What are the control methods for crown gall disease in chrysanthemums?
Answer content: Key points of chrysanthemum cultivation and management techniques 1. Site selection: Chrysanthemums like moisture and fear drought, and cannot tolerate waterlogging. If soaked by heavy rain for 48 hours after planting, they will all die. Therefore, choose land that is fertile, convenient for drainage and irrigation, and has sufficient light. If converting rice fields for planting, reinforce the main surrounding embankment and ensure sufficient drainage and irrigation capacity, so as to avoid...
I. Symptoms of damageChrysanthemum crown gall disease mainly occurs at the root crown or on lateral roots and branches. The diseased part produces tumors of varying sizes. In the early stage, the tumors are grayish brown, with a smooth surface and not hard texture. Later they become brown, and the surface becomes rough or cracked and hard, causing the aboveground part to lose green color or turn yellow, with small leaves that fall early. Diseased plants grow slowly or die.
II. Morphological characteristics of the pathogen
The pathogen is Agrobacterium tumefaciens, called crown gall wild bacterium or crown gall soil bacterium, belonging to bacteria. The bacterium is short rod-shaped, single or in pairs, size 1.2-3 x 0.4-0.8 (micrometers), with 1-5 peritrichous flagella, Gram stain negative, can form capsules, no spores; grows rapidly on nutrient agar medium, colonies white, round, with neat edges, shiny and semi-transparent. In liquid medium it appears as light cloudy turbidity, with a thin film on the surface. Physiological and biochemical characteristics: does not liquefy gelatin; coagulates milk but does not peptonize it; reduces litmus milk to blue; slightly reduces nitrate; slightly produces acid but no gas on glucose, fructose, arabinose, galactose, mannitol, and salicin; does not hydrolyze starch. The bacterial growth temperature range is 0-37°C, optimum temperature 25-30°C, lethal temperature 51°C for 10 minutes. The bacterium tolerates a pH range of 5.7-9.2, optimum pH 7.3.
III. Disease characteristics
Crown gall bacteria overwinter in the cortical layer of tumor tissue or mixed into the soil. The bacteria can survive in soil for more than one year and spread through rainwater and irrigation water. In addition, underground pests such as white grubs, mole crickets and nematodes can also spread the bacteria. The bacteria invade through insect wounds, mechanical injuries and injuries caused by other root diseases. After invasion, they colonize only in cortical tissue. During parasitism they secrete ?-indoleacetic acid to stimulate surrounding cells to divide faster, enlarge in volume and form crown galls. After host cells become tumors, they can continue to expand even without the pathogen. Temperature and humidity are the main conditions for infection by crown gall bacteria. Bacterial infection and disease occurrence increase with rising soil moisture, and decrease otherwise. Tumors are closely related to temperature: at 28°C tumors grow quickly and large, above 31-32°C they do not form, and below 26°C they form slowly and small. Alkaline soil favors disease occurrence, while acidic soil is unfavorable to the disease. At pH 6.2-8 the bacterium can still maintain its pathogenicity, and below pH 6 the disease is mild or absent. Heavy clay soil and poor drainage lead to severe disease; careless cultivation or damage by underground pests causing wounds all favor bacterial invasion and increase the chance of disease. Contaminated seedlings are the main route of long-distance spread.
IV. Control methods
(1) Strict quarantine. For seedlings suspected of disease, soak in 72% agricultural streptomycin sulfate 1500-fold solution for half an hour or in 1% 96% copper sulfate for 5 minutes, rinse clean and then plant.
(2) Implement crop rotation for more than 2 years in severely affected areas.
(3) Breed and use disease-resistant varieties.
(4) Strengthen field management, cultivate carefully, avoid various wounds, and promptly remove severely diseased plants. For lightly diseased plants, irrigate with antibacterial agent 402 diluted 300-400 times with water, or cut off the tumors and smear with 72% agricultural streptomycin sulfate soluble powder 1000-fold solution, which has a certain therapeutic effect. When necessary, use methyl salicylate-iodine solution, that is, mix 50 parts methanol, 20 parts glacial acetic acid, and 12 parts iodine flakes evenly and apply to the tumor surface and 3 cm around it, which can kill bacteria on the tumor surface.
(5) Biological control. It has been reported abroad that a non-pathogenic Agrobacterium radiobacter strain K84 isolated from the roots of plants with crown gall disease can produce agrocin 84, a nucleotide bacteriocin, which can effectively inhibit tumor tissue formation. Treating seeds or roots with K84 preparation has a very good control effect on crown gall disease.