Citrus Huanglongbing, also known as yellow shoot disease and yellow blight, is a domestic quarantine object. Mainly distributed in low-altitude areas of Taiwan Province, Hainan, Guangdong, Guangxi, Fujian, Yunnan and other provinces (regions). Ganzhou, Jiangxi, Chenzhou, Hunan, Wenzhou, Zhejiang, Xichang, Ferry, southern Guizhou and other places have also occurred. The disease was first discovered in Chaoshan area of Guangdong province more than 0/00 years ago. The first outbreak occurred in 1930, and 70-80% of citrus reticulata suffered from it. The second epidemic occurred around 1943, and the third epidemic occurred around 1955. 1960- 1978, citrus production in many provinces in South China was seriously damaged. Within 2-3 years after the onset of the disease, the citrus orchard completely lost its production capacity and economic cultivation value. New citrus orchards adjacent to seriously ill orchards are often destroyed before they are put into production. The harm and control of Huanglongbing is the main problem in citrus production in southern and southwestern parts of China, and it is the biggest obstacle to the large-scale development of citrus.
Symptoms (Figure 2- 1)
Figure 2- 1 Citrus Huanglongbing
1. diseased branches 2-4. Unhealthy fruit. Healthy fruit 6-7. Mottled diseased leaves 8. Evenly yellowed diseased leaves 9. Thrombotic diseased leaves 10- 1 1. Absence of diseased leaves
This disease can occur all year round. Symptoms are manifested in branches, leaves, flowers, fruits and roots. Early symptoms of bamboo shoots appear in spring, summer and autumn. Summer bamboo shoots are the most common, followed by autumn bamboo shoots, and bamboo shoots are the least. Young trees also have a few diseases in winter bamboo shoots. But its severity is the heaviest in spring shoots and the lightest in summer shoots.
Leaf symptoms are the main basis for differentiating Huanglongbing. The most typical symptom in the early stage of the disease is the appearance of one or two branches or several yellow buds in the dense green crown, commonly known as "yellow head of a chicken" in Guangdong. However, the symptoms on the leaves are different at different onset periods. In the early stage of summer and autumn shoots, individual or partial leaves of new shoots turn yellow and appear mottled. After the leaves turn green, they turn yellow from the vicinity of the main lateral vein, the base or edge of the leaves, and the yellow part gradually forms yellow-green mottle, and then the whole leaves turn yellow after the leaves turn green. The leaves do not turn green during the growth process, showing uniform yellowing. The main lateral veins remain green, and only the mesophyll between veins turns yellow, which is similar to zinc deficiency or manganese deficiency. Among the etiolated types, variegated type is the most representative and the main feature of Huanglongbing. On the first-grade diseased branches, mottled yellowing and uniform yellowing leaves are the majority, and the new shoots and leaves on the diseased branches are mainly mottled yellowing and yellow-lacking.
Symptoms of Huanglongbing on spring shoots, although the newly harvested spring shoots can turn green normally in that year, after May, some or most of the new leaves of diseased trees turn green and yellow. The veins of diseased leaves are yellow-white or light green, and the leaves are hardened, mottled yellow-green or even yellow or green. The symptoms of spring seedlings develop rapidly. After the onset of spring shoots in that year, 4-5-year-old young ponkan or banana citrus trees will become ill in winter.
The symptoms in summer, autumn and winter are basically the same. There are 1 to several shoots on the banana tree, and the new buds and leaves can't turn green, and the yellowing is not obvious, and the main veins are light and slightly shiny. After ripening, mesophyll becomes even yellow-green or yellow-white or yellow-green variegated. Leaves are leathery, thicker than healthy leaves, rough to the touch, dull, swollen veins, partially corked and cracked. A few summer, autumn and winter buds turn from green to yellow only after the leaves are mature, the veins of diseased leaves are enlarged and the mesophyll is mottled with yellow and green. The disease tip is hard, the skin is yellow and white, and the blade insertion angle is small. Most of the diseased leaves on ponkan trees first turn orange yellow, which is similar to element deficiency yellowing, and then turn into uniform yellowing or mottled yellowing. The diseased leaves of lemons and grapefruit often have yellow and green spots. The diseased leaves of tangerine and citrus showed more variegated types. Like banana oranges, the diseased leaves in the new buds turn yellow evenly.
In summer and autumn, some leaves of new shoots turn yellow, showing obvious symptoms of "yellow shoots", and then the lower branches and branches of other parts of the crown become ill one after another, which is the characteristic of new diseased trees. "Yellow bamboo shoots" mostly appear at the top of the tree crown, and then gradually expand. General 1-2 years later, the whole plant becomes ill. With the onset of big branches, the symptoms of leaves are mainly mottled and uniform yellowing, while the latter is mainly mottled and yellow-lacking. New buds can be drawn from the diseased branches of mild plants, but the new buds are short and weak, and the leaves are small.
In the middle and late stage, the diseased leaves of "Yellow Shoot" that occurred in that year fell off in late autumn, and the varieties with strong disease resistance often did not fall off in late autumn, but all the leaves fell off before germination in the following spring, and the diseased branches died. Some undead diseased buds germinate earlier, and new buds grow short and thin. In the wide-skinned oranges of bananas and ponkan, some diseased branches have small, narrow and hardened diseased leaves, most of the veins are swollen, yellow-green or yellow-white, and the leaves are orange-yellow. It is characterized by "iron deficiency and zinc deficiency". On the diseased leaves of citrus and grapefruit, there are many symptoms of "zinc deficiency", but the leaves are not obviously smaller, the leaves are not thick but soft, and the veins are not swollen. The diseased leaves of tangerine mostly show the symptoms of "lack of nutrients", which are slender and hard, the main lateral veins are swollen, the leaves are cork, and the leaves are curled towards the leaves.
/kloc-After the new shoots of 0/2-year-old young trees are "yellow shoots" in the same year, the whole tree will turn yellow in the next shoot-out period or winter, but it will take 2-3 years for adult trees or diseased trees with large crowns to turn yellow. When the top of a main branch becomes Huang Shi, it gradually expands from top to bottom and then spreads to another main branch nearby. Branchlets at the base of the main branch turn yellow first, and then expand from bottom to top. This transmission process seems to be related to the activities of pathogens in trees.
On the symptoms of flowers and fruits, diseased trees tend to bloom earlier than healthy trees, with large flowers and small flowers, a large proportion of deformed flowers and leafless flowers, short and plump petals, yellow and dull, and finally almost all of them fall off, with no or little fruit. Even if the results are not ripe, they are all gone.
The diseased tree has many flowers, few top leaves, small and deformed fruits (the navel of the fruit often inclines to one side), smooth skin, little juice, light and sour taste, uneven yellow and green when coloring, forming "painted face" fruits (such as Fu orange, October orange, etc. ) or colored near the pedicel, and the rest is turquoise, which becomes "red nose fruit".
At the root, there was no obvious change at the initial stage of the disease, and with the aggravation of the disease, the root gradually rotted. The severity of root rot of diseased trees corresponds to the severity of branch disease. The fibrous roots of diseased trees decreased, and the root tips rotted and browned. Later, the big roots gradually rotted, and there were fewer new roots. By the late stage of the disease, all the taproots rot, the cortex falls off and the whole plant dies.
Yellowing symptoms caused by Huanglongbing are different from those caused by other reasons. The lack of nitrogen and water damage caused the yellowing of citrus, and the whole plant had no "mottled" symptoms, and the leaves were soft and unchanged. Lack of trace elements is also yellowing of the whole plant, and there is no "mottling" symptom; Yellowing caused by foot rot and Anoplophora glabripennis. Or by girdling the bark, the leaves or veins of the whole plant turn yellow, and there is no "mottling" symptom in the later stage. However, the yellowing caused by Huanglongbing gradually spread from several "yellow buds" to the whole plant, causing yellow-green "mottling" symptoms on the leaves.
cause of a disease
According to the results that the pathogen of Huanglongbing was sensitive to tetracycline antibiotics and penicillin, the disease was spread by citrus psylla, and the pathogen of Citrus Huanglongbing was Rickettsia-like. Most bacteria are round, oval and spindle-shaped, and a few are irregular. Its periphery consists of three layers of membranes, the thickness of the boundary membrane is13-33 nm, the cell size is160-830 nm, and the spindle cell can be as long as160-830 nm. Mitochondrial structure of ribosomes and DNA-like proteins. This pathogen is different from spiroplasma and mycoplasma, which are only 7- 10 nm thick and insensitive to penicillin.
Pathogens have poor heat tolerance. Treating diseased scions or seedlings with hot and humid air at 49℃ for 50-60 minutes can obviously reduce the occurrence of diseases. 13, when the field temperature reached above 40℃, the symptoms disappeared temporarily, but after the temperature dropped, the symptoms reappeared after a period of time.
At present, only the host range of Huanglongbing is found to be Murraya murraya, which harms citrus, kumquat and Fructus Aurantii, and Murraya murraya.
Occurrence law
The primary infection sources of Huanglongbing are mainly diseased plants and seedlings in orchards. Long-distance transmission mainly depends on the transportation of diseased seedlings and diseased scions. In the old ward, it mainly comes from diseased trees in the orchard. Adult and nymph of citrus psyllid. ) Suck the sick juice, and the swimming cycle of the bacteria in the psylla is about 30 days. After breeding in psyllid, the pathogen enters the saliva of sucking mouthparts, and spreads to healthy plants when it flies to disease-free citrus trees to feed, causing diseases. The incubation period of the pathogen in Eureka lemon and sweet orange seedlings is about 10 month. The incubation period of disease transmission by diseased bud grafting is 8- 10 months. Pathogens propagate and move in the phloem of trunk, branch and leaf cortex, and the crown appears yellowing and mottling, and finally it is serious until death. Therefore, after diseased trees or seedlings appear in an area or orchard, the citrus psyllid spreads in a short distance (100-200m), which makes the incidence rate of an orchard reach 70- 100% in 2-3 years, and eventually leads to the destruction of the whole orchard. When the distance is greater than 1km, the spread of psyllid can be slowed down when there are shelter forests, Woods or large buildings and rivers.
Juice friction will not infect diseases, and neither will soil. The seeds of bergamot and Limonium sinense may spread the disease, which needs further experiments to confirm.
Epidemic conditions of citrus Huanglongbing
The population density of citrus psyllid is positively correlated with the occurrence of Huanglongbing. For example, in a state-owned citrus farm in Boluo County, Guangdong Province, in the 20 years before 1953- 1974, citrus psylla rarely occurred. There were 960,000 citrus trees in a 2 1500 mu citrus farm, and the incidence rate was less than 1%,1977-. In addition, the investigation in three citrus orchards in Chenghai county, Guangdong province also shows that there are many pests and diseases; The relationship between many diseases and many insects (Table 2- 1). Because of the large population of psyllid, there are many opportunities to spread diseases and the disease is serious. In addition, due to the few diseased branches and leaves, poor growth, few fruits, good light, many branches, long germination time and abundant food, psyllids mostly concentrate on these diseased trees to harm and suck, and then pass the disease source to healthy plants.
Table 2- 1 Relationship between population density of citrus psyllid and Huanglongbing
Relationship between the distribution of citrus psyllid and the occurrence of Huanglongbing. In the past, it was thought that geographical latitude and altitude were closely related to the occurrence of Huanglongbing, that is, most orange areas in the north of Xichang, Sichuan, Guilin, Guangxi, Shibai, Ganzhou, Jiangxi, Fu 'an, Fujian and Fuzhou did not have the occurrence and harm of Huanglongbing, which was the northern line of disease distribution. There is little or no disease at high altitude. The research results in recent years have proved that the distribution of Huanglongbing is completely controlled by the distribution and density of psyllids. For example, in Guangxi, from south to north, psyllids have gone from existence to non-existence, and Huanglongbing has gone from serious to non-occurrence. In Xichang, Sichuan, from low to high mountains, psyllids are from many to few, and Huanglongbing is from severe to mild. There are a large number of psyllids in the orange orchard at an altitude of 1465m in Binchuan, Yunnan, and almost 100% of the orange trees are infected. In the orange orchard at an altitude of 1600m, there are few psyllids, and only a few percent of the plants have Huanglongbing. In areas far from the northern limit, the monthly temperature of 1 month is mostly below 7℃, so psyllid can't overwinter in these places, let alone lay eggs and breed, so Huanglongbing can't happen, even if it happens, it can't spread, and eventually it will die with dead trees. At high altitude, the winter temperature is low, foggy, humid, short sunshine, large temperature difference between day and night, orange trees grow vigorously, and bamboo shoots are picked less often. These environmental conditions are not conducive to the survival and occurrence of citrus psylla, so Huanglongbing will not be popular. The living range of citrus psyllid is the area where Huanglongbing occurs. In an area, psyllids often occur before Huanglongbing. The host of citrus psylla can not only harm citrus and murraya murraya, but also harm Guangdong wine cake gluten and smelly grass. Whether all the plants harmed by citrus psyllid have Huanglongbing pathogen or Huanglongbing-like diseases needs further study and proof.
The population number of psyllid in orange orchard and the appearance of new citrus shoots are consistent with the peak period of Huanglongbing. Generally speaking, productive orange orchards produce spring shoots from February to April, summer shoots from June to July, and autumn shoots from August to September. The germination period of the three new shoots is the three peak periods of the occurrence and damage of the adult psyllid and the three peak periods of Huanglongbing, and the mature and aging period of the three new shoots is the low tide period of the population decline of psyllid. In the orange area of South China, psyllid can reproduce all the year round when there are buds sprouting all the year round, so the occurrence and harm of Huanglongbing is particularly serious. As long as there are buds, various insect states can be seen every year. Without the buds of citrus, larvae and nymphs can't survive. Bud can stimulate the development of adult psyllid eggs. Without buds, the eggs in the body cannot mature. The process of adult ovary maturation in winter and spring is closely related to the germination of citrus varieties. In a word, the population density of psyllid is influenced by food, climate, environment, spraying and natural enemy activities, among which food and climate are the most important. In general, the population density of diseased trees and weak trees is 10 to dozens of times higher than that of healthy trees, so Huanglongbing occurs seriously in diseased orchards, aging orchards and young orchards near diseased orchards. When the incidence of young orchards exceeds 10%, or the rate of diseased plants in adult orchards reaches more than 20%, orchards will be destroyed in pieces within 2-3 years. On the contrary, if there are few diseased seedlings and trees in citrus orchards, they can be cut down and burned in time, and cultivation management should be strengthened, and attention should be paid to the prevention and control of infectious insect psylla, then Huanglongbing will occur slowly and the harm will be less.
Relationship between light conditions and disease occurrence
In citrus orchards with few shaded leaves and plenty of sunshine, the air convection is strong, the relative humidity is low, and there are few barriers, so the activities of psyllids are frequent, and Huanglongbing spreads rapidly and is seriously harmful (Tables 2-2 and 2-3). On the contrary, citrus trees planted in budding forests have fewer psyllids and fewer diseases. Generally, citrus planted in the open space in the forest is not sick, and the condition is alleviated after the diseased trees are shaded. In a shady environment, the water is abundant, the humidity is high, the air temperature and the temperature on the leaves are relatively low and stable, and the light is mostly scattered, which is beneficial to the growth of citrus, and the leaves are large and dark green. In addition, the number of shoots is small, and there are not many psyllids, which are generally not pathogenic.
Table 2-2 Relationship between population density and incidence of psyllid under different light intensities
Table 2-3 Incidence of Murraya murraya under Different Lighting Conditions
The development and harm of Huanglongbing in mountain citrus orchards with shelterbelts, forests and mountains are slow and light; On the contrary, in the plains and low hilly areas where there are no isolation conditions such as shelterbelts, the disease occurs early, spreads quickly and does serious harm.
Relationship between tree age and infection rate
Generally, old trees have stronger disease resistance than young trees, and the infection of Huanglongbing develops slowly. The old tree has a large crown, and the pathogen causes the whole plant disease for a longer time than the young tree. If a new citrus orchard is built near the seriously ill old citrus orchard, or young trees are replanted in the seriously ill citrus orchard, the newly planted young trees often die faster than the old trees. Fruit farmers in Chaoshan, Guangdong Province call it "planting before death, planting before planting". The reason is that the newly planted young trees shoot more times than the old trees, and the citrus psylla has more opportunities to spread poison on the young trees, so it often dies first.
The more diseased trees there are in the orange orchard, the more serious the disease is. When a few plants in the orchard began to get sick, most of these scattered branches were in the sunny part of the crown. In late autumn, winter and early spring, psyllids like to live on branches in the southeast and southwest with good light and high temperature. Moreover, the sprouting time of diseased branches on diseased trees is earlier than that of healthy trees, and psyllids concentrate on these diseased buds for feeding, spawning and wintering. The next spring, when the healthy trees around dried up the new buds, the adult psyllid began to do harm, spread diseases, reproduce and Huanglongbing became more and more serious. However, due to the weak flying ability and small range of activities, psyllid has formed a disease heart that spreads around after its onset. In order to prevent the spread of diseases, diseased trees and psyllids must be thoroughly removed.
Relationship between cultivation management and Huanglong disease
After a bumper harvest, if management fails to keep up, the vitality and disease resistance of citrus trees will be weakened, and Huanglongbing will easily occur in the following year, and it will rapidly decline and die. On the contrary, orange orchards with good water and fertilizer management, timely irrigation and drainage, thorough control of psylla, no drought and no waterlogging have less diseases and slow speed.
Disease resistance of citrus varieties
Among the existing citrus varieties, they are all susceptible to Huanglongbing in different degrees, among which banana orange, ponkan orange, tea orange and Nian orange are the most susceptible, and Wenzhou mandarin orange, sweet orange, pomelo and lemon are the most resistant. Lime is less affected. Fructus Aurantii is the lightest and has no obvious symptoms in the field.
The effect of rootstock on scion disease resistance is generally not obvious. Plants grafted on common rootstocks such as lime, bitter orange, lemon and Fuju, and plants grafted on rootstocks such as sweet orange, bitter orange, orange and rough lemon are prone to diseases, and there is no obvious difference in the severity of diseases.
Prevention and cure method
The research results of more than 40 years have proved that plant quarantine is an important administrative regulation to protect epidemic-free areas and new areas. Establishing disease-free nursery, cultivating and planting disease-free seedlings are the basis of preventing Huanglongbing. The key measures to prevent the occurrence and spread of Huanglongbing disease are to eliminate psyllids that spread the disease in time and dig out the diseased trees thoroughly. When developing new orchards in epidemic areas, we should plant them in isolated places as far as possible to avoid and reduce the harm of citrus psylla and the spread of diseases, and reduce the occurrence of Huanglongbing. On the premise of implementing the above measures, strengthen cultivation management, create an ecological environment that is not conducive to pathogens but beneficial or harmless to hosts, make plants grow healthily, inhibit the infection activities of pathogens, and achieve the goals of early fruiting, high and stable yield and long life in orange orchards.
1. Strictly implement plant quarantine.
It is forbidden for the seedlings and scions in the ward to enter the disease-free protected area, and the propagation materials such as seedlings, seeds and scions shall not be introduced into the ward without the consent of the ward.
2. Establish a nursery without pests and diseases, and cultivate healthy seedlings without pests and diseases.
Establish provincial, county and township (village) three-level pest-free (quarantine) nurseries. Their respective tasks are: to establish a provincial pest-free mother collection center, to be responsible for cultivating pest-free mother forests, and to provide pest-free basic seedlings and disease-free scions for pest-free nurseries in various counties. The disease-free nursery at county (city) level is responsible for expanding the breeding of disease-free basic seedlings and disease-free scions provided by provincial nurseries, providing improved scions without pests and diseases for registered nurseries at township (village) level, or directly supplying disease-free seedlings to production units and fruit tree specialized households. Disease-free nursery at township (village) level uses disease-free scions or seedlings provided by county-level nursery to cultivate disease-free seedlings, which are supplied to fruit farms and fruit farmers for planting.
The mother orchard of the provincial pest-free center should be built in the mountainous and forest areas more than 5 kilometers away from citrus, preferably surrounded by mountains and windbreaks (Chinese fir, Casuarina equisetifolia, Ligustrum lucidum, bamboo, etc. ) planted around the park, and windbreaks such as Metasequoia glyptostroboides, Chinese fir and Ligustrum lucidum should be planted every 50 meters in the park.
City (county), township (village) level four pests (quarantine) nursery site, in principle, according to the requirements of the provincial nursery, at least 2 kilometers away from the citrus forest, shelter forest, the distance can be closer.
Technical measures for establishing provincial pollution-free-center cutting mother tree garden;
(1) Select 8- 10-year-old healthy and good-quality trees, collect seeds, put the seeds into bamboo baskets or cloth bags, pre-soak them in warm water at 50-52℃ for 5 minutes, and then soak them in hot water at 55-56℃ for 50 minutes. When soaking the seeds, the seeds should be submerged by hot water for more than 3 cm, and the seeds should be continuously stirred to make them evenly heated. After the seeds are taken out, they are cooled in cold water, then spread and dried, and then planted in isolated places to cultivate disease-free seedlings. After the results, disease-free single plants with high yield and high quality were selected as mother trees, and disease-free seedlings were propagated by cutting. The rootstock seeds are also treated with hot water at 55-56℃ for 50 minutes according to the above method, and then the rootstock seedlings are planted and cultivated.
(2) In a disease-free garden far away from the disease area, scions are selected from trees with high yield and good quality, which are over 10 years old, and grafted and planted in an isolated nursery. After 4-5 years of observation, a single plant with high yield, good quality and no disease was selected as the scion mother tree to cultivate disease-free seedlings. This method is also more reliable. However, the limited variety resources may not meet the needs of variety diversification and high quality and high yield in production. In addition, there may be some cryptovirus diseases.
(3) In disease-free or lightly diseased orchards in epidemic areas, single plants with high yield, high quality and healthy appearance of disease-free varieties are selected as excellent parent trees of scions, and candidate seedlings of disease-free parent trees are cultivated. The specific method is as follows: after the scion is washed with soapy water, it is treated with streptomycin 700ppm and 1% ethanol or liquor for 30-60 minutes, then with hot water at 47-50℃ for 6- 12 minutes, once every 24 hours, continuously for 3 times, and then with tetracycline 2000.
(4) Shoot-tip grafting is adopted to remove germs and cultivate candidate seedlings of mother trees.
Methods and standards for determining candidate seedlings of disease-free mother trees. Candidate seedlings are mainly free from Huanglongbing, which requires no decline disease and peeling disease, and strive to be free from quarantine and dangerous pests such as canker and citrus psylla. Except for canker and citrus psyllid, there are four inspection standards for other diseases: ① check when the symptoms of Huanglongbing are most obvious in autumn and winter every year, and those who have not found the typical symptoms, recession and peeling of Huanglongbing for three consecutive years and have good gardening performance; ② After rapid diagnosis by microscope and examination by electron microscope, no red lump, prokaryotic microorganism and linear virus plasmid were found. Slice the petiole by hand, put it on a glass slide, drop a few drops of 0. 1% saffron solution and dye it for 2 minutes. After absorbing the dye solution with filter paper, drop a few drops of 95% ethanol to decolorize it, then absorb the ethanol with filter paper, and finally drop distilled water to cover the sheet. Microscopically, no bright red to dark red lumps were found in phloem (colorless to light red in health). Or take the vein and make it into ultra-thin slice sample by conventional method, and examine it under electron microscope; ③ Candidate seedlings were negative in bioassay. Seedlings of banana, ponkan (Huanglongbing), eureka lemon and Mexican lemon (recession) after 50 minutes of hot water treatment at 55-56℃; Taking the seedlings or cuttings of Citron (used for split skin disease) as identification plants, each plant was grafted with 2 buds or shoots (length 10 cm, with buds, used for Huanglongbing) by abdominal grafting method. Each sample was grafted with 3-5 identification plants and observed continuously for 3 years. Banana and ponkan seedlings have no symptoms of Huanglongbing, Eureka lemon and Mexican lemon seedlings have no symptoms of decline, and citron seedlings have no symptoms of peeling. ④ Those with no positive reaction in serum test of anti-degenerative diseases. The stem bark of spring shoots was taken from the candidate seedlings, dried, and determined by enzyme-linked method with the antiserum of citrus decline disease prepared in advance.
Candidate seedlings that fully meet the requirements of the above four conditions can be designated as basic seedlings of disease-free mother trees. After the propagation of disease-free basic seedlings, they are planted in the isolated central cutting mother garden or the city (county) cutting mother garden, which is called disease-free mother tree and is specially used to provide disease-free scions for registered nurseries and cultivate disease-free seedlings.
3. Development and protection of new areas
(1) Planting disease-free seedlings
The new development zone must plant disease-free seedlings cultivated by scions provided by provinces, cities (counties), and it is strictly forbidden to plant citrus seedlings from other sources. And check the citrus orchard at any time. Once a diseased plant or a suspected diseased plant is found, it should be cut down immediately, and pests such as citrus psylla should be thoroughly controlled.
(2) Delineate new districts and constituencies by city (county).
The new district should strictly implement quarantine, establish a nursery free of pests and diseases, and cultivate disease-free seedlings.
4. Prevention and control of diseases and insect pests in wards
(1) When establishing a new orchard in an old ward, it should be more than 500m away from the old orchard. Because the new orchard shoots many times, citrus psylla will lay eggs on the tender buds, which is easy to spread diseases, so it is impossible to build a new citrus orchard next to the old endemic area.
(2) A new orchard cannot be established until the whole plant in the seriously ill area is cut down and cleared for one year.
(3) Thoroughly control the citrus psylla which spreads diseases.
(4) Dig out the diseased trees in time to prevent the spread of diseases and insect pests during the growing period of citrus, and patrol the citrus orchard at any time, and dig out the diseased trees as soon as they are found. In particular, digging out diseased trees in time is the key measure to prevent the occurrence of psylla and the spread of Huanglongbing. The earlier and more thorough digging, the better the effect, otherwise it will affect the healthy growth of neighboring orchards. Diseases and insects often go hand in hand. If you find 1 diseased trees (after digging them out), you should use drugs to prevent and control several surrounding trees in time. Sporadic nursery should be sprayed with pesticides to prevent psyllids from spreading diseases. If only psyllids are found, but no diseased trees are found, pesticides should be used in time to prevent disease outbreaks. After the lightly diseased tree saws off the diseased branches, it should also be sprayed with pesticides frequently.
(5) Replanting big seedlings in time to ensure stable production. Old citrus orchards with an incidence rate of less than 20% and new citrus orchards with an incidence rate of less than 10% should be replanted in time after the diseased trees are dug up to ensure high and stable yield of citrus orchards. Where the incidence of newly-built citrus orchards exceeds 10% and the incidence of old citrus orchards exceeds 20%, no replanting is allowed.