer also helps. Feeding trees with potash and phosphate fertilizers greatly increases disease resistance, but overfeeding with nitrogenous fertilizers increases the risk of infection, especially during the budding period.
The room with seedlings should be ventilated as often as possible, observing the humidity regime and avoiding drafts.
Control measures
A real and especially effective method of treating powdery mildew is spraying apple trees every 3-4 days with special preparations, as well as copper oxychloride, colloidal sulfur, potassium permanganate, or a mixture of soda ash and soap.
Topaz gives a positive and dynamic effect in the treatment, protecting all organs of the tree from disease. This fungicide can significantly reduce the cost of removing and cutting infected shoots and significantly reduce the harmfulness of secondary infection. To suppress the primary infection of the pathogen, Topaz treatment is carried out at the beginning of the growing season according to the instructions. When the first signs of the disease appear, treatments are carried out at intervals of 6-12 days. In the main period of the disease, which begins during the flowering period, 2-3 sprays are carried out immediately. No more than four sprays with this fungicide should be carried out per season. Processing should be stopped after the suspension of the secondary growth of the apple tree.
There are also other control measures such as zoning and selection of resistant varieties of apple trees. Removal and cutting in early spring or autumn of shoots that are affected by the disease. As well as eradicating chemical treatment in the autumn, when the leaves fall off completely. Abundant, but not excessive watering, keeping the soil constantly moist, will make the tree stronger and more resilient.
Video “Powdery mildew of an apple tree”
This video will tell you about the chemical treatment of apple trees, as a method of dealing with powdery mildew.