Causes and Control Methods of Bacterial Angular Leaf Spot
According to the Huaping County Organic Late-Maturing Mango Research Center, this year's temperatures in Huaping differ from previous years: in the early period (February-April) they fluctuated between high and low; from May to mid-June, temperatures were relatively high with severe sun exposure; this caused severe outbreaks of bacterial angular leaf spot in some mango-growing areas. Fruit growers sprayed pesticides again and again but still could not bring it under control.
Bacterial angular leaf spot mainly damages leaves and fruit. On leaves, the initial symptoms are water-soaked light green spots that later turn pale brown and appear angular because they are restricted by the leaf veins. In the later stage, the lesions turn grayish white and tend to perforate. When humidity is high, white slime is produced on the lesions. On stems and fruit, the lesions are initially water-soaked and nearly round, later turning pale gray; cracks often form in the center of the lesions, and bacterial ooze is produced under moist conditions. The fruit rots in the later stage and gives off a foul odor.
1. Why is angular leaf spot on mango so difficult to treat?
In summary, there are the following reasons: 1. The vast majority of fruit growers emphasize treatment over prevention. Most fruit growers rely on spraying pesticides to control mango pests and diseases, spraying only after disease occurs, and often use
organic copper fungicides, inorganic copper fungicides, agricultural antibiotics, thiazoles and microbial preparations, etc., to spray the leaves of fruit trees or the fruit. Because the pathogenic bacteria lurk inside the tree body and, when disease develops, emerge from the inside out, spraying chemicals can only cure the surface, but the pathogenic bacteria inside the tree body are not killed, so once suitable weather occurs the disease recurs. This chemical control method can only achieve a temporary effect; the lesions dry up after 1-2 days and appear to have healed, but once cloudy and rainy weather or high-humidity weather occurs, the lesions begin to exude bacterial ooze again. This method, which treats the symptoms but not the root cause, requires repeated spraying, wasting time, effort and money, with unsatisfactory results.
2. Poor awareness of prevention and protection. During production, some fruit growers carelessly discard diseased branches, leaves and fruit, so year after year the sources of disease in the orchard accumulate excessively, and once an outbreak occurs it is very difficult to control.
3. Angular leaf spot can spread through the air. Fruit growers generally prevent and treat disease individually rather than through joint prevention and control. Since planting areas are mostly contiguous and orchards are connected to one another, once adverse weather occurs, a large area becomes affected.
4. Unscientific use of chemicals. Many fruit growers do not have an adequate grasp of pesticide knowledge, often causing phytotoxicity, and phytotoxicity can also trigger large-scale outbreaks of angular leaf spot.
2. How can bacterial angular leaf spot of mango be controlled scientifically?
First of all, one must know the occurrence pattern of bacterial angular leaf spot; understand the disease-triggering factors, and adopt corresponding effective control measures, with prevention as the main approach and integrated control, so that the disease can be controlled at the budding stage and yield twice the result with half the effort. Bacterial angular leaf spot, also known as bacterial black spot or bacterial canker, is an important and frequently occurring disease of mango. It often causes large numbers of leaf spots and fruit spots, seriously affecting tree growth as well as fruit appearance and commercial value. Anthracnose fungi and stem-end rot fungi often invade the fruit through the lesions, inducing extensive fruit rot during storage.
(1) Symptoms: Angular leaf spot mainly damages mango leaves, branches, flower buds, flowers and fruit
On leaves, water-soaked small spots first appear and gradually enlarge and turn dark brown; the edges of the enlarging lesions are often restricted by leaf veins and appear angular; sometimes multiple lesions merge into larger lesions. The surface of the lesions is slightly raised, often surrounded by a yellow halo, and the midrib and petiole may also be affected and split longitudinally. On branches, the lesions are dark brown and canker-like; when a lesion enlarges and encircles a young branch, it can cause the shoot to die. On fruit, water-soaked small spots appear first, then enlarge and turn dark brown, with a raised surface and cankerous cracking. The common symptoms of the affected parts are: dark brown lesions, raised surface, often with a yellow halo around the lesions, and when humidity is high, the diseased tissue often exudes a sticky fluid. In addition, on highly susceptible varieties it can also cause flower buds and leaf buds to die. The wounds formed by this disease can also become entry points for anthracnose fungi and stem-end rot fungi, inducing extensive fruit rot during storage.
(2) Occurrence pattern: Angular leaf spot is a bacterial disease. Diseased leaves, diseased branches, diseased fruit, diseased residues in the orchard, infected seedlings, and host weeds in or around the orchard are the primary infection sources of bacterial angular leaf spot of mango. The pathogen can spread and disperse through airflow, infected seedlings, wind, rainwater, etc. The pathogen enters through wounds and natural openings such as stomata on leaves and fruit to cause disease. The optimum temperature for pathogen development is 20-25°C; high temperature and frequent rain favor the occurrence of this disease. Generally, areas with strong winds, windward zones or low-lying orchards are more severely affected, while wind-sheltered orchards and orchards at higher elevations are less affected. At present, the main mango varieties differ somewhat in resistance to bacterial angular leaf spot, but there is no immune variety.
(3) Control measures
1. Strengthen quarantine. Prevent the pathogen from spreading with infected seedlings, scions and fruit.
2. Carry out disease prevention measures well. (1) Strengthen fertilizer and water management, improve the soil environment, and strengthen the tree body: rationally combine organic fertilizer, microbial fertilizer, medium and trace element fertilizer, and functional compound fertilizer to improve the soil environment, enhance the formation of soil aggregates in the orchard, activate soil organic matter, increase beneficial bacteria, inhibit and kill harmful pathogenic bacteria in the soil, improve the reasonable nutrient supply to the tree body, strengthen tree vigor, and improve the overall resistance of the tree body. (2) Do a good job of orchard sanitation: during production, promptly prune off diseased leaves, diseased branches and diseased fruit, and burn or deeply bury them in a centralized manner; after harvest, clear diseased residues from the orchard and do a good job of orchard cleaning to reduce sources of infection. (3) Carry out tree shaping and pruning at the right time and to an appropriate degree, so that the canopy and orchard remain well ventilated and light-transmitting, reducing the chance of disease. (4) At the dry-wet transition period, that is, before and after wind and rain, promptly spray copper preparations or other bacterial agents, 1-2 times in succession, for prevention. (5) If it is during the fruit stage, you can supplement pure organic calcium fertilizer on the leaves 2 times in advance, together with bacterial agents, to effectively prevent the occurrence of angular leaf spot.
3. Wind protection. If conditions permit, you can try planting windbreak forests around the orchard to reduce the impact of strong winds and heavy rain, which can lessen the disease.
4. Remedial measures after disease occurrence. It is recommended to rotate the use of organic copper fungicides, inorganic copper fungicides, agricultural antibiotics, thiazoles and microbial preparations and other bacterial agents. Spray at least 2 times in succession, especially after wind and rain; it is best to use a bacterial agent each time. A brief introduction to bacterial agents is as follows:
a. Organic copper fungicides: common organic copper preparations include quinoline copper, thiabendazole copper, copper ammoniate, copper abietate, copper succinate, nonyl copper, thiasen copper, etc. This class of fungicides has good control effects on some bacterial diseases. Quinoline copper and thiabendazole copper are used more often. The advantages of this class of fungicides are that they are safer and generally do not cause phytotoxicity, and can also be used during the flowering and young fruit stages; their scope and timing of use are broad, and they are suitable for fruit trees, rice, vegetables, etc.; their copper content is relatively low, they do not cause mite proliferation, and copper accumulation is small; they have good miscibility and are convenient to use, reducing the burden. The disadvantage of organic copper preparations is that the price is generally relatively high.
b. Inorganic copper fungicides: common inorganic copper fungicides include: copper hydroxide, cuprous oxide, Bordeaux mixture, copper oxychloride. The advantages of this class of fungicides are that they act quickly, are relatively low in price, and have a comparative cost advantage. Their disadvantages are that they easily cause phytotoxicity, and are prohibited or restricted during the flowering and young fruit stages; their miscibility is relatively poor, and traditional copper preparations are mostly alkaline, cannot be mixed with other pesticides, and need to be used alone, increasing labor costs, and it is very easy to miss the optimal disease control period. Among them, neutral (pH=7) inorganic copper preparations have better miscibility.
c. Antibiotic fungicides: antibiotic fungicides, with the more commonly used agricultural streptomycin, kasugamycin, zhongshengmycin, jingangmycin, oxytetracycline hydrochloride, cycloheximide, etc., all being currently widely applied major antibiotic varieties. When using agricultural antibiotics, suitable application methods should be chosen according to different plant diseases. For plant diseases transmitted through soil, they can be used as soil disinfectants; for plant diseases transmitted through seeds and seedlings, antibiotics can be used for soaking, such as seed soaking, root soaking, seedling soaking, etc., or spraying on the seedbed; for diseases on the aboveground parts of agricultural and forestry crops, plant spraying can be chosen at an appropriate time; when used on fruit trees, they are mainly made into oil paste to control bacterial canker. The advantages of this class of fungicides are that they are relatively widespread on the market, can still be sold without advertising, and have relatively good systemic and therapeutic effects; the disadvantages are that they have a long history of use and the products have strong pesticide resistance.
d. Thiazoles: Thiazoles include bismerthiazol, thiazole zinc, etc. They were first used to control rice bacterial leaf blight, and in recent years, because the area of cash crops has expanded and bacterial diseases occur commonly, they have regained vitality; this class of compounds has no bactericidal activity under in vitro conditions, and their control of soil bacteria is limited; homogenized competition is common, and they are common products for controlling plant bacterial diseases. (5) Microbial preparations: active microbial preparations, such as Paenibacillus polymyxa, Pseudomonas fluorescens, Bacillus subtilis and Bacillus spp., have become newly developing microbial preparations. Such biological pesticides are generally safe for the environment and have an improving effect on soil, and can safeguard the health and safety of food, animals, plants and humans, but farmers' market acceptance is somewhat poor.