Precautions for Soilless Cultivation of Flowers and the Occurrence and Control of Crown Gall Disease in Seedlings

Crown gall disease, also known as root crown gall disease, is one of the most common root diseases affecting garden flowers and nursery stock. According to statistics, crown gall disease has hosts in as many as 60 families and over 300 species of plants. It occurs in flowering plants and trees such as cherry blossoms, peaches, Chinese roses, roses, ginkgo, carnations, dahlias, begonias, plum blossoms, lilacs, and geraniums, as well as fruit trees such as grapes, plums, apricots, pears, apples, and kiwifruit. It is a major difficult problem in terms of control.

Damage symptoms. It mainly affects the root crown, main roots, and lateral roots. In nursery stock, it mostly occurs at the junction where the scion and rootstock heal. In the early stage of the disease, small nearly round tumor-like growths appear, which gradually enlarge, harden, develop a rough and cracked surface, and change in color from light to dark brown or black-brown, with the interior of the tumor becoming lignified. The tumors vary in size; large ones may be as big as a fist or larger, and the number ranges from a few to more than a dozen. Because the root system is damaged, diseased flowering plants and trees grow slowly, and in severe cases the entire plant dies. The disease is very stubborn; even if the cancer tumors are removed, they often grow back.

Disease occurrence pattern. The pathogenic bacteria overwinter in the cortical layer of the tumor tissue, or when the tumor tissue rots and ruptures, the bacteria mix into the soil. The cancer-causing bacteria in the soil can also survive for more than 1 year. The bacteria spread through water flow, underground pests, grafting tools, and farm implements used in operations. Long-distance spread can occur through the transport of diseased flower and tree seedlings and cuttings. The bacteria invade the host through wounds. Insect wounds, mechanical injuries caused during cultivation, cut openings on cuttings, grafting wounds, and other injuries can all become routes for bacterial invasion. The occurrence of the disease is greatly related to soil temperature and moisture. High soil moisture favors bacterial infection and disease development; a soil temperature of 22°C is most suitable for tumor formation, and when the soil temperature exceeds 30°C, tumors can hardly form. Soil and pH are also related to disease occurrence. Alkaline soil favors the occurrence of crown gall disease, while acidic soil has less disease. Soil that is heavy in texture, low-lying, and poorly drained has more severe disease, and continuous cropping of flowers and trees leads to severe disease. In addition, in flower and tree nursery land with extensive cultivation and management, many underground pests and soil nematodes, and various mechanical injuries, the disease is more severe. If the wounds of cuttings do not heal well during temporary planting, the seedlings produced are also prone to the disease.

Control methods. First, strengthen quarantine. For nursery stock leaving the nursery or introduced from outside, diseased plants should be discarded. For suspicious nursery stock, they can be soaked in 500-2000 PPM streptomycin solution for 30 minutes, or soaked in 1% copper sulfate solution for 5 minutes, or soaked in 2% lime water for 1-2 minutes. After rinsing with clean water, plant them. When establishing plantings, avoid introducing nursery stock or cuttings from diseased areas. When diseased nursery stock is found, it should be thoroughly removed and burned. Second, strengthen cultivation management. In severely diseased areas, implement crop rotation for more than 2 years or sterilize the soil with chloropicrin before planting. In neutral or slightly alkaline soil suitable for the occurrence of crown gall disease, apply more organic fertilizer to increase soil acidity and improve soil structure; during intertillage, weeding, and other operations, try to minimize root injury or damage to the base of stems and vines of flowers and trees; pay attention to timely control of underground pests and soil nematodes to reduce insect wounds; after rain, pay attention to draining accumulated water, reducing soil moisture, promoting the growth of flowers and trees, and improving disease resistance. Third, scrape off diseased tumors or remove diseased plants. When it is found that a nursery or planted flowers and trees are infected with the pathogen, the surrounding soil should be promptly opened up, and the tumors should be thoroughly excised with a scraper until disease-free xylem is exposed, and the wounds should be protected with high-concentration lime sulfur mixture or Bordeaux mixture to prevent reinfection. Severely diseased plants that cannot be treated should be promptly removed and the remaining roots thoroughly collected and burned in a centralized manner. For lightly diseased plants, "402" antibacterial agent at 300-400 times dilution can be used to irrigate the roots, or after excising the tumor body, 500-2000 PPM streptomycin or 500-1000 PPM oxytetracycline, or 5% ferrous sulfate can be applied to the wounds. It is also reported that applying methyl-ice-iodine solution (50 parts methanol, 25 parts glacial acetic acid, 12 parts iodine flakes) to the tumors has a therapeutic effect. Biological control can also be carried out by applying Agrobacterium radiobacter 84. Fourth, grafting should avoid letting wounds contact soil, and grafting tools can be sterilized with 75% alcohol or 1% formaldehyde solution.