Why is wheat turning yellow in patches, and can it recover?

AbstractThe reasons wheat turns yellow in patches are: soil drought, shallow plough layer under straw return to the field, soil nutrient deficiency, relatively high sowing density, sowing too deep, herbicide phytotoxicity, damage from prolonged low-temperature freezing, and the occurrence of pests and diseases. Wheat yellowing is a common phenomenon during the wheat growing period. Slight yellowing over a small area will not cause yield loss, but severe yellowing over a large area must be taken seriously. Scientific management in the middle and late stages must be seized to increase the thousand-grain weight and recover the yield loss caused by yellowing.

1. Yellowing caused by drought

1. This type of wheat generally occurs in fields with insufficient soil moisture before sowing where sowing was rushed to catch the moisture. When the water the wheat root system absorbs from the soil finds it relatively difficult to compensate for consumption, its growth and development are affected, which causes the leaves to turn yellow, and in severe cases death occurs.

2. In the field, the whole wheat field turns yellow fairly uniformly, tillering is delayed, growth is slow or stagnant, new leaves grow short and small, and roots are small and fine, so sowing requires adequate soil moisture.

2. Shallow plough layer under straw return to the field

1. Generally, returning corn straw to the field helps improve the soil and build up soil fertility. At present, the amount of straw returned to the field is relatively large, and the plough layer is generally less than 20cm, causing part of the wheat in the field, after emergence, to have its root system set in the loose straw, making it difficult to absorb nutrients from the soil.

2. This causes the wheat seedlings to turn yellow, so deep ploughing and deep turning are advocated, with deep ploughing once every 3 years to a depth of 25cm.

3. Nutrient deficiency

1. Insufficient nitrogen fertilizer or insufficient phosphorus fertilizer will both cause wheat to turn yellow. Nitrogen-deficiency yellowing in wheat is caused by sowing too early, insufficient nitrogen fertilizer application, or low content in the base fertilizer, which causes the wheat leaves to turn yellow.

2. About 5kg of urea per mu needs to be applied as a topdressing during the greening stage, and another 12-15kg of urea per mu during the standing-up or jointing stage. When wheat seedlings are deficient in phosphorus, 10kg of diammonium phosphate or 45-50kg of calcium phosphate can be applied per mu in furrows.

4. Relatively high wheat sowing density

Relatively high wheat sowing density leads to an excessively large wheat population, which affects wheat photosynthesis. When nutrient supply is insufficient, the leaves turn yellow, so sowing must be done in appropriate amounts according to the season and temperature.

5. Sowing wheat too deep

Sowing wheat too deep causes the wheat to emerge slowly and the mesocotyl to be too long. During the emergence process, a large amount of nutrients is consumed, and yellow and weak seedlings appear at the wheat seedling stage.

6. Harm from herbicide phytotoxicity

Generally, more farmers apply wheat herbicides in early to mid-March. If the pesticide is improperly diluted, too many agents are mixed, and in addition some herbicide components themselves cause a certain degree of damage to wheat, or the field is damaged by herbicide drifting in, the detoxification function of the wheat declines, which causes yellowing.

7. Damage from prolonged low-temperature freezing

There is also prolonged low temperature and few sunshine hours, so the overall condition of the wheat seedlings is relatively weak, and they show yellow leaves under the influence of low-temperature freezing damage. Management needs to be strengthened after the cold wave arrives, and foliar fertilizer should be sprayed at the same time.

8. Occurrence of pests and diseases

1. The main wheat pests and diseases are sheath blight, take-all, and root rot, all of which are fungal diseases. In the early stages of occurrence, they can all show yellow leaves, but the sites where the lesions occur and the shapes of the lesions differ somewhat.

2. Sheath blight mainly harms the leaf sheath; take-all mainly harms the stem base and roots; and root rot occurs only at the roots, but all are soil-borne diseases. Underground pests can also harm wheat and cause yellow leaves, so seed dressing with chemicals and soil treatment are needed.

9. Salinization of irrigation water quality

Some wheat fields are close to rivers, ditches and canals, and the water resources are polluted with excessive heavy metals, which is also a cause of wheat yellowing.