What are the main diseases of citrus and their control?

1. Citrus Huanglongbing (see Figure 2- 1), also known as yellow shoot disease, is an important plant quarantine object, mainly distributed in Fujian, Guangdong, Guangxi and other provinces (autonomous regions), and also occurred in parts of Yunnan, Sichuan, Hunan, Jiangxi and Zhejiang. Taiwan Province province is called damping-off disease, which is also found in some foreign countries and young trees. In the ward, a new kind of garden without an old orchard around can be seriously ill after being planted for 89 years.

Figure 2- 1 Citrus Huanglongbing

1. Element-deficient etiolated leaves 2, 3 and 4. Mottled yellow leaves.

(1) At the initial stage of symptoms, "yellow buds" and mottled leaves of diseased trees are typical symptoms of Huanglongbing. At the early stage of the disease, several leaves with new buds on the green crown turned yellow, showing obvious "yellow buds". This "yellow tip" often appears at the top and periphery of the crown. Subsequently, the lower branches of the disease tip and the branches of other parts of the crown became ill one after another. The disease can occur all year round, and bamboo shoots in summer and autumn are the most common diseases, followed by bamboo shoots. When summer and autumn buds come, before the new buds turn green, they often turn into "yellow buds", and the leaves turn yellow or mottled evenly. The onset of spring shoots is that after the newly harvested spring shoots turn green normally, with the maturity of spring shoots, the leaves begin to fade from the veins and gradually spread out, forming mottled yellowing.

Yellowing leaves can be divided into three types: ① Uniform yellowing leaves. It often appears in early diseased trees and trees with summer and autumn shoots, and the leaves are even yellowish green, but this symptom stays in the branches for a short time. So it's hard to see in the field. (2) mottled yellow leaves. The leaves show uneven yellow and green patches, and the shape and size of the patches are uncertain. Starting from the vicinity of the vein, it is particularly easy to turn yellow from the base of the midvein and the top of the lateral vein (that is, the green part of the leaf), gradually expand to form yellow-green spots, and finally make the whole leaf yellow-green. This kind of leaf is easy to find on diseased branches in spring, summer and autumn, and on diseased trees in the morning, middle and evening. (3) Yellowing leaves lacking elements. The mesophyll near the veins and veins is green, and the mesophyll between veins is yellow. Similar to the lack of trace elements such as zinc, manganese and iron, this leaf appears in the middle and late stage of the diseased tree. The above three types are often used as the basis for field diagnosis of Huanglongbing trees, because mottled yellowing leaves can be seen in the shoot, early, middle and late stages of various diseased trees, and the symptoms are obvious.

In addition to the symptoms of yellowing leaves, diseased trees will also have fallen leaves, untimely shoots, short and weak shoots, small leaves, swollen midvein in some varieties, and partial cork cracking. The diseased tree has sparse crown, many dead branches, short plants, early and many flowers, short and small petals, thick and light yellow, and dull, and many flowers on the twigs often gather into a ball, which is called "flower ball" in Guangdong. The fruit setting rate is low, the fruit becomes smaller and deformed (such as long or skewed fruit shape), and some varieties (such as Fuju and October Orange). ) It turns orange-red early near the pedicel, and the coloring is uneven. Fujian is commonly known as "red nose fruit". The roots of diseased trees were normal in the early stage, and rotten roots appeared in the later stage. Rot starts from fibrous roots and gradually decays to big roots, xylem turns black and root bark falls off.

(2) Pathogen As for the pathogen of Huanglongbing, according to the successful results of grafting transmission test, it was confirmed as a virus disease in 1950s. In 1960s, according to the typical symptoms caused by citrus recession virus, such as veins, veins, pits, dwarfing and lobules, which reappeared on the indicator plants of Mexican lemon and jatropha curcas, it was considered that the pathogen was probably an exotic citrus recession virus. However, according to the different symptoms, host susceptibility and geographical distribution, some people think that the diseases of the two are different, which was later confirmed by the observation results of electron microscope. In 1970s, people discovered that the pathogen of Huanglongbing was sensitive to tetracycline antibiotics by soaking scions and injecting trunk, and thought that the pathogen was bacteroides instead of virus. Later, someone observed the diseased leaves with ultra-thin section electron microscope, and saw pathogens in the phloem tissue of the diseased leaves, with the size of 150 ~ 650 nm and the boundary membrane of 20 nm, while the thickness of the conventional bacteroid membrane was less than 20 nm, which should be classified as.

The pathogen of Huanglongbing is not only sensitive to tetracycline antibiotics, but also sensitive to penicillin, and hyperthermia also has obvious curative effect. Soak the diseased scion in 1000 mg/L tetracycline or penicillin for 2 hours, and then graft. All or most of the cultivated seedlings showed no symptoms. Hot air treatment of diseased seedlings at 49℃ for 50 minutes can keep the heat treatment effect for more than 50 ~ 60 months.

(3) germs can be spread by grafting, but not by juice friction and soil infection. If grafting is carried out on diseased trees, seedlings are generally diseased plants, and the long-distance transmission of Huanglongbing mainly depends on the transportation of scions or seedlings of diseased plants. There are some signs that seeds may also spread. The natural transmission medium in the field is citrus psylla. After sucking up the sap, the citrus psyllid turns to healthy trees to suck and spread. The circulating period of the pathogen in psyllid is about 20 days to 1 month, and the shortest is 2 days. Larvae and adults over 3 years old can spread.

(4) Prevention and control methods Chemical prevention and control can alleviate the symptoms of Huanglongbing, but it cannot be eradicated, and comprehensive prevention and control must be implemented. ① Looking for new methods and disease-resistant materials to cultivate disease-resistant citrus rootstocks and scions. ② Strengthening the detection and control of citrus psyllid and cutting off the transmission route are important links to prevent the epidemic of Huanglongbing. Using yellow stick insect can not only monitor psyllid, study its biology and ecology, but also study the growth and decline of natural enemies of citrus psyllid. With the help of Murraya murraya, the host plant for trapping citrus psyllid, the effect will be better, and both adults and nymphs can be caught. By monitoring and strictly controlling citrus psyllid, it can be basically eliminated. The control of citrus psyllid mainly depends on spraying chemicals, including 40% dimethoate emulsion1000 ~ 2,000 times, 20% trichlorfon crystal 800 times, 25% imiphos EC 400 times, Derris extract 200 times and rosin mixture 15 ~ 20 times, which have good control effects on nymphs and adults. ③ Strictly implement the quarantine system. It is forbidden for scions and seedlings from epidemic areas to flow into new areas and non-epidemic areas. The new area will use disease-free seedlings and scions, as well as seedlings of unknown origin, regardless of their quality and symptoms. According to the plant quarantine regulations, it should be burned. ④ Establish disease-free nursery and cultivate disease-free seedlings. According to the pathogenic characteristics and transmission characteristics of the disease. The disease-free nursery should be selected in the non-epidemic area without citrus psylla. If the nursery is built in an epidemic area, there must be isolation conditions, that is, the nursery should be more than 5 kilometers away from the citrus orchard, and it is best to have natural conditions (such as mountainous areas and forest areas) to block it. Before the nursery is completed, the scattered citrus plants or hosts of citrus psyllids such as murraya should also be eradicated.

2. Citrus scab (see Figure 2-2) is a common disease in citrus planting areas in central and northern subtropics of China. Zhejiang, Fujian, Taiwan Province, Jiangxi, Sichuan, Yunnan, Guizhou, Guangxi, Guangdong and other provinces (autonomous regions) have occurred. Leaves fall when leaves are damaged, branches grow poorly when damaged, fruits fall off easily when damaged, or diseased fruits are small and deformed, with poor quality.

Symptoms (1) The disease mainly harms new shoots, leaves and young fruits, and also harms flower organs. Oil-soaked spots appeared on the damaged leaves at first, then gradually expanded, showing waxy yellow to yellowish brown, and then turned to gray to grayish brown, forming conical scab-like cork-like lesions with a diameter of 0.3 ~ 2 mm, protruding to one side, like horns or funnels, and having a rough surface. Disease can occur on both sides of the leaf, but it mostly occurs on the back of the leaf, does not penetrate both sides, and the lesion is scattered or even flaky, which makes the leaf deformed and twisted when the harm is serious.

Figure 2-2 Citrus Scab

1. sick leaves 2. Consequence 3 The lesion increased by 4. pathogen

The symptoms of new shoots are similar to those of leaves, but the process is not obvious, the lesions are scattered or connected together, and the branches are short and twisted. Petals will fall off quickly when damaged. After the fruit is damaged, many scattered or clustered tumor-like protrusions often grow on the peel. Young fruits are easy to rot and fall off in brown, and slightly larger fruits are easy to produce yellow-brown cork-like protrusions. Deformities often fall early, the fruit is big, and the diseased spots often become inconspicuous, but the skin is thick and juicy. In the late stage of fruit disease, large skin tissue in the affected area is necrotic and falls off like tinea, and the underlying tissue is cork-shaped, and the cortex is thin, which is often easy to crack after long-term sunlight exposure.

(2) The pathogen (Sphaceloma fawcetti Jenk) is a kind of fungus, which belongs to a semi-ignorant fungus and has asexual and sexual phases, and the sexual phase has not been found in China. Umbilical spores and conidia are produced in asexual stage. Molecular spores are densely arranged on the disk. Conidiophore is short, cylindrical, with thin ends and 0 ~ 2 diaphragms. Conidia are terminal, unicellular, colorless, rectangular, oval or kidney-shaped, and each end contains an oil spot.

(3) The pathogen overwinters in diseased branches and leaves with hyphae, and it is rainy and humid in the following spring. When the temperature reaches above 15℃, the hyphae on the old diseased spot begin to move and produce conidia. After wind and rain spread, the sprouting tube invades the young leaves, flowers and young fruits of spring shoots, and the pathogen can continue to produce conidia for reinfection. Long-distance transmission is through diseased seedlings and scions.

There are great differences in disease resistance among different citrus varieties. Generally, oranges are the most susceptible to diseases, followed by limes, lemons, bitter oranges, oranges and grapefruit. Sweet orange has strong disease resistance, while some varieties of sweet orange and kumquat can be immunized. The senescence degree of citrus tissue is also related to disease resistance. Usually, young leaves of new shoots are most susceptible to disease before spreading, and the leaf width is about 1.65 cm, which means disease resistance, and it is also most susceptible to disease in the young fruit period shortly after flowering. When the young fruit of the new leaf matures, it is not easy to get sick. Temperature and humidity have a decisive influence on the occurrence and prevalence of this disease. The range of onset temperature is 65438 05 ~ 24℃, and the optimum temperature is 20 ~ 265438 0℃. When the temperature exceeds 24℃, the disease will stop, so it is serious in spring shoots and young fruit stages, and it is generally not easy to get sick in summer shoots and autumn shoots. Continuous rain or fog in the early morning is conducive to the germination and invasion of bacteria, leading to a large number of diseases.

(4) Prevention and treatment methods The comprehensive prevention and treatment of the disease should be based on chemical prevention. (1) The focus of chemical control and spraying protection is on young leaves and young fruits. Spraying chemicals to protect the spring shoots before the new buds germinate to 2 mm long, and spraying chemicals to protect the young fruits when the flowers wither by 2/3. The medicament can be selected as follows: 0.5% ~ 0.8% dosage of Bordeaux solution, 500 ~ 700 times dosage of 70% Pujunke wettable powder, 600 times dosage of 30% copper oxychloride, 300 ~ 500 times dosage of 40% polysulfide suspending agent, 50% carbendazim wettable powder 1000 times dosage, and 70% thiophanate methyl. 1000 times of 50% benomyl wettable powder, 500 times of 50% chlordane wettable powder, 500-800 times of 75% chlorothalonil wettable powder, copper soap solution (copper sulfate 0.5 kg, rosin mixture 2 kg, 200 kg). According to the specific situation of the disease, the number of spraying is determined, generally once every 10 ~ 15 days. (2) Reduce the source of bacteria, combine with pruning in winter and spring, cut off dead leaves and too dense branches, remove the dead leaves on the ground, and reduce the initial source of infection. ③ Strengthen the management of fertilizer and water, prevent other pests and diseases as early as possible, promote the trees to be strong, sprout neatly, and enhance the disease resistance. ④ When new orchards are built, disease-free seedlings are selected, and scions in diseased areas are soaked in 800 times solution of 50% benomyl wettable powder for 30 minutes, which has good sterilization and disinfection effect.

3. Citrus Anthracnose (see Figure 2-3) is widespread in citrus producing areas, which can cause defoliation, dead branches, split branches and fruit rot.

Figure 2-3 Citrus Anthracnose

1. Get sick and leave 2. Ill branch 3. Unhealthy fruit. pathogen

Symptoms (1) The disease mainly damages leaves, branches and fruits, as well as branches, stems, flowers and fruit stalks of seedlings. Leaf diseases are generally divided into chronic (leaf blight type) and acute (leaf spot type). Chronic type mostly occurs in the wounds caused by mature leaves and liriomyza sativae, especially in dry season, and the diseased leaves fall off slowly. Lesions mostly occur at the edge or tip of the blade, forming a nearly round irregular shape, with a diameter of 3 ~ 20 mm, light taupe and brown edge, and the boundary with the healthy part is very obvious. In the later stage or when the weather is dry, the center of the lesion is dry, faded to grayish white, and the surface is densely raised, arranged in concentric wheel-shaped small black spots. In rainy and humid weather, many small orange sticky liquid spots overflow from the black spots on the lesion. Acute type mainly occurs on young leaves in high temperature season after rain, and diseased leaves rot and fall off quickly, which often causes serious defoliation of the whole plant. Light blue or dark brown spots are mostly produced at the leaf margin and tip or along the main vein, which is quite similar to hot water burns. Rapidly expand into corrugated plaques filled with water, generally 30 ~ 40 mm in diameter, and sometimes vermilion sticky liquid spots are produced on the diseased spots. Sometimes arranged in the form of wheels. Sick leaves fall off easily.

There are two symptoms on the branches. One is that the branches spread downward from the top of the branches, and the diseased parts are brown, and finally they gradually die. The boundary between the dead part and the healthy part is obvious. After death, the branches are gray with many black spots scattered on them. The other is in the middle of the branch tip, starting from the axillary bud at the base of the petiole. At first, the lesion was light brown oval, and then it expanded into a long spindle. When the diseased spot is cut around the branch tip, the diseased tip will die immediately. Flowers are diseased, and the stigma of pistil is damaged, browned and rotted after flowering, resulting in flowering.

When the fruit stalk is damaged, it turns pale yellow first, then brown and dry, showing a dead pedicle shape, and the fruit falls off. The onset of young fruit was initially dark green oily irregular spots, and later spread to the whole fruit. The lesion was sunken and turned black, turning into a stiff fruit hanging on the tree.

The damage of seedlings usually begins at 6.6 ~ 9.9 cm from the ground or at the joint, forming irregular dark brown spots, leading to the top of the trunk dying and extending to the dry branches.

(2) The pathogenic bacteria are fungi. It belongs to subfamily Hemifungi. The small black spots in the sick area are the gathering places of bacteria. The umbilicus on the diseased fruit is surrounded by bristles, dark brown and transparent at the top. In diseased parts of branches and leaves, apetalous usually do not produce bristles. Conidia are arranged in a grid, colorless, cylindrical and indehiscent. Conidia are cylindrical or oval, colorless and single-celled, often with 1 ~ 2 oil globules. The optimum temperature for pathogen growth is 2 1 ~ 28℃, and the lowest temperature is 9 ~ 15℃. The optimum temperature for conidia germination is 22 ~ 27℃, and the lowest temperature is 6 ~ 9℃.

(3) Occurrence regularity: The pathogen of the disease is a kind of weak parasitic bacteria with latent infection characteristics. Pathogens mainly overwinter on diseased branches, diseased leaves and diseased fruits with hyphae. In the following spring, conidia germinated and invaded from diseased tissues spread by wind, rain or insects, and after a certain period of latent expansion, tissue lesions were caused.

The disease can cause harm to citrus in the whole growing season, but it usually occurs less in spring shoots and more in summer shoots and autumn shoots. Symptoms rarely appear in citrus orchards with normal climatic conditions and average management level, but there are a large number of adherent cells in the appearance of organs and tissues of healthy plants. When the climate is abnormal, the cultivation management is extensive or other pests and diseases are harmful, the tree is weak, which can make conidia germinate the germinating tube, the appressorium germinate the slender invasive filaments, invade the epidermal cells, develop into hyphae, and spread until symptoms appear.

(4) Control methods According to the latent infection characteristics of pathogenic bacteria, comprehensive control measures focusing on strengthening cultivation management and improving tree vigor were adopted. ① Strengthening cultivation management. Strengthening tree vigor and improving disease resistance are the key to control the disease. According to the actual situation of orchards around the country, we can prescribe the right medicine. For example, expand holes, dig deep, plant green manure, apply more organic manure, supplement phosphorus and potassium fertilizers, supplement trace fertilizers, pay attention to timely irrigation and drainage, pay attention to pruning, and prevent and control other pests and diseases in time. Secondly, clear the garden in winter, cut off diseased branches and fruits, remove diseased fruits from the ground, and reduce the source of germs. ② Spray protection. Spray it once in spring, summer, autumn and winter, and spray it again at the early stage of the disease to protect it, reduce or prevent its direct invasion and form attached spores. Common fungicides can effectively prevent conidia from germination. The bactericides are: 0.5% Bordeaux solution such as lime, 50% wettable powder for sterilization 500-700 times, 70% thiophanate 1000 times, 50% carbendazim 1000 times and 65% zineb for sterilization 500 times. Add water to 40% mirex 500-fold solution, 25% carbopol emulsion 500-600-fold solution and jinggangmycin (same as 1 ~ 1.5 kg tea bran powder), boil, cool, filter, add 47.5 kg water, add 50 g jinggangmycin and 2 g salt, fully dissolve, and then.

4. Citrus canker Citrus canker (see Figure 2-4) is one of the important diseases of citrus and the object of plant quarantine at home and abroad. The disease occurred seriously in Guangdong, Fujian, Taiwan Province, Guangxi, Sichuan and other provinces (autonomous regions), and also distributed locally in other citrus producing areas. After plants are infected with diseases, the damaged fruit quality is reduced, the seedlings are damaged, and the growth is hindered. In severe cases, a large number of leaves and fruits fall off, causing huge losses. In addition to citrus, it can also parasitize a variety of weeds.

Figure 2-4 Citrus Canker

1. sick leaves 2. Consequence 3 The lesion increased by 4. The diseased branch tip 5. Pathological tissue anatomy 6. pathogen

Symptoms (1) The disease damages leaves, shoots, fruits and sepals, and forms cork-like process lesions. At first, the lesion of the injured leaf is needle-shaped, yellow and oily (obvious when observing the leaf in the sun), and then it expands into a nearly round lesion with a diameter of 3 ~ 5 mm. The two sides of the leaf are raised, cork-like, rough, taupe and cracked into a crater. The edge of the lesion is oily with yellow halo around it. In severe cases, the leaves will fall early. The diseased spots on branches and fruits are similar to those on leaves, but the diseased spots are more prominent, the potholes are more obvious, the cork is more solid, hard and rough, and there is generally no yellow halo. The diseased fruit is limited to the peel and does not harm the pulp, so the diseased fruit is not deformed, but it is easy to fall off.

(2) Pathogen [Monomonas orange color. The pathogen of this disease is a Mucor flavipectus bacterium. The bacteria are short rod-shaped, round at both ends, with flagella at one end, movable, capsule, no spores, gram-negative reaction and aerobic. On PDA medium, the colony is bright yellow, round and smooth with narrow leucorrhea around it. On the beef juice peptone medium, the colony is round, waxy yellow, shiny, complete, slightly convex and sticky. The optimum temperature for the growth of pathogenic bacteria is 20 ~ 30℃, and the suitable acidity range is pH 6. 1 ~ 8.8.

(3) Pathogenic law: The pathogen overwinters in the diseased spots of leaves, branches and fruits, overflows from the diseased parts when the conditions are suitable in the following spring, spreads by wind and rain and insects, and invades through the stomata, lenticels and wounds of the host. Diseases that cause leaves, shoots and fruits. Long-distance transmission mainly includes diseased seedlings, scions and fruits.

The high temperature and rainy season is conducive to the reproduction and spread of germs, so it is serious in subtropical areas. In citrus orchards, summer shoots are the most serious, followed by autumn shoots and spring shoots are the lightest. Fruit is seriously ill. In coastal areas, after the typhoon and rainstorm every year, there is often a peak period of illness. Typhoon and rainstorm cause many wounds of hosts, which is beneficial to the invasion and spread of germs. The harm caused by leaf miner is also beneficial to the serious occurrence of the disease. Species and varieties have different disease resistance. Generally, oranges are the most susceptible to diseases, followed by oranges, oranges and kumquat. Irrational fertilization and excessive application of nitrogen fertilizer can induce diseases, and seedlings and young trees are prone to diseases. The older the tree, the lighter the disease.

(4) Control method ① Implement plant quarantine system. When transporting seedlings, scions, rootstocks, seeds and fruits, plant quarantine conditions should be strictly implemented, and introduction from disease-free areas and new areas is strictly prohibited. Cultivate disease-free seedlings. Cut down the diseased plants immediately when they are found, and remove weeds around the diseased trees at the same time. (2) Comprehensive prevention and control measures should be implemented in epidemic areas. The focus of spraying is the germination stage and young fruit stage in summer and autumn. Generally, spray it for the first time when the new shoot length is 1.6 1 ~ 3.3 cm, spray it for the second time when the leaves turn green, and spray it for 2 ~ 3 times continuously, and protect the young fruits every 10 day, 30 days and 50 days after flowering. The chemicals are: 30% copper oxychloride suspension 700 times, agricultural streptomycin 700 ~ 900 international units/ml, 50%DT wettable powder (copper dibasic acid) 700 times, 14% colloidal copper aqueous solution 300 times, 0.5% lime double Bordeaux solution, copper soap solution (CuSO40.5 kg, rosin). After each typhoon and rainstorm, it is necessary to spray drugs in time, strengthen the prevention and control of diseases and pests such as leaf miner, and reduce the wounds invaded by germs. (3) Strengthen the management in the growing period and winter. Rational fertilization to control the growth of summer shoots. Do a good job in clearing the garden in winter, cut off the branches and leaves of pests and diseases, collect fallen leaves, dead branches and fruits, burn them centrally, reduce the source of the disease, and spray 0.5 Baume lime-sulfur mixture in combination with other pest control.

In production, canker and scab are easily confused. Their differences in leaves are as follows:

Symptoms and characteristics of canker scab symptom presentation period: the new bud extends from tender leaf to tender bud, and the lesion has spread to both sides of the leaf before the tender leaf and the leaf are not fixed. The lesion is circular on one side of the blade, often protruding to the back of the blade, splitting into a cone, and the undivided lesion is crater-shaped.

attached table

Plan (continued)-1

5. Citrus Black Spot, also known as scab, occurs in Guangdong, Fujian, Taiwan Province, Zhejiang, Sichuan, Yunnan and other provinces. The disease mainly harms fruits, and the quality of diseased fruits is degraded, which makes them intolerant of storage and transportation, and even causes fruit drop and rot in severe cases. Besides citrus, it can also harm pomelo, sweet orange, lemon, orange and other fruit trees.

(1) Symptom Black Spot damages fruits, branches and leaves, mainly near-ripe fruits. The focus is a round spot, with a diameter of 1 ~ 5 mm, mostly 2 ~ 3 mm, with reddish brown spots, and then the middle of the focus is sunken, grayish brown to gray or lavender, and some are scattered with black spots; The edge of the lesion is slightly convex, dark reddish brown to dark brown, only hurting the peel and not going deep into the pulp. Too many diseased spots will lead to fruit drop. During storage and transportation, it will continue to get sick and lead to decay. The leaves are damaged, the disease is mild, and the symptoms are similar to those of fruits.

(2) The pathogen of citrus black spot is a fungus (Phoma citri-carpa Mcalp), which belongs to the subfamily Hemifungi. There are two stages: sexual and asexual, and the small black spots on the diseased spots are conidia of asexual generations of bacteria. Conidia are nearly spherical, dark brown and have a hole. There are two kinds of conidia in conidia, one is oblong or ovoid, with colorless single spore, and the other is short rod-shaped with slightly enlarged ends and colorless single spore. These two conidia are not produced in the same spore apparatus. The growth temperature of the pathogen is 15 ~ 38℃, and the optimum temperature is 25℃.

(3) Occurrence regularity: The overwintering conidia in diseased fruits and diseased branches and leaves are the primary infection sources. In the following spring, conidia are distributed in conidia, mainly spread by wind and rain, and invade young leaves and young fruits. After the invasion, the incubation period is as long as 3 ~ 12 months, until the fruits and leaves are close to maturity and symptoms appear. Produce conidia on the lesion for reinfection. According to the observation in Guangdong and Fujian, the pathogen invaded the young fruit within one and a half months after flowering, and symptoms began to appear from the end of July to the beginning of August. The peak period of fruit diseases is from the end of August to the beginning of 65438+ 10, and the development of fruit diseases basically stops in 165438+ 10. This disease is very serious in a hot and humid climate. The stuffy and unventilated orchard is seriously ill. Generally, the incidence of spring shoots is less, summer and autumn shoots are more, the incidence of fruits of young trees aged 4 or 5 years is less, the incidence of trees over 7 years old is more, and the incidence of old trees is even worse. Orchard management is good and diseases are light.

(4) Control methods To prevent black spot disease, comprehensive control measures should be taken, mainly spraying chemicals to protect fruits, supplemented by clearing fields and strengthening cultivation management. 1 spray to protect the fruit. Master spraying medicine for 2 ~ 3 times continuously within 30 ~ 40 days after flowering, spraying 0.5% lime double Bordeaux solution or 50% carbendazim wettable powder 1000 times solution, and strengthening the control of mites after spraying Bordeaux solution. ② Reduce the source of bacteria. Pruning is combined with garden cleaning, cutting off diseased branches and leaves, and burning dead leaves and falling fruits in a centralized way. Then spray 0.8 ~ 1 baume mixture of lime and sulfur for disinfection. ③ Strengthening cultivation management. It is necessary to increase the application amount of organic fertilizer, strengthen the management of water and fertilizer, enhance the tree vigor and improve the disease resistance.

6. Citrus foot rot Citrus foot rot, also known as skirt rot, is almost spread all over the world in citrus producing areas. This is an important disease on citrus. It occurs in citrus producing areas in China. It mainly causes the cortex at the base of the trunk to rot, leading to the death of plants.

(1) symptom citrus foot rot (see figure 2-5) occurs at the base of citrus trunk, and young trees planted too deeply often start to get sick at the joint, causing cortex rot and fibrous root death. Most lesions occurred in the roots and neck, the lesion cortex was amorphous, the lesion layer turned brown, rotted, smelled of distiller's grains, and brown colloid often flowed out, and the lesion part could reach xylem. When it is dry, it condenses into blocks, and when it rains, it dissolves and washes away. In the hot and rainy season, the disease spots spread rapidly in vertical and horizontal directions, reaching the base of the trunk about 30 cm from the ground and down to the root group, causing a lot of rot of the main roots, lateral roots and fibrous roots; Lateral expansion can completely rot the bark of root neck, leading to "ring cutting" and plant death. When the weather is dry, the diseased part is cracked and the boundary with the healthy part is obvious. On the crown corresponding to the location of the disease, the leaves lose their luster and become smaller. When the disease is serious, the leaves turn yellow, and the midvein and lateral vein of the leaves turn yellow, which is easy to lose leaves. With the aggravation of the disease, the leaves of the whole crown are sick, the branches wither and the trees are weak. Plants tend to blossom and bear fruit in large numbers, drop fruit early, or turn yellow and sour in advance when the fruit is small. Sometimes the fruit of diseased trees becomes bigger, the skin is thick and rough, and it is colored early.

Figure 2-5 Citrus Foot Rot

1. pathogen 2. Root 3. Lesion 4. pathogen

(2) The pathogen of citrus root rot is caused by several Phytophthora. It is reported that Phytophthora parasitica is the main pathogen in Sichuan and Phytophthora citricola is the main pathogen in Hunan. Parasitic Phytophthora, the sporangium is terminal, ovoid, ovoid or pear-shaped, with mastoid, sexual reproduction heterozygosity, producing oospores. Its growth and development temperature is 10 ~ 35℃, and the optimum temperature is 25 ~ 28℃.

(3) Pathogenic law: Pathogenic bacteria overwinter on diseased plants and diseased residues in soil with hyphae and chlamydospores. In the following year, with the increase of temperature and rainfall, hyphae in the diseased spots continue to harm healthy tissues, and at the same time, sporangium is formed, swimming spores are released, which spread with water and infect new plants through wounds. The resistance of different kinds of citrus to this disease is obviously different. The most susceptible are oranges, kumquat, lemons, sweet oranges, Vietnamese oranges and Sihui oranges. Among them, there are oranges, big-pointed oranges, red-skinned oranges, annual oranges and Shantou sour oranges. The resistance is bitter orange, orange, grapefruit, lime, red bean leaves and wine cake grapefruit. High temperature and rainy weather are conducive to the occurrence and prevalence of the disease. Generally, citrus orchards with heavy soil, poor drainage, too deep planting and too dense planting are seriously ill. The wound at the base of trunk caused by human factors or insect pests is beneficial to the occurrence of this disease.

(4) The control methods should be based on disease-resistant rootstocks, strengthening cultivation management, thoroughly controlling trunk pests, chemical control and other comprehensive control measures. ① Using disease-resistant rootstocks is the most effective and economical measure to prevent diseases in newly planted orchards. Disease-resistant rootstocks include bitter orange, orange, lime and pomelo. However, the best combination of rootstocks and the use of disease-resistant rootstocks should consider the interference of other diseases and soil conditions, such as the incompatibility of grapefruit rootstocks grafted with sweet oranges; Fructus Aurantii is sensitive to split skin disease, but not tolerant to alkaline and high calcium carbonate soil. Lime rot is very sensitive. Therefore, in the selection of disease-resistant rootstocks, we should comprehensively consider various factors and adjust measures to local conditions, and avoid paying attention to only one thing and one thing. At the same time, when grafting with disease-resistant rootstocks, it is necessary to properly improve the position of grafting interfaces. ② Replace rootstocks, select three disease-resistant rootstocks from different directions at the base of infected rootstocks, and combine pruning, digging roots and fertilizing outside the roots to promote the recovery of diseased trees. This method is applied to young trees and the effect is obvious. (3) drug treatment, disease season, field disease survey, found that the diseased tree should be stripped of root and neck soil, scraped off the rotten part, cut the diseased part vertically, and reach the xylem with an interval of 65438±0cm；; Then apply 200 times solution of 25% metalaxyl wettable powder (Domil, Fungus and Methamidophos) or 90% triethylaluminum phosphate wettable powder (Phytophthora, Phytophthora and Phoxim)/0/000 times solution. It is reported that 100 ~ 500 mg/L paclobutrazol has a good control effect after removing the rotten parts. The experience of fruit farmers in some areas is that after the diseased trees are scraped clean, the soil with bacteria at the root of the trunk is dug out and filled with new soil from river sand. It will heal after 4 ~ 6 months and grow a lot of new roots. ④ Strengthen orchard management. Do a good job in irrigation and drainage system to prevent water accumulation in orchard; It is necessary to prevent and control longicorn beetles and gibbons in time; Agricultural operations should avoid damaging the bark at the bottom. Reasonable close planting and timely pruning can ventilate the orchard, reduce the air humidity and reduce the occurrence of the disease.

7. Citrus resin disease Citrus resin disease (see Figure 2-6) is also called gummosis. Because of the different symptoms, onset sites and periods, there are also names such as stem rot, sand skin disease and black spot disease. It mainly harms branches, leaves and fruits, especially branches, causing cortex rot and causing branches to die. The disease is widely distributed in China and has occurred in Zhejiang, Fujian, Jiangxi, Guangdong, Guangxi, Sichuan and other provinces (autonomous regions).

Figure 2-6 Citrus Resin Disease

1. The diseased fruit of Shapi 2. Exaggerated symptoms of Shapi 3. Symptoms of sticky branches. Sarcoidosis leaves 5. pathogen

(1) Symptoms of resin disease often vary with the injury site and environmental conditions. After (1) glue and dry branches were damaged, the cortex was necrotic and there were dark brown oil spots in the early stage.