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In recent weeks, several farmers have raised concerns about the abnormal development of summer corn in certain regions, where the tassels and leaves are not unfurling properly. Some fields show severe deformities, resembling a "cow tail" or "big whip" shape, prompting questions on how to address this issue. To provide clarity, Shao Lixia, a senior agronomist from the Longyin County Agricultural Bureau, has conducted an in-depth investigation and carried out prevention and control trials in the affected areas. Here's what she has to say.
Currently, summer corn in central and southern provinces is entering the critical "big bell mouth" stage, but some plots are showing unusual symptoms. The heart and leaves are not developing as expected, with many plants displaying twisted, deformed, or even broken structures. These issues are commonly referred to as "cow tails" or "big whips," which can significantly impact yield and quality.
**Main Causes**
1. **Thrips and Swedish Fly Damage**
Farmers who peeled back the outer leaves often found that the base of the leaves had been damaged, with holes and shriveled tissue. The main symptoms include distorted leaves, reduced size, and visible holes. Some leaves also show a transparent film on the surface, while the heart may contain mucus or be completely broken. This damage is primarily caused by thrips and Swedish flies during the 5-6 leaf stage of corn growth. Our survey between July 8th and 11th found that some fields had up to 500–1,000 thrips per plant, leading to yellowing and twisting of the leaves. In some cases, the leaves were silvery white, deformed, or even broken, resulting in "cow tail" shapes and the formation of tillers. Corn planted in lighter soils or with insufficient fertilizers and pesticides tends to suffer more severely. The thrips and Swedish flies feed on the heart of the plant, causing it to fail to unfold and twist into a "cow tail."
2. **Herbicide Injury After Emergence**
Another common cause is the misuse of post-emergence herbicides, particularly those containing nicosulfuron. Symptoms include stunted growth, twisted hearts, and inability to unfurl. According to a survey conducted on July 20th, 20% of corn fields showed signs of phytotoxicity. Some plants had curled leaves, rotten hearts, and yellowing foliage, with a 10% rate of rolled leaves. One reason for this is the low purity of nicosulfuron products in China, which can lead to chemical burns. Additionally, spraying at high temperatures or repeated applications can also cause injury. If herbicides are applied after the corn has more than five leaves, the liquid may accumulate in the heart, causing deformation, white leaves, or a rotten core. This can result in dwarfed plants and hinder normal tassel development, especially in hybrid varieties that are sensitive to the chemicals.
**Control Measures**
1. **Immediate Intervention**
For affected plants, farmers should cut off the top 1/4 of the leaves using scissors or manually remove the twisted heart. Afterward, apply foliar fertilizer and gibberellin (90%) at a rate of 1 gram per 15–30 kg of water. Alternatively, spray "Tianfengsu" (brassica lactone) at 1,000 times dilution, or use "Double Effect King" at 40 ml per 30–45 kg of water. A mix of 6,000 times Love More + 0.2–0.3% potassium dihydrogen phosphate + 1% urea + zinc fertilizer can also be used. Repeat this process every 5–7 days, and consider applying it twice if necessary.
2. **Pest Control**
If the issue is due to pests like thrips and Swedish flies, it’s essential to apply insecticides during the 5–6 leaf stage. Use a mixture of 10% imidacloprid at 1,000 times dilution or 5% acetamiprid at 1,500 times, along with agricultural streptomycin at 500–800 times. Adding gibberellin or high-efficiency foliar fertilizer can enhance effectiveness. Spray every 5–7 days and repeat the treatment twice.
3. **Herbicide Management**
To prevent herbicide-related damage, always use high-quality post-emergence herbicides and avoid application during high temperatures. It’s also important to apply them at the right time to prevent late-stage injury. Choosing the correct timing and product can significantly reduce the risk of crop damage.
By taking these steps, farmers can mitigate the effects of these issues and help their corn crops recover and grow normally.