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Hotbeds and greenhouses have long enjoyed well-deserved respect among gardeners. It is not difficult to maintain a microclimate favorable for growing plants in such structures. Recently, the popularity of such structures has increased even more thanks to the use of polycarbonate. With its excellent characteristics, this material has transferred the greenhouse from a purely functional structure to the category of landscape design elements that are not dismantled in warm weather. However, with all its undeniable advantages, in some situations greenhouses turn into a hotbed of various diseases, since they create favorable conditions not only for the development of plants, but also for the reproduction of various pathogens. One of them is the notorious Phytophthora.
How to detect disease
Determining the infection of plants with phytophthora is quite simple. This disease affects mainly nightshade crops, so you should look for signs of its manifestation when examining tomatoes, eggplants, peppers, potatoes and physalis planted in a greenhouse. However, strawberries can also be attacked by the pest. Late blight begins with the appearance of dark gray-brown spots on the leaves of plants. At high humidity, a fluffy white coating forms on the back of the leaf plates. Then dark spots appear on the stems.
Scheme of the late blight development cycle
Affected leaves dry up, flower stalks also dry and inflorescences turn black. The fruits of diseased plants are covered with brown-brown spots that are hard to the touch, which grow over the surface and in depth, and then soften and begin to rot. On the tubers of late blight-infected potatoes, grayish areas with wrinkled skin appear, under which, when cut, “tongues” of reddish pathogen-affected tissue can be seen extending deep into. No wonder late blight is also known as brown rot or potato rot. At first sight, normal tubers and ripening green fruits affected by it become inedible even after a time after harvest, and diseased plants often die. In the middle of the XNUMXth century, an outbreak of this disease affected almost the entire potato crop in Ireland, causing famine and mass immigration in the country. 
What causes phytophthora
The causative agent of the disease is oomycetes, which have long been listed among fungi, and today stand out as a separate group of special mycelial organisms. Phytophthora spends most of its time in the form of spores, and only when favorable conditions occur, it begins to actively multiply. The combination of high humidity and air heated to +25 – +30 ° C, the most “favorite” of phytophthora, often occurs in the greenhouse. Under these conditions, spores can germinate and infect the plant in just an hour. A sharp fluctuation in temperature throughout the day, when a rather hot day is replaced by a cool night, causes heavy dew and fog to form. This is a favorable period for phytophthora activity.
Unlike polycarbonate structures, film-covered greenhouses are more prone to disease outbreaks, since copious condensation forms on the inside of the film during temperature changes. Phytophthora spores are washed off when watered into the soil, where they can survive even severe frosts. So the disease can appear in the greenhouse along with the soil. Spores persist for a long time in the remains of infected plants, so they should certainly be removed outside and burned. Even improperly processed inventory can become a carrier of the disease.. Over long distances of tens of kilometers, spores are carried by the wind. So the pest can get into the greenhouse, which is quite far from the source of the disease. Phytophthora can also enter along with seedlings purchased on the market, since a visually beautiful plant does not guarantee that it is not a carrier of the disease during the incubation period. 
We treat plants
To date, there is no such tool, the use of which would completely get rid of phytophthora. Since it is in a state of spores for most of its life cycle, neither chemical nor biological preparations completely destroy it. However, their use together with some other methods leads to a significant decrease in the number of spores and inhibition of pest activity. That is, the fight against phytophthora to a greater extent consists not in the treatment of diseased plants, but in the prevention of infection. Since spores can be almost everywhere, in the spring the greenhouse should be fumigated with special sulfuric smoke bombs. Also, walls and ceilings can be treated with microbiological preparations such as Fitosporin-M, Radiance or Baikal EM.
Unlike chemical fungicides, these agents destroy pests due to the colonies of living microorganisms that are part of them, for which late blight spores are food. It is necessary to fight the disease when it is found in the greenhouse by removing and burning the affected leaves and other parts of the plants. After that, foliar spraying of all solanaceous crops with the mentioned microbiological preparations or copper-containing fungicides should be carried out: copper sulphate, copper oxychloride, Bordeaux liquid, Barrier, Oxyhom, Ordan and others. These substances are also used for regular preventive treatment of plants. Seedlings should be carefully sprayed before planting in the soil of the greenhouse, and then do it again after two weeks.
It should be remembered that after the use of chemicals, vegetables can be eaten no earlier than three weeks after the last spraying. Among folk remedies, the processing of nightshade planted in a greenhouse with an infusion of garlic, which is prepared by adding 40 grams of chopped garlic to a bucket of water and insisting for a day, is popular. The plant is sprayed in this case weekly. Since at air temperatures above +30 ° C the development of the pest practically stops, you can fight it by periodically arranging a steam room, closing all doors and windows for several hours on a sunny day, and then ventilate the greenhouse well. Since excess moisture contributes to the active reproduction of phytophthora, the plants should be watered only under the root. But it is highly undesirable to use underground irrigation or mulch the soil with polyethylene in a greenhouse. 
Unlike open ground, in a greenhouse it is rarely possible to avoid close proximity of plants when planting. This contributes to the rapid infection with phytophthora of all nightshade crops. Therefore, if due to the size of the greenhouse it is necessary to use thickened planting patterns, then it is necessary to form plants into one stem. Since the disease often affects old leaves, the protective treatment consists in removing them during regular inspection of plantings.
A good way to protect plants from the appearance of phytophthora is the nightshade crop rotation, in which they are planted in the same greenhouse in the spring with an interval of three to four years. Also, a fairly effective method to avoid infection will be the cultivation of hybrids that are particularly resistant to infection or early ripening varieties, the crop of which ripens before the start of the period of pest activity. Seeds for planting in a greenhouse should be purchased from those manufacturers who carry out their pre-treatment from pests and diseases.
How to treat the soil
Since phytophthora in the greenhouse is stored mainly in the soil, the main fight against it is carried out precisely on this “battlefield”. The main thing in this case is to choose a way to treat the soil in such a way as to most effectively suppress the source of infection, that is, to destroy as many spores as possible.
The lack of copper salts in the soil contributes to the long-term preservation of pathogen spores, therefore, when infected plants are found in the fall after harvesting, the top layer of soil about 5 centimeters thick is removed from the greenhouse, and in spring the soil should be treated with a copper-containing fungicide or microbiological preparation.
The introduction of chemicals into the soil (Ordan, Oxyhom, Bordeaux liquid, etc.) requires refraining from using vegetables for food for a month after their use, but microbiological preparations (Fitosporin-M, Mikosan, Radiance, Baikal EM, etc. .) You can work the soil at any time. You should not use them only during the flowering period, when it can harm the bees flying into the greenhouse for nectar.
Video “Processing greenhouses with your own hands”
If you want to know how and by what means greenhouses should be treated from phytophthora, then watch this video.
