Symptoms and Integrated Control Measures of Plum Root Nodule Disease
Symptoms and Integrated Control Measures of Plum Root Nodule Disease
Fund projects: Sichuan Province 13th Five-Year Breeding Research Funding Project (2016NYZ0034); Sichuan Province 12th Five-Year Breeding Research Funding Project (2011NZ0098-8).
1 Symptoms
In the early stage of plum root nodule disease, the above-ground symptoms of affected plants are not obvious, mainly manifesting as slow growth, stunted plants, leaf yellowing, and premature leaf drop. In the initial stage of plum root nodule disease, grayish-white tumor-like growths form on the diseased parts, with a rough surface and soft internal tissue. As the nodules gradually enlarge, the epidermis dies off and turns brown to dark brown, while the internal tissue becomes hard and lignified. The diameter of the nodules varies. If the root collar or main root has large or numerous nodules, the number of roots decreases, and the ratio of nodule diameter to trunk diameter exceeds 1, the plant's vigor declines and yield decreases. If the root collar or roots have exceptionally large nodules, causing blockage of transport, the number of roots is extremely reduced, and the tree vigor is extremely weakened or even leads to death [Jishan Huayao][.
2 Transmission Routes and Causes
2.1 Transmission Routes
Plum root nodule disease has extensive transmission routes, including rain, wind, insects, transportation, and human farming activities. When introducing seedlings or rootstocks from epidemic areas, the root nodule pathogen is brought in and spreads over short distances and small ranges through rain, unreasonable irrigation, underground pests, and farming activities. The transportation and sale of infected seedlings is the main route for long-distance spread of the root nodule pathogen.
2.2 Causes
Plum root nodule disease is prone to occur in spring and summer, with many causes, and the main causes are pathogen-carrying seedlings and unreasonable cultivation and management measures. First, pathogen-carrying seedlings. When introducing seedlings or rootstocks, the seedlings themselves carry the pathogen, and no disinfection treatment is carried out at planting. Second, improper cultivation and management measures. Improper soil and fertilizer management during cultivation, excessive application of chemical fertilizers in orchards, and no or only a small amount of organic fertilizer applied; unreasonable drainage and irrigation in orchards, failure to promptly and reasonably drain excess moisture during continuous rainy weather, and unreasonable irrigation during drought; mechanical injuries caused during intertillage, weeding, grafting, and other farming operations, and failure to promptly control underground pests and other improper cultivation and management measures can easily lead to the occurrence of this disease.
3 Integrated Control Measures
3.1 Strict Seedling Testing and Disinfection to Control the Pathogen from the Source
When introducing normal seedlings from non-epidemic areas, because the root nodule pathogen has a certain incubation period, it cannot be judged by visual observation alone. All introduced rootstocks and seedlings must be disinfected before soil heeling-in and planting. That is, soak the portion below the graft union of the seedling or the entire root system of the rootstock in 75% alcohol for 30 s, then soak in 0.1% mercuric chloride solution for 6-8 min, and rinse clean with water; or soak in K84 bacterial solution for 5-7 min.
For self-propagated seedlings, first select a plot where root nodule disease has never occurred as the nursery, then treat seeds with K84 during sowing. When seedlings leave the nursery, seedlings visibly bearing nodules must be resolutely removed and destroyed, and usable seedlings should be soaked in K84 bacterial solution using the same treatment method as above.
3.2 Reasonable Cultivation to Create Environmental Conditions Unfavorable for the Reproduction and Spread of the Root Nodule Pathogen
3.2.1 Scientific orchard establishment. Root nodule disease is severe in plots with heavy clay soil and poor drainage. Therefore, it is not suitable to establish orchards in low-lying wet fields or flatland areas in mountainous water-collection line zones; instead, orchards should be established with deep furrows and high beds. On gentle slopes, orchards should be established by gathering soil into ridges to improve the soil, enhance soil aeration and moisture drainage capacity, and create an environment unfavorable for pathogen reproduction.
3.2.2 Scientific fertilization. Reduce the amount of chemical fertilizer applied in plum orchards and increase the application of organic fertilizer. Alkaline soil favors disease occurrence, while acidic soil is unfavorable to disease occurrence. Therefore, organic fertilizer and physiologically acidic fertilizer should be added to plum orchards on alkaline soil to adjust the soil pH to slightly acidic, which is unfavorable for pathogen reproduction.
3.2.3 Timely and reasonable drainage and irrigation. Rainwater and irrigation water are the main media for the spread of the root nodule pathogen. In plum orchards, especially flatland plum orchards, drain promptly after heavy rain, and irrigate reasonably during drought. Do not use "connected drainage and connected irrigation," thereby cutting off the routes by which the pathogen spreads through rainwater and irrigation water.
3.2.4 Scientific farming operations and timely control of underground pests. During intertillage and weeding, try to avoid causing wounds on the root collar; during seedling propagation and top grafting for variety replacement, pay attention to protecting the graft union, disinfect tools with 75% alcohol, and avoid aggravating susceptibility to disease due to farming operations. Underground pests such as white grubs and mole crickets can damage the plum tree root system, leading to infection by the root nodule pathogen. Therefore, underground pests must be controlled promptly to reduce the occurrence of root nodule disease.
3.2.5 Timely control when diseased plants are found. When nodules are found on the root collar or graft union of fruiting trees, cut them out promptly, and apply 5°Bé lime-sulfur mixture, K84 bacterial solution, agricultural streptomycin 400-500 times solution, or 3% colloidal copper succinate 300 times solution to the wounds for sterilization. The excised nodules must be taken out of the orchard and destroyed in a centralized manner.