What are the precautions for using fungicides?
1. Concentration: When spraying with liquid formulations, it is often necessary to mix or dilute the agent with water to an appropriate concentration. Too high a concentration will cause phytotoxicity and waste, while too low a concentration will be ineffective. For some non-wettable or difficult-to-wet powders, a small amount should first be added and stirred into a paste, then water added to prepare the mixture; a wetting agent may also be added during preparation.
2. Spraying time: Spraying too early will cause waste or reduce control efficacy, while spraying too late means a large amount of the pathogen has already invaded the host, and even if a systemic therapeutic agent is sprayed, the gain will be limited. Based on the pattern of disease occurrence and the conditions at the time, or based on short-term forecasts, spraying should be done in time to protect the plants before disease occurs or just as it begins.
3. Number of sprays: The number of sprays is mainly determined by the length of the pesticide's residual effect period and meteorological conditions. Generally, spray once every 10—15 days for a total of 2—3 sprays, with an additional spray after rain. Cost should be considered to save on pesticide use.
4. Spraying quality: The spray volume should be appropriate. Too little cannot provide thorough protection for all parts of the plant, while too much wastes pesticide and may even cause phytotoxicity. Spraying requires fine droplets and even application, and all parts of the plant that need protection, including both the upper and lower surfaces of leaves, should be covered.
5. Phytotoxicity: There are many reasons why spraying causes phytotoxicity to plants. Agents with stronger water solubility are more likely to cause phytotoxicity, and different crops have different sensitivities to agents. For example, Bordeaux mixture generally does not cause phytotoxicity, but it can also cause phytotoxicity in copper-sensitive crops. Legumes, potatoes and cotton are sensitive to lime sulfur mixture. Different developmental stages of crops also respond differently to agents; generally, the seedling stage and the booting and flowering stages are prone to phytotoxicity. It is also related to meteorological conditions, generally with the effects of temperature and sunlight being more obvious. High temperatures, strong sunlight, heavy fog or high humidity all easily cause phytotoxicity.
6. How to mix: Generally, pesticides that readily decompose and lose efficacy when encountering alkaline substances cannot be mixed with alkaline substances. For example, alkaline fungicides such as Bordeaux mixture and lime sulfur mixture cannot be mixed with 1605, dimethoate, dichlorvos and the like. Agents that produce a chemical reaction after mixing and can cause phytotoxicity also cannot be mixed and applied. A small number of pesticides show a synergistic effect after mixing. For example, when dimethoate is mixed with neutral and acidic fungicides such as zineb, wettable sulfur and colloidal sulfur, not only is the efficacy unaffected, but it is slightly improved.
7. Resistance: Long-term use of a single agent (mainly systemic fungicides) will cause pathogens to develop resistance, making the agent ineffective. To avoid this problem, different types of agents can be used alternately, or systemic fungicides and traditional fungicides can be mixed and used together.