It has long been believed: the most dangerous disease of solanaceous, including potato, late blight disease - is transmitted only with planting material. In our soil, under our conditions, the harmful fungus does not overwinter. However, in recent years, this truth from agro-educators seems to have been called into question. Potato late blight has acquired new properties and seems to have gone over to the counteroffensive. Therefore, it is necessary to deal with late blight all year round.
What is late blight
Phytophthora (Phytophthora - comes from Greek, literally means "plant destroyer") - parasitic fungi that cause brown rot in plants. Now found their 80 species.
Late blight is a fungal disease of plants caused by late blight.
In Europe, phytophthora was first discovered in 1830 and spread over the entire territory in ten years - from 1843 to 1847 a real epidemic erupted here (agronomists say “epiphytotism”).
In the 1840s, the effects of the late blight epidemic reached a catastrophic level in Ireland, where the population was most dependent on this staple food. The result of epiphytotics was the death of about 1 million people from starvation and the emigration from the Netherlands of more than 1 million people. Even today, the Irish population remains less than three-quarters of what it was in the early 1840s. These dramatic consequences of the late blight epidemic have arisen due to the lack of chemical and genetic methods to combat this disease.
In our country, late blight disease is ubiquitous. It does the most harm to potato plantings in the northwestern part of Russia. An outbreak of late blight is usually associated with rainy weather. But especially quickly, the disease devours potato plantings if cool, wet nights alternate with warm, wet days. With such favorable weather for him, the fungus can spread hundreds of kilometers. And since potatoes have a long vegetative period, are susceptible to illness throughout the summer, the danger of epiphytotia is always and everywhere where comfortable conditions for the development of the fungus develop.
Potatoes usually get sick in the second half of the growing season when the plants bloom. First, the leaves on the lower side are affected - there are more stomata. The fungal hyphae penetrate into these natural holes with thin processes. From the moment of infection of the plant to the first symptoms of the disease, only 4 days pass. At the beginning of mass flowering, dark brown spots with a grayish bloom appear from the bottom of the plant's lower leaves. Their sizes are increasing rapidly. In a few days, the tops can turn into a brown rotting mass.
The causative agent of late blight
The causative agent of the disease is a lower fungus parasitizing on a living plant, capable of developing on dead plant debris of potatoes or tomatoes. The mushroom overwinters in the form of its vegetative body - mycelium - in infected tubers, plant debris. The homeland of this mushroom is South America, where the potato came from.
Phytophthora fungus parasitizes most of its life on living potato tissues - leaves, stems, tubers in the form of mycelium. From diseased seed tubers, he quickly switches to seedlings, and from them - to potatoes of a new crop. Slightly depressed spots of lead color, irregular in shape, appear on the peel. Under the skin of such a spot, the tuber tissue is first reddish in color and then darkens. With severe infection, the stain liquefies.
In the soil, the fungus usually does not last long. But this is usual. In recent years, a more aggressive form of late blight has appeared, which was previously found only in Mexico. Now the disease may appear earlier, before the budding phase - flowering. Its harmfulness has greatly increased. A characteristic sign of late blight of potatoes or tomatoes was a massive lesion of the stems. The development of the disease is noted even under conditions that were previously considered unfavorable for late blight. Significantly decreased resistance to fungus of zoned potato varieties.
An increase in the harmfulness of late blight is associated with changes in the composition and properties of the causative agent of the disease. If earlier he hibernated only in the form of mycelium on tubers, now sexual structures are formed in the affected tissue - the so-called oospores. They can accumulate in the soil, becoming an additional source of infection. Such contaminated soils for many years become unsuitable for planting solanaceous.
Of particular danger is the fact that, in addition to potatoes and tomatoes, the causative agent of late blight parasitizes on some wild species of the nightshade family. For example, on a bittersweet nightshade. Moreover, such an “intermediate host” enhances the formation of oospores, and itself serves as a reservoir of infection. The first symptoms of the disease are now manifested at any stage of growth, development of plants, starting with seedlings. The first infectious spots are often found both on the upper and lower leaves, so on the stems at the base or at the point of growth. It happens that even young shoots or shoots are already completely infected. They die off immediately.
The affected tops with oospores of the fungus, falling into the soil, will serve as a constant source of accumulation of resting structures. They retain their viability there for several years. After a winter dormant period at low temperatures, oospores sprout in the spring at high soil moisture, infecting healthy potato sprouts. Thus, the primary foci of infection are created, giving rise to the development of the next phytophthora population. When tubers are planted in such soil, they also become infected with late blight. The disease can occur at any stage of growth and development of potatoes.
Control measures and prevention of late blight
We must deal with late blight almost all year round. It is never too late to check and, if necessary, sort out seed potatoes, remove diseased, damaged tubers. In time, you should buy protectants that will be needed for spring pre-plant dressing. This is TMTD - an 80 percent wettable powder, polycarbacin, Dita M-45, nitrafen, formalin.
Stock up on mineral fertilizers at the same time. Their use can dramatically - one and a half to two times - reduce the spread of late blight. I would just like to warn those who, when using mineral fertilizers, adhere to the rule of butter you will not spoil the porridge. Alas, an excess of nitrogen in the soil, not compensated by potassium and phosphorus, facilitates the spread of the disease, significantly reduces the number of healthy potatoes.
Even novice gardeners know the high efficiency of late blight, an accessible technique for everyone - spring germination of tubers for 20-25 days.
It is worth recalling that in years with conditions especially favorable for the development of late blight, it is impossible to do without using a complex of chemical plant protection, and you cannot get a good crop.
In household plots for processing potatoes, it is allowed to use a 5 percent Azocene - 10 g of the drug is consumed per 10 liters of water. The term of the last processing is 20 days before harvesting. A maximum of four times the treatment.
Medex (a mixture of copper sulfate with soda ash - 1: 1) is used throughout the growing season. The consumption rate is 100-150 g per 10 liters of water. The last treatment is 15 days before harvesting. Four treatments are possible.
Polycarbacin 80 percent take 40 g per 10 liters of water, the last treatment - 20 days before the start of cleaning. Fourfold processing.
Copper chloride 90% solution (40 g per 10 liters of water) - 20 days before harvesting, five treatments during the growing season.
60% arceride - 50 g, 20 days before the start of harvesting, three times processing.
Well, and most importantly - where the new form of late blight began to rage, it is necessary to abandon the common practice of growing potatoes from year to year in one place, to ensure spatial isolation of solanaceous crops.