What causes secondary branching and deformed scapes in garlic? Here's how to prevent it
Every year during the scape-elongation stage of garlic, a certain proportion of garlic plants develop secondary branching and deformed scapes, seriously affecting the yield and quality of garlic scapes. In recent years, we have carried out investigations and research, and have explored effective agricultural measures to control the occurrence of secondary branching and deformed scapes in garlic, which have played a very significant role in improving the economic benefits of garlic growers.
1 The harm caused by secondary branching and deformed scapes in garlic
Secondary branching in garlic is also called secondary growth or branching, and growers call it "horsetail garlic". There are generally two phenomena: first, branches differentiate from the outer leaf sheath, forming multi-clove garlic; second, branches differentiate from the inner leaf sheath, forming loose-clove garlic. After secondary branching occurs, the main scape of the garlic becomes shorter and thinner, and may even fail to emerge, and scape yield drops by 70%, basically losing its marketability. In severe years, the incidence of secondary branching in "Ershuizao" reached 90%; the incidence in "Sanyuehuang" reached 45%. In 2005, 33 hectares of "Ershuizao" garlic in Yuhua Town, Dafeng City, basically suffered a total loss of garlic scapes due to severe secondary branching, causing huge losses to garlic growers.
Deformity of garlic scapes means the scape becomes thicker and shorter, elongation is poor, and the scape becomes swollen and bent, losing marketability; growers call it "deformed scape". According to many years of multi-site surveys, deformed scapes in Ershuizao occur every year, with an incidence of 5%-15% in ordinary years and 30%-40% in severe years. The incidence of deformed scapes in Sanyuehuang garlic is relatively low, generally 1%-2% in ordinary years and 5%-10% in high-incidence years.
2 Main causes of secondary branching
The causes of secondary growth in garlic are complex. Apart from varietal factors, differences in climate and cultivation factors are the main causes.
2.1 Climatic factors
Although garlic is highly tolerant of low temperatures, it is relatively sensitive to temperature. In particular, the vernalization stage of garlic has low requirements for temperature and time. Generally, after sowing, garlic plants can pass through the vernalization stage after 1-2 months at 0-10°C. Through our experiments, when Ershuizao garlic seed was treated at 5-10°C for 30 days before sowing, the secondary branching rate reached 53.54%, while the secondary branching rate of untreated garlic seed was only 8.51%, which is 6.29 times that of the control. This indicates that garlic that has undergone low-temperature treatment has already passed through the vernalization stage and produced branches. From 2003 to 2004, a warm winter climate favorable for garlic growth and development appeared before winter; growth was vigorous before winter, pseudostems were thick after winter, leaves were wide and thick, and secondary branching occurred relatively severely.
2.2 Cultivation factors
① Sowing too early and excessively vigorous pre-winter growth easily produce secondary branching. In recent years, garlic growers have blindly pursued early marketing of garlic scapes, and at the same time, because of the large area, they sowed early in order to stagger farm work, causing seedlings to develop early and branching to increase. According to a 2004 survey of sowing dates for Sanyuehuang garlic, in fields sown at the end of August and the beginning of September, the incidence of secondary branching was 60%, while in Sanyuehuang garlic sown in mid-to-late October, the incidence of secondary branching was only 5%. This indicates that delaying the sowing date can significantly reduce the probability of secondary branching in garlic.
② Effect of sowing density on the amount of branching. If garlic sowing density is too sparse, it will lead to lateral growth in the early stage, thickening of the pseudostem, a large disk, and easy development of lateral buds into branches, especially prominently in Ershuizao. According to a 2003 survey, a grower in Yuhua Town, Dafeng City, planted 4669 sq m of Ershuizao garlic, with 37,500 plants per 667 sq m in equal row spacing, and applied 100 kg of phosphate fertilizer, 75 kg of cottonseed cake and 30 kg of urea as base fertilizer. Before overwintering, the pseudostem reached 2 cm in diameter, 75% of plants developed branching, and finally because most garlic scapes lost marketability, the field was plowed up and replanted.
③ Improper fertilization can also cause an increase in secondary branching in garlic. Garlic is relatively tolerant of fertilizer and has a high demand for nutrients. However, if too much nitrogen fertilizer is applied in the base fertilizer, under early sowing conditions a growth peak will form before winter, and a large number of branches will be produced after winter. If spring fertilizer is applied too early, nitrogen in the plant will be excessive, and insufficient scape fertilizer later will also promote increased branching, making the main scape thin and reducing yield.
3 Main causes of deformed scape formation
The formation of deformed scapes in garlic is mainly caused by damage from garlic viral disease. In affected plants, the development of new leaves is hindered, the plant type becomes shorter, leaves and pseudostems are deformed and twisted, the garlic scape cannot emerge, and swollen necks occur. The sources of the virus are, first, transmission and damage by green peach aphids and other vectors, and second, because garlic is propagated asexually, the propagation material carries the virus.
4 Measures to prevent secondary branching and reduce deformed scapes
4.1 Sow at the appropriate time and plant at a reasonable density
In order to ensure that garlic can grow well before winter and grow normally after winter, garlic is generally required to grow to 5-6 leaves, with a pseudostem diameter of 0.5-0.6 cm and a seedling height of about 20 cm before winter. According to these requirements, in normal years, open-field cultivation of Ershuizao should be sown in late September, and plastic-film cultivation should be sown in mid-October; open-field cultivation of Sanyuehuang should be sown in early-to-mid October, and plastic-film cultivation should be sown from late October to early November. At this time, seedlings emerge about 15 days after sowing, which is conducive to forming robust seedlings before overwintering.
When sowing, the plant and row spacing should be 6 cm x 15 cm, with about 60,000 plants per 667 sq m as suitable, and the sowing rate is generally about 200 kg per 667 sq m. Ershuizao should be appropriately densely planted, with a density of 70,000-80,000 plants per 667 sq m. This can not only improve the yield and quality of garlic scapes and garlic bulbs, but also effectively prevent the occurrence of secondary branching in garlic.
4.2 Use plastic-film cultivation
In the early and middle stages of garlic growth, it is relatively sensitive to temperature, rainfall and light. In recent years, with warm winters and cold springs, relatively large temperature changes and frequent alternation of dry and wet conditions have directly affected the stable growth of garlic and weakened disease resistance, leading to varying degrees of disease and differing incidence rates of branching and deformed scapes between years. According to observations over the past ten years, whenever temperatures in winter and spring are relatively high and field humidity is high, disease occurrence is more severe, and secondary branching and deformed scapes in garlic are more serious. When temperatures are low and there is little rain in winter and spring, disease is lighter and secondary branching is less. Since the promotion of plastic-film cultivation in 1998, multi-site surveys have shown that garlic branching and deformed scapes have both shown a clear downward trend. Especially in 2004, a year with a major outbreak of secondary branching, the incidence of secondary branching was significantly reduced wherever plastic-film cultivation was used.
The reason plastic-film mulching can control the occurrence of branching and deformed scapes is, first, that plastic film has a warming and heat-preserving effect, improves the ecological environment around garlic roots, and at the same time effectively prolongs the vernalization stage of garlic.
4.3 Scientifically manage fertilizer and water
Garlic should be based mainly on base fertilizer, supplemented by topdressing; based mainly on organic fertilizer, supplemented by quick-acting fertilizer; increase the application of phosphorus and potassium fertilizer, control the amount of nitrogen fertilizer, and actively promote soil-testing formula fertilization technology. In terms of application methods, sufficient base fertilizer should be applied, winter fertilizer should be applied broadly, and scape fertilizer should be applied heavily, preventing excessive growth before winter so that garlic overwinters at the appropriate age, grows steadily in the middle stage, and bolts with strong seedlings, while preventing premature senescence in the later stage. This achieves uniform thickness of garlic scapes, high scape yield and good quality; enhances stress resistance and reduces disease. For general fields, apply 1000 kg of organic fertilizer, 40-50 kg of phosphate fertilizer, and 15-20 kg of urea or 150 kg of high-quality garlic formula fertilizer per 667 sq m as base fertilizer. Apply 1000 kg of decomposed human and livestock manure or 15 kg of urea per 667 sq m as winter fertilizer; apply 15-20 kg of urea per 667 sq m as scape fertilizer to promote garlic bolting.
4.4 Disease control
Garlic diseases mainly occur in the middle and later stages, and the occurrence of diseases will also aggravate secondary branching and deformed scapes in garlic. The main garlic diseases are leaf blight, leaf blight caused by Phytophthora, and leaf spot. Use 70% thiophanate-methyl 800-fold solution, 25% mancozeb 400-fold solution, 75% chlorothalonil 500-fold solution, 50% iprodione 1000-fold solution or 50% procymidone 500-fold solution for alternating spraying in the early stage of disease onset. Soaking seed and spraying leaves with 100-fold solution of 83 Zeng Kang Ji can effectively reduce the occurrence of deformed scapes.