Upon the prolonged regular use of the same fungicides, pathogens often become adapted to them, and their resistance to the fungicide used grows. For instance, an increase in the resistance of phytophthora to copper formulations was noted.
The repeated application of Bordeaux mixture and copper oxychloride would have a negative effect on the plants being protected, causing depression of photosynthesis, dehydration of the leaves, inhibition of the growth of shoots, and damage to fruits, especially if the plants were treated after the fruits were formed. To diminish the negative effect of copper-containing formulations on plants, treatments with them are alternated with the use of derivatives of dithiocarbamic acid, which are not phytotoxic and, as a rule, facilitate the sanitation of the plants.
Upon the prolonged use of zineb against grapevine mildew, however, improper growth and more intense infection with oidium was noted in the second and third years.
Although the spectrum of fungicidal action of these groups of substances is close, for individual pathogens, their toxicity and nature of action are different. For example, dithiocarbamic acid derivatives are the most effective against the blue mould of tobacco, whereas the pathogen of this disease exhibits an increased resistance to copper-containing substances.
The mixtures of zineb with copper oxychloride and of metiram with copper oxychloride (poly-chom) were not inferior in their effectiveness to their constituent parts when used individually, while in a number of cases, they were more effective, especially if the copper-containing substances predominated in a mixture. The advantages of the mixtures used to control grape mildew displayed themselves, especially clearly in epiphytotic years. The mixtures inhibited mildew and limited the development of oidium and grey rot.
The active ingredient, a mixture of zineb and copper oxychloride, is a bluish-green odourless powder. It is marketed in the form of an 80 per cent wettable powder containing 65 per cent copper oxychloride and 15 per cent zineb.
Cuprosan persists better on the surface of treated plants than zineb, is superior in its effectiveness to the Bordeaux mixture, and does not produce burns. It is used for treating apples, pears, plums, peaches, apricots, and currants. Gooseberries, raspberries, grapevines, sugar beets, potatoes, tomatoes, watermelons, cantaloupes, cucumbers. For the low-volume aerial spraying of grapevines, cuprosan is used at a rate of 5 kg ha with the addition of the anti-vaporizer AI-4P. The plants are sprayed during their vegetation period with 0.4 per cent suspensions. The harvest time is 20 days.
Cuprosan is moderately toxic to humans and warm-blooded animals (LD50 for rats is 400 mg/kg). Its cumulative properties are pronounced, and the cumulative coefficient is 0.9. The maximum tolerated concentration in the air of the working zone is 0.1 mg/m3 and in water basins, 0.1 mg/litre. The tolerance level in vegetables, fruits, grapes, and melons is 0.1 mg/kg (with respect to the copper oxychloride).
Polychom is a mixture of metiram with copper oxychloride. It is marketed in the form of an 80 per cent wettable powder containing three parts of 75 per cent metiram and one part of 90 per cent copper oxychloride. It is recommended for the treatment of apples, pears, grapevines, sugar beets, potatoes, and tomatoes and helps during their vegetation period to control the same diseases against which copper oxychloride and metiram are effective. Treatment is conducted using 0.4 per cent suspensions at a rate of 2.4-8 kg/ha. The harvest time is 20 days.
The author is an associate professor (retd.) & former head of the department of botany at Ananda Mohan College.