Causes of crop injury and its symptoms
Although the causes of phytotoxicity are complex, they can generally be summarized into three main aspects. First, incorrect use of pesticides or the presence of impurities in them. Second, the use of degraded or contaminated pesticides that contain harmful substances. Third, a lack of proper knowledge about pesticide application techniques, such as spraying at inappropriate growth stages, unfavorable weather conditions, or using improper methods.
The main symptoms of phytotoxicity include:
1. **Spot Type**: This type of damage typically appears on leaves, stems, or fruit surfaces. Common signs include brown spots, mottled spots, and net-like patterns. For example, rice leaves may show irregular browning after improper application of butachlor. Watermelon leaves treated with Jinggangmycin can develop small yellow spots, while Bordeaux mixture may cause brown net-like spots on apple skin.
2. **Yellowing Type**: This occurs when chlorophyll production is disrupted, leading to leaf yellowing. It can range from partial to complete yellowing of the plant. For instance, fenthion applied to watermelons may cause yellowing on new shoots, while Simazine can lead to yellowing starting from the leaf edges and spreading inward.
3. **Malformation Type**: This includes leaf curling, stem thickening, root swelling, and malformed fruits. For example, wheat seedlings exposed to dimethyltetrachlorobenzene may show swollen roots, and cotton plants treated with herbicidal ether may have blistered leaves.
4. **Withering Type**: This symptom affects the entire plant and is often caused by improper herbicide use. Glyphosate, for example, can cause rice seedlings to yellow and die, while chlorotoluron can lead to wilting and death of watermelon seedlings.
5. **Growth Stagnation Type**: This occurs when plant growth is slowed down due to pesticide exposure. Improper use of green mellon in rapeseed can result in slow growth and reduced branching, affecting yield.
6. **Infertility**: This happens when pesticides interfere with reproductive processes. For example, rice shoot green can cause male infertility in rice pollen, leading to empty panicles.
7. **Shedding Type**: This involves the dropping of leaves, flowers, or fruits. Copper preparations on peach trees can cause defoliation, while ethephon misuse in hawthorn can lead to fruit drop.
8. **Fruit Quality Reduction**: This results in smaller, misshapen, or discolored fruits. Ethylene toxicity in watermelons can cause dark red, odoriferous melons, while copper-based sprays on tomatoes may lead to brown spots on the fruit surface.
**Phytotoxicity vs. Disease**:
Phytotoxicity differs from diseases in several ways. Spot-type phytotoxicity usually appears irregularly across the field, while physiological diseases tend to affect uniform areas. Fungal diseases have distinct centers of infection, whereas phytotoxic spots vary in size and shape. Yellowing from phytotoxicity tends to spread quickly, while nutrient deficiency causes gradual yellowing. Virus-induced yellowing often has a mosaic pattern and affects the whole plant systemically.
To prevent phytotoxicity:
- Always conduct small-scale trials before large-area application.
- Follow dosage instructions carefully and apply at the right time.
- Clean equipment thoroughly, especially after using herbicides, to avoid cross-contamination.
If phytotoxicity occurs, quick action is essential. Regular crop monitoring after pesticide application is crucial. Remedies include adding fertilizers, irrigation, and using plant hormones like gibberellin to reduce damage. For example, applying 20–30 ppm gibberellin after dimethyltetrachlorobenzene exposure can help alleviate symptoms.
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