Three Major Types of Phytotoxicity to Guard Against in Cotton Cultivation

Diagnosis and identification of phytotoxicity from insecticides and fungicides. The severity of phytotoxicity from insecticides and fungicides depends not only on the cotton variety, growth stage and morphological characteristics, but also on the type of pesticide, as the symptoms of damage differ for each. For example, dichlorvos has an extremely strong vaporizing effect; spraying twice with a 1000-fold solution can scorch cotton leaves dry. After phytotoxicity from lime sulfur mixture, the leaves and boll husks of the cotton plant develop scorched patches, and the leaves soon dry up and drop off. Lime sulfur mixture is a strongly alkaline pesticide; Bordeaux mixture can only be used 7 to 10 days after its application. If lime sulfur mixture is sprayed within 1 to 2 months after spraying Bordeaux mixture, the cotton leaves will develop black copper sulfide deposits, causing phytotoxicity. In short, when using insecticides and fungicides on cotton, a certain interval should be maintained between successive applications of several pesticides (especially inorganic pesticides), to prevent phytotoxicity caused by reactions between pesticides with different modes of action. In addition, the severity of cotton phytotoxicity is also affected by the environment; for example, when pesticides are sprayed under high temperature, the large amount of water evaporation caused by the heat increases the pesticide concentration, causing spots, yellowing and curling of cotton leaves, and even scorching and death.

Diagnosis and identification of phytotoxicity from herbicides. There are more than 30 common types of weeds in cotton fields. In cotton production, improper use of herbicides often induces phytotoxicity, and the symptoms are often variable and diverse, including growth inhibition, bending, twisting and curling of stems and leaves, shortened internodes, thickened leaves, chlorosis, albinism, necrotic spots and deformities. For example, after diuron is absorbed by cotton, it is difficult for it to translocate upward; if it is used when cotton has two true leaves, the phytotoxicity rate of cotton will exceed 90%. After phytotoxicity from prometryn, the young leaves of cotton fade, lose chlorophyll and wither. After phytotoxicity from trifluralin, the 2nd and 3rd true leaves of the cotton plant may shrink and become smaller; in severe cases, the cotyledons of cotton become dark green, thickened and brittle, the stem base thickens, the plant becomes shorter, and even the growing point may necrose and lateral branches proliferate. In addition, cotton is extremely sensitive to 2,4-D; in production, phytotoxicity is often caused by using a sprayer that has been used for 2,4-D without washing it clean before spraying cotton fields (or by drift into adjacent cotton fields when 2,4-D is sprayed on other crops). Affected cotton plants show smaller and narrower leaves, twisted veins and petioles, shrinkage and deformities, often resembling chicken claws.

Diagnosis and identification of phytotoxicity from plant growth regulators. Improper use of plant growth regulators can also often cause artificial phytotoxicity to cotton plants. For example, chlormequat is a trace, highly efficient substance that inhibits crop cell growth (without inhibiting cell division); at suitable temperatures, it can effectively control vegetative growth and promote reproductive growth. However, cotton is relatively sensitive to chlormequat; if the mixing concentration is too high or the dosage is too large, it will inhibit cotton growth, making the plants too short, impairing ventilation and light penetration, causing easy shedding of squares and bolls, and deforming cotton bolls.