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Methods for controlling freeze injury in sweet potato cultivation
Sweet potato freeze injury, also called hard-heart sweet potato. [Disease diagnosis] When storage roots are frozen, at first the skin color shows no obvious difference from healthy roots, only slightly losing its luster. Later, the skin can be seen to take on a slightly dark, lusterless appearance; pressing the frozen part lightly with a finger gives a springy feeling; cutting open a frozen sweet potato shows that the flesh near the skin rapidly turns brown, does not exude white milky sap, and squeezing it by hand releases clear water. Generally, based on the speed at which the flesh turns brown, the degree of freezing can be estimated; the faster the browning, the more severe the freezing. Frostbitten storage roots often form a hard heart, taste bitter, and will not cook soft. After storage roots are frozen, they are easily attacked by weakly parasitic fungi, causing rot. ⟦4 <p>Sweet potato freeze injury, also called hard-heart sweet potato. </p><p></p><p>[Disease diagnosis] When storage roots are frozen, at first the skin color shows no obvious difference from healthy roots, only slightly losing luster. Later, the skin can be seen to take on a slightly dark, lusterless appearance; pressing the frozen part lightly with a finger gives a springy feeling; cutting open a frozen sweet potato shows that the flesh near the skin rapidly turns brown, does not exude white milky sap, and squeezing it by hand releases clear water. Generally, based on the speed at which the flesh turns brown, the degree of freezing can be estimated; the faster the browning, the more severe the freezing. Frostbitten storage roots often form a hard heart, taste bitter, and will not cook soft. After storage roots are frozen, they are easily attacked by weakly parasitic fungi, causing rot. </p><p>[Cause] When storage roots are exposed to a low-temperature environment below 9°C for a relatively long time, their physiological functions are disrupted, the flesh tissue is frostbitten, a hard core forms, and the skin and tissue become necrotic. However, after freezing, a period of time must pass before rotting begins. The lower the temperature, the more severe the freeze injury. The longer the low temperature lasts, the more severe the freeze injury. When storage roots suffer freeze injury, rotting generally has two peaks: one is when harvesting is too late in production and the roots freeze in the field, called freeze injury outside the storage pit, and rotting begins about 15 days after being put into the pit; the other is during the middle and late stages of pit storage, when freezing occurs because the pit is shallow or cold protection and insulation are poor and the pit temperature is low, followed by rotting. Generally, at 4-5°C, rotting begins after lasting half a month, and at 6-7°C, rotting occurs only after 1 month. </p><p>[Control measures] (1) Harvest and put into the pit at the proper time to prevent storage roots from freezing. The suitable harvest time for sweet potato should be when the local average temperature of the ten-day period drops to about 14-15°C. Harvest promptly on a sunny day, and the storage roots should be put into the pit on the day of harvest. (2) Disinfect the storage pit. Before the storage roots are put into the pit, a layer of soil should be scraped from the walls of an old pit, and then chemicals should be used for disinfection: fumigate with 30-40 milliliters of 402 water-based fungicide per cubic meter of space, or spray the walls around the pit with a 50-fold formalin solution; in either case, it must be sealed for two days, then opened for ventilation, and used after another two days. (3) Carefully select storage roots. Any storage roots that are diseased, insect-bitten, or frostbitten by frost should be removed and must not be put into the pit. (4) During harvesting, transport, and pit storage, handle them carefully, lifting and setting them down gently, and prevent wounds as much as possible. (5) Manage the temperature and humidity of the storage pit well. To prevent storage roots from freezing and ensure safe storage, the pit temperature should be kept at 11-13°C and the relative humidity at 75%-80%.</p>
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