Differences Between Herbicide Damage and Fertilizer Damage in Azaleas and How to Tell Them Apart

When azaleas suffer herbicide damage and fertilizer damage, they share some characteristics but also differ in their manifestations. What they have in common is that both are physiological injuries, causing leaves to wither and drop within a short time, but the patches do not slowly expand, nor do pathogens appear on them.

The differences between herbicide damage and fertilizer damage are as follows: when azaleas suffer herbicide damage, they show sensitivity to a certain pesticide or a certain class of pesticides. For example, after a pesticide is sprayed on the leaves, within a few days the parts of the leaves touched by the spray solution turn yellow, wither and drop, seriously affecting the plant's growth, but the roots are not damaged, and it generally does not easily cause the death of the whole flowering plant. Moreover, the time from the appearance of herbicide damage spots to leaf drop is somewhat longer than with fertilizer damage (note that there are exceptions, such as plum blossoms, whose leaves all drop within 1 to 2 days after spraying dimethoate and similar solutions). Because it is an acid-loving flower, strongly alkaline pesticides have obvious side effects on it. In addition, when using pesticides on azaleas, first, the concentration should be low; second, avoid the hours around noon in high-temperature seasons to prevent herbicide damage. Azaleas like fertilizer but cannot tolerate it, and alkaline fertilizers must not be used, otherwise fertilizer damage easily occurs, injuring roots and damaging leaves. For example, after mistakenly applying raw fertilizer, concentrated fertilizer, heavy fertilizer or basal fertilizer, the fine hairy fibrous root cells of azaleas are prone to reverse osmosis, causing the tender roots to lose water, shrink and die, and within 1 to 2 days the leaves can become scorched, or even all drop off, and in severe cases the whole plant dies. If the fertilizer solution concentration is somewhat too high, timely soaking and rinsing for rescue will prevent death, but it will still seriously damage the plant's "vitality," and it will be difficult to recover in a short time. For this reason, topdressing azaleas should use thin fertilizer applied frequently, and one must never blindly apply raw fertilizer, concentrated fertilizer or heavy fertilizer. Generally, the concentration of organic fertilizer is about 10%, that is, diluted 10 times with water for watering. The concentration of chemical fertilizer is about 0.3%, used for watering the roots, and the concentration for foliar topdressing is about 0.1%. In addition, whether it is Rhododendron simsii or Western azaleas, they are also relatively sensitive to monopotassium phosphate compound containing certain trace elements, and it should not be used to spray leaves, otherwise it easily causes leaves to scorch and fine branches to die, which will likewise seriously affect their growth or survival. In the high-temperature season from July to August, azalea growth stalls, and fertilization should be stopped. In late autumn and early winter, for species whose plants enter dormancy, fertilization should be stopped in winter. Under normal circumstances, on the morning of the second day after fertilizing, water once with clear water; the purpose is to dissolve the fertilizer in the soil so as to facilitate absorption by its root system.

The situations of fertilizer damage and herbicide damage in other flowers are basically the same as in azaleas, but pesticides to which some flowers are relatively sensitive must be avoided. For example, pomegranate and cruciferous flowers avoid fenitrothion; plum, peach, cherry blossom, pomegranate, apricot, citrus, chrysanthemum, coral flower, golden shrimp plant, shrimp plant, golden-vein justice, golden-vein Aphelandra, and net-vein grass avoid dimethoate and omethoate; cedar avoids Supracide; plum, peach and cherry blossom avoid trichlorfon; pear, persimmon, peach and plum avoid chlorothalonil; pear, peach and plum avoid lime sulfur mixture; citrus, peach, apricot, pear, plum and persimmon avoid Bordeaux mixture; cherry and Anthurium avoid malathion; ponytail palm avoids methamidophos; peach blossom is sensitive to triadimefon; plum blossom is allergic to monocrotophos and carbofuran. For the above flower and tree species, one must be careful when using pesticides: first, avoid using them as much as possible; second, it is best to use them during the leaf-drop period; third, use them at reduced concentration, so as to keep herbicide damage to a minimum.