What are the main storage-period diseases of garlic scapes and garlic bulbs?

In Jinxiang County, Shandong Province, the garlic planting area has been around 500,000 mu in recent years, accounting for 40% of the county's cultivated land area. The county has 700 large and small constant-temperature warehouses, with a storage capacity of 450,000 t. Every year from late April to early-to-mid May, garlic scapes and garlic bulbs are harvested in a concentrated period. Apart from a small amount consumed locally or shipped out, most of the garlic scapes and garlic bulbs are stored. Losses occur during storage, and rot caused by various diseases is the main cause of loss during the storage of garlic scapes.

1 Diseases of garlic scapes during storage

1.1 Gray mold

1.1.1 Symptoms The scape tip is the most susceptible to infection; a thin white mold layer grows on the withered and declined tissue, then spreads to the bracts. The diseased part is pale yellow and water-soaked, densely covered with a white to gray mold layer. The base of the scape stalk and other wounded or diseased parts are also easily affected. In the early stage, the lesions are elongated lengthwise along the scape stalk, pale brown, and sunken; in the later stage, the lesions expand, rotting from the base upward, or rot at other parts to form broken strips, with a white to gray mold layer. There is a strong moldy odor.

1.1.2 Occurrence pattern The disease is caused by a fungus. Its suitable growth temperature is 15-21°C. At a relative humidity above 95%, especially under conditions where water droplets are present, the germination rate of the pathogen spores is high. The pathogen is mainly brought into storage with the garlic scapes from the field, and pathogens remaining in the warehouse are also related to the occurrence of the disease to a certain extent. The pathogen mostly infects from both ends of the garlic scape and from wounded or otherwise diseased parts.

The suitable storage temperature for garlic scapes is 0°C. However, at low temperatures around 0°C, the pathogen still has a certain level of activity. If the temperature is on the high side, it not only favors infection and reproduction by the pathogen, but also accelerates the shrinkage and aging of the scape bracts and the base of the scape stalk, reducing disease resistance and accelerating the development of the disease.

In controlled-atmosphere storage with a relative humidity of 90%-100%, the withered scape tips are in a moist state, and are especially prone to disease and rot after being soaked by condensed water.

Under high-oxygen conditions, the disease is severe. An oxygen concentration of 3%-5% can better inhibit mold and rot than 6.5% or above.

1.2 Viral disease

1.2.1 Symptoms Yellow spots form on garlic scapes, or elongated yellow stripes form lengthwise along the scape stalk, also called "flower face" or "yellow spot." They are more common in the middle and lower parts of the garlic scape and less common near the scape bract. As the storage period lengthens, the tissue at the yellow spots becomes sunken and necrotic, and becomes infected with mold and rots.

1.2.2 Occurrence pattern Garlic scape viral disease is mainly caused by complex infection by two or more viruses, including Garlic mosaic virus and Garlic latent virus. Among them, Garlic mosaic virus alone can cause obvious stripe mosaic symptoms in garlic. Garlic mosaic virus is transmitted through successive generations via seed garlic, and is mainly transmitted non-persistently by aphids in the field. Virus multiplication in the plant body is not limited to the aboveground leaves, pseudostems and garlic scapes, but can also transfer and spread to the underground bulbs. In the garlic growing season, high temperatures and drought, poor management conditions, large aphid populations, and continuous cropping or adjacent cropping of Allium host plants lead to severe disease.

Diseased garlic scapes are all brought with disease from the field, and in the middle and late storage stages, the yellow spots become sunken, necrotic and rotten. When precooling is not timely and the storage temperature is on the high side, the disease develops rapidly.

1.3 Carbon dioxide injury and low-oxygen injury

1.3.1 Symptoms Carbon dioxide injury causes small yellow spots to appear on the garlic stalk, which then expand into round or irregular lesions that are sunken; afterward, the garlic stalk becomes a water-soaked soft strip, commonly known as "boiled garlic scape," and the scape bract becomes necrotic. The sunken spots often join into patches, and when infected by pathogens they rot to form broken strips, and the packaging bag has a strong alcoholic odor. Low-oxygen injury mainly appears as the scape bract fading to white or yellow, the green of the scape stalk becoming darker, and the tip becoming soft, resembling slight water loss.

1.3.2 Cause Caused by unsuitable high carbon dioxide or low oxygen conditions. Generally, injury occurs when the carbon dioxide concentration in the storage environment exceeds 10%-15%. Before symptoms appear in injured garlic scapes, the permeability of their tissue cells increases greatly, causing physiological disorders. As the storage period lengthens, the tolerance of garlic scapes to carbon dioxide weakens, and they are susceptible to injury in the middle and late storage stages. Low-oxygen injury occurs under conditions where oxygen remains below 1% for a long time.

1.4 Garlic bulb dry rot

1.4.1 Symptoms The base of the garlic clove is yellow-brown, gradually shrivels and dries, and an orange-red mold layer may be produced on the diseased part.

1.4.2 Occurrence pattern Garlic dry rot is a fungal disease. The suitable growth temperature for the pathogen is 25-28°C, and the suitable temperature for disease onset is 28-32°C.

The pathogen enters the warehouse with the garlic bulbs, and these diseased garlic bulbs rot during storage. Wounds at the stem base and roots favor invasion by the pathogen. When garlic encounters high soil temperatures near harvest, the disease is most severe. When the storage-period temperature is around 28°C, garlic is most prone to rot, while at 8°C it is very mild.

2 Control methods

2.1 Strict quality requirements before storage Selecting products from areas with no disease or little disease is currently an effective measure to avoid viral disease, and is also very effective for preventing other infectious diseases. During the growing period, pay attention to the control of gray mold, dry rot, and so on. From 1-5 days before harvest, spray 50% procymidone wettable powder at 1500 times dilution, or 40% compound carbendazim suspension concentrate at 800 times dilution, or 50% iprodione wettable powder at 1000 times dilution, or 70% thiophanate-methyl wettable powder at 600 times dilution, etc. Harvest at the appropriate time on sunny days, avoiding rain, sunscald and injury. Quickly precool to 0°C.

2.2 Chemical mold prevention Before use, the cold storage room should be fumigated for 24 hours with 8-10 g of sulfur per cubic meter plus twice its amount of sawdust, or sprayed with 1%-2% formalin. The racks, packaging bags, etc. used should be washed with 0.5% bleaching powder solution and used after air drying. Absorbent paper or cotton balls soaked with 50% sec-butylamine liquid formulation can be placed in the packaging bags, with the amount calculated at 0.1 ml per kilogram of garlic scapes.

2.3 Control storage conditions The ambient temperature is -0.5°C-0.5°C, oxygen 3%-5%, and carbon dioxide 5%-8%.