Causes of crop injury and its symptoms
Although the causes of phytotoxicity are complex, they can generally be summarized into three main categories: First, incorrect use of pesticides or contamination with impurities. Second, the use of degraded or contaminated pesticides containing harmful substances. Third, lack of proper knowledge in pesticide application, such as spraying at inappropriate crop growth stages, unfavorable weather conditions, or improper methods.
The main symptoms of phytotoxicity include:
1. **Spot Type**: This type typically appears on leaves, stems, or fruit surfaces. Common manifestations include brown spots, mottled spots, and net-like lesions. For example, applying butachlor incorrectly to rice fields may cause irregular browning on rice leaves. Similarly, Jinggangmycin on watermelon leaves can result in small yellow spots, while Bordeaux mixture may create brown net-like spots on apple skin.
2. **Yellowing Type**: This occurs mainly on leaves and stems, often due to interference with chlorophyll production. Symptoms range from partial yellowing to complete yellowing of the plant. For instance, fenthion on watermelon can cause yellowing of new shoots, while Simazine can lead to yellowing starting from leaf edges and spreading across the entire leaf.
3. **Malformation Type**: This affects stems, leaves, and roots, causing symptoms like curled leaves, clumped growth, swollen roots, or deformed fruits. For example, dimethyltetrachlorobenzene can cause swelling in wheat seedlings, while herbicidal ether may lead to blistered cotton leaves. Tomato plants exposed to 2,4-D may develop hollow or malformed fruits.
4. **Withering Type**: This affects the whole plant and is often caused by herbicides. Glyphosate applied to rice seedlings can lead to yellowing and death, while chlorotoluron injury on watermelons may result in leaf scorching and plant death.
5. **Growth Stagnation Type**: This leads to slow or stunted growth. Improper use of green mellon in rapeseed may reduce branching and affect yield, while paclobutrazol in paddy fields can delay heading if not used properly.
6. **Infertility Type**: This occurs during reproductive stages, leading to reduced fertility. For example, rice shoot green can cause male sterility, resulting in empty panicles and lower yields.
7. **Shedding Type**: This affects fruit trees and some dicots, causing leaf or fruit drop. Copper preparations on peach trees may lead to defoliation, while ethephon misuse in hawthorn can cause fruit drop.
8. **Fruit Quality Reduction Type**: This affects fruit size, appearance, and quality. Ethylene toxicity in watermelons may cause discoloration and odor, while copper-based sprays on tomatoes can lead to brown spots on fruit surfaces.
**What's the difference between phytotoxicity and disease?**
Phytotoxicity differs from physiological diseases in that its symptoms are usually irregularly distributed, while diseases tend to show consistent patterns. Spot-type phytotoxicity also differs from fungal diseases, as it lacks a clear center and has variable spot shapes. Yellowing from phytotoxicity often progresses rapidly, unlike nutrient deficiency-related yellowing, which is slower and more uniform. Virus-induced yellowing may show systemic symptoms and mixed healthy and diseased plants, while phytotoxicity tends to be localized.
Deformities from phytotoxicity are often widespread, whereas virus-induced deformities are usually sporadic and may involve symptoms like mottled veins or wrinkled leaves. Wilt from phytotoxicity typically develops gradually, without a central point, while disease-induced wilting is often linked to root blockage and may appear suddenly under high evaporation.
Delayed growth from phytotoxicity is often accompanied by other symptoms, while physiological issues may manifest as root problems and yellow or dark green leaves.
The key difference between phytotoxicity and disease is that phytotoxicity shows specific symptoms without disease signs, while diseases may have no visible symptoms or present systemic effects.
To prevent phytotoxicity, follow these practices:
1. Conduct small-scale trials before large-area application to determine the correct dosage, timing, and method.
2. Strictly control pesticide use, including selecting the right product, following recommended concentrations, and applying at the right time.
3. Clean equipment thoroughly after using herbicides, especially when switching between different chemicals. Use lye for plastic tanks and ferrous sulfate for metal ones. Ensure nozzles and other parts are also cleaned.
After an incident, regular field inspections are crucial, especially after using herbicides or growth regulators. To mitigate damage:
- Apply fertilizers to help recover from leaf spots or yellowing.
- Use irrigation to reduce the impact of certain herbicides.
- Spray gibberellin after exposure to certain herbicides to counteract their effects.
These steps can significantly reduce the negative impact of phytotoxicity on crops.
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