What's wrong with watermelon?

(1) Watermelon fruit rot (also known as watermelon "dehydration" disease).

Watermelon is not only an essential fruit for China people to cool down in summer, but also one of the important fruits on the table all year round. Because of this, the area of watermelon is not only growing, but also developing in the direction of high quality and early maturity. The protected area of watermelon is increasing. Although grafting has solved the problem of continuous cropping, a new problem-"watermelon dehydration" has appeared in recent years. Basically, it happened in protected greenhouses, with a large area and becoming more and more serious. Shandong, Shenyang, Hebei, Beijing, Hainan, Jiangsu and other places have occurred. Watermelon dehydration is a serious problem that threatens watermelon production in protected areas and must be solved.

(1) harmful symptoms.

A. In recent years, a new disease of watermelon has appeared, which is called "dehydration" and "pulp collapse", also called "purple pulp" or meat quality deterioration. This disease damages watermelon fruit, and the diseased fruit completely loses its edible value and commodity value, so no one can buy it and eat it. The incidence of the disease is generally 5% ~ 20%, and the serious situation is as high as 70%, which has caused huge economic losses to melon farmers.

B. Watermelon "dehydration" shows symptoms. The roots, stems and leaves of diseased plants are the same as those of normal melons, but the sound is dull when knocking, like the tapping sound of a cork, which is different from normal mature melons and raw melons. After cutting open, it was found that the pulp was purple, soaked and soft, and it could smell sweet and sour at the same time. Judging from the appearance of the fruit, the diseased melon can only be seen from the pedicel, which is dark brown in color, and the melon hair falls off earlier without any symptoms.

② The reason of watermelon dehydration. Some people think that it is a bacterial disease, and some people think that it is caused by green mottle mosaic virus (CGMMV), but according to the research and analysis, it is considered that the disease is a physiological disease under the environmental conditions of the reserve in early spring. The main cause of the disease is the "steamed melon" caused by high temperature and high humidity after watering watermelon during the discoloration period. The mechanism is that high temperature and high humidity promote the production of excessive ethylene in pulp, cause abnormal breathing, accelerate the ripening process and make the meat quality worse.

A. there are obvious differences among varieties. The varieties with thin skin and hard and crisp meat are seriously ill, such as "Artemisia argyi" of Shanxi Dazheng, and some series of varieties that look like Xin Jing 1; The varieties with thick skin and crisp meat are mild, such as real Xin Jing 1 and Japanese Ruby. Late-maturing varieties such as Admiralty Guanlong, Xinhongbao and Black Melon series were selected, and the disease was very light or no diseased melon was found.

B.the protective greenhouse is the most serious. Under the cultivation conditions of solar greenhouse or small arch shed, there are different degrees of diseases; Watermelons cultivated in the open field rarely get this disease.

C. the reason of humidity temperature.

A.there is too much water. Overflow irrigation causes long-term saturation of soil moisture, which reduces oxygen around roots, suffocates, turns yellow-brown, and even decays, which is one of the important reasons for watermelon dehydration.

B.the moisture is too small. Too dry soil can also cause "water detachment", which is generally called "drought detachment" The reason is that the soil is too dry, and if the temperature is high, it will also reduce the absorption capacity of roots and lead to the death of some roots.

C.the temperature is too high. When the watermelon grows to a certain size, it is the period when the watermelon fruit accumulates sugar and other nutrients. For example, if the temperature in the greenhouse is too high, the watermelon fruit will consume too much nutrients, resulting in physiological imbalance and discoloration of the pulp, resulting in "dehydration".

D. Timing of irrigation. When watermelon is shaped, too much irrigation or too much soil moisture will lead to discoloration of the flesh inside the fruit and inedible. Because watermelon seeds are developing, they need nutrition, and at the same time secrete hormones to stimulate meat color changes and accumulate sugar. At this time, if the water content is too large, the hormone concentration will decrease, sugar can not be accumulated, and the seed development is incomplete, resulting in incomplete or withered seeds. This is another important factor that leads to watermelon dehydration.

D. soil and fertilizer.

A. soil. Cohesive soil is more serious than loam, loam is more serious than sandy loam, and sandy loam is more serious than sandy soil. For example, most of the land in Panggezhuang Town, Daxing District belongs to sandy land, and watermelon dehydration is rare. Most of Yangzhen Town and Beiwu Town in Shunyi District of Beijing belong to sandy loam, and most of them use plastic greenhouses to grow watermelons. In 2006, 80% of greenhouses were dehydrated to varying degrees, with an average occurrence rate of 20% ~ 35% and a serious degree of 70%. Similarly, the soil of some villages in Yufa Town of Daxing belongs to loam or sandy loam, and the phenomenon of watermelon "dehydration" is obvious. Most of the land in the south of Tongzhou District belongs to saline-alkali land, and watermelon "dehydration" is more serious. The same is true in Shandong and Shenyang.

B. fertilizer. Too much nitrogen fertilizer can also cause watermelon "dehydration". Watermelon is an edible fruit and needs more phosphorus and potassium fertilizer. On the other hand, topdressing is not paid attention to, and it is mostly used as a disposable base fertilizer, which leads to insufficient nutrition in the later stage. Or too much fertilizer is used, which leads to the rapid growth of nutrients and insufficient nutrients delivered to fruits, resulting in the phenomenon of "dehydration".

E. rhizome. "Dehydration" has a lot to do with the roots of watermelon. Because most of the nutrients and water needed by watermelon are absorbed and supplied by the root system. If the rootstock is too early and the root system is aging, it will cause the normal growth and development of watermelon. As a rootstock, gourd or gourd itself has a physiological disease called "acute wilt", which will cause watermelon "dehydration". Pumpkin also has such problems. For example, if the pumpkin grows too fast, the plant grows too fast or absorbs too much water, which will cause the watermelon fruit to "dehydrate".

③ Effective cultivation techniques to solve the dehydration problem of watermelon.

A. select varieties.

A. choose watermelon varieties. Choose Xin Jing 1 variety and Japanese ruby suitable for greenhouses and small and medium-sized greenhouses; Choose late-maturing varieties in the open field, such as Jinzhong Guanlong, Xinhongbao, and black-skinned melon series, and the disease is very light or no diseased melon is found.

B. select rootstock varieties. Choose varieties that are not prone to premature aging and have great growth potential. For example, in 2006, Cangshang Village in Beiwu Town used "Wannianchun" watermelon rootstock, which is a hybrid of gourd and gourd, and there is almost no "dehydration".

In 2007, they continued to expand the planting area.

B. temperature and humidity management. Timely ventilation, dehumidification and cooling. Keep the air fresh at ordinary times, and control the shed temperature below 30℃. During the period from 0/5 to 25 days after fruit setting, watermelon swells and begins to change color, which is the key period of management. At this time, as long as the temperature in the shed is not lower than 20℃, fresh air should be used for ventilation. After watering, all the film gaps on the roof should be opened, and there should be no water vapor in the canopy for three consecutive days. In case of high temperature weather, the lower parts of both ends and sides of the shed should be opened to speed up the air circulation and reduce the temperature in the shed.

C. water resources management.

A. changes in irrigation methods. Popularize drip irrigation technology or small water irrigation, the width of watermelon and melon border in greenhouse is generally 80 ~ 100 cm, and the whole border is watered when watering. The main reasons for excessive humidity in greenhouse are flooding and wide watering surface. If conditions permit, drip irrigation technology should be gradually popularized, and if conditions permit, the border planting should be as narrow as possible, so as to reduce the watering area and the one-time watering amount. At the same time, the ground in the shed should be covered with plastic film to reduce water evaporation, thus prolonging the watering cycle and reducing the humidity in the shed. Watermelon should be watered frequently in the middle and late growth period to prevent flooding.

B. master the irrigation period. When watermelon begins to set, watering should be strictly controlled. Irrigation before solidification to prevent water shortage during solidification. If there is serious water shortage in a certain period of time, the greenhouse temperature must be reduced before irrigation. As sandy soil, water should be more frequent, and attention should be paid to reducing the greenhouse temperature. When the edge of watermelon seeds turns black, you can water them boldly. Generally speaking, keep the soil moist and don't alternate between wet and dry.

D. formula fertilization. Pay attention to late topdressing to promote plant health. Farmers in producing areas often ignore the use of potassium fertilizer, while the supply of nitrogen fertilizer is seriously surplus, which is not conducive to the healthy growth of watermelon. Fertilization should strictly follow the law of watermelon fertilizer demand, and nitrogen, phosphorus and potassium should be used together in the ratio of 3∶ 1∶4 to promote plant health and root system development, thus improving plant stress resistance. It is forbidden to use urea alone when topdressing.

E. leveling the land to reduce water accumulation in the field. Before watermelon planting, the land in the shed should be leveled to make the upstream and downstream of the cultivation bed have a certain slope to ensure smooth watering. Drainage pipes should be set downstream to drain excess water out of the shed when watering. This is not only beneficial to the growth of watermelon, but also can reduce the humidity in the shed and reduce the occurrence of watermelon "purple meat" disease.

F be good at observing, finding out in time and taking measures as soon as possible. At the early stage of the disease, the stem of the melon turns yellow or red, and the leaves of the plant are well preserved. At this time, the air should be released as soon as possible to reduce the temperature. If the temperature is kept around 1 week, some diseased melons can return to normal pulp.

(2) Watermelon wilt.

Fusarium wilt is a common disease of watermelon, so it is named because it causes Fusarium wilt. Fusarium wilt is a soil-borne disease, which is an important disease in solar greenhouse and greenhouse watermelon in winter and spring, and the incidence rate is as high as 90%.

(1) harmful symptoms. During the whole growth period of watermelon, all parts of the ground may be hurt. The peak of watermelon Fusarium wilt in Jiangsu and Zhejiang provinces is from mid-June to late June, and in North China it is from mid-May to late June. Leaves and melons suffered the most, but mainly damaged the stem base. When the cotyledons of seedlings are damaged, small spots first appear in the water, and then expand into large blue-gray circular spots. If it invades from the leaf edge, it will expand into a large arc spot and spread to the whole cotyledon quickly, causing cotyledon death. The stem of the seedling was damaged, and water spots began to appear, and then quickly spread up and down, and soon the whole plant died of soft rot; After the leaves were damaged, they were initially light brown water spots, and then gradually expanded into round, nearly round or irregular black spots with a diameter of 1 ~ 2 cm. If the leaf margin is damaged, it will form dark brown arc and wedge-shaped large spots, the diseased part is dry, and the surface is sometimes scattered with black spots, that is, conidia and ascomycetes of pathogenic bacteria; The rattan stems were damaged, which mostly occurred at the branches at the base of the stems in the early stage, showing water-stained gray-green spots, and gradually spread to all nodes along the stems. At first, the affected part showed oval or strip-shaped brown pits, and continuously secreted yellow colloid. After drying, it condensed into dark brown to black granular colloid, which adhered to the surface of the affected part and had many dense black spots, which withered the leaves of lianas. Cross-cutting the diseased stem, it can be seen that the epidermis around the stem turns brown and the vascular bundle turns brown; The fruit was damaged, initially as small spots like water stains, and later expanded into round, dark brown pits. On some varieties of fruits, the surface of the diseased spots is cracked in a star shape, and the inside is cork-shaped dry rot, which will rot after blackening, and many scattered black spots will be produced on the diseased spots.

② Pathogen. Watermelon Fusarium wilt is caused by Fusarium oxysporum infection.

③ Infection cycle of Fusarium wilt. Pathogens mainly fall into the soil with the disease and disability, overwinter in incompletely decomposed manure, and seeds can also carry bacteria. The carrier rate of seeds is 5% ~ 30%, and they can survive for more than 18 months. The survival period of the pathogen on the diseased remains varies with different wintering sites. It can survive for 3 months in water and wet soil, and for more than 8 months on the diseased remains in dry land. In the following spring, conidia and ascospores were released for primary infection. The disease of cotyledons is caused by bacteria in seeds. The conidia formed on the diseased body spread through wind, rain and irrigation water, invaded from stomata, water holes or wounds, and repeatedly infected and spread.

④ Pathogenic conditions. The occurrence of diseases is closely related to temperature, humidity and agricultural measures. The mycelium can grow at 5 ~ 35℃, and the optimum temperature is 25℃. Conidia can germinate at 5 ~ 40℃, and the optimum temperature is 26 ~ 30℃. At 28℃, the germination rate reached 75.5% in 8h and 965438 0.7% in 24h. Conidia germinated only 4% when the relative humidity was 90%, and germinated and developed 47.7% in water droplets 12 hours. Rainfall and rainfall times are the leading factors of disease occurrence. In the rainy season, the temperature is above 30℃ and the rainfall is above 100 mm, which is the peak of the disease. Watermelon sown early in Jiangnan (usually sown before mid-March) is in the rainy season, and it is the most susceptible and serious. The northern area occurs in Gua Tian, Gua Tian, where plants grow vigorously, the density is too high, the irrigation is too high, the drainage is not smooth, and the humidity is high. With the increase of continuous cropping years, the disease will be aggravated year by year, and partial or large application of nitrogen fertilizer can aggravate the disease.

⑤ Preventive measures. Watermelon Fusarium wilt is one of the most important diseases in watermelon production, which often leads to serious lack of seedlings or broken ridges, resulting in serious yield reduction. The most important measure to control watermelon Fusarium wilt is crop rotation, which usually takes 4 ~ 5 years. However, due to the phenomenon of continuous cropping in rural areas, it is more difficult to realize 4-5 years rotation. Therefore, strengthening the prevention and control of watermelon Fusarium wilt in continuous cropping fields (including fields with insufficient rotation years) is an important measure to achieve stable and bumper harvest of watermelon.

A. select disease-resistant varieties. In recent years, scientific research departments have strengthened the breeding of disease-resistant and disease-resistant watermelon varieties, such as disease-resistant Sumi, disease-resistant Suhongbao, Jingkang No.2, Xin Jing 1, Kangzheng No.2 and Kangzheng No.3.

B. seed disinfection. Before sowing, the following methods can be used: a. Soak the seeds in warm soup: soak the seeds in warm water at 55 ~ 60℃ for 20 minutes; B) Seed soaking with liquid medicine: 40% formaldehyde 150 times of seed soaking for 30 minutes, or 500 times of 50% carbendazim wettable powder 1 hour; C, seed dressing with chemicals, that is, 0.20%-0.30% of the dry seed weight is used for seed dressing with dixone, seed dressing double or carbendazim.

C. grafting to prevent diseases. Watermelon Fusarium wilt has obvious host specialization, and the effect of preventing grafting is remarkable. Commonly used rootstocks for watermelon grafting are: gourd, wild watermelon "Warrior" south rootstock 1, gourd and so on. Gourd rootstock and gourd rootstock have good grafting affinity with watermelon, and the survival rate of grafted seedlings is high. Grafting methods include butt joint, insertion, split joint and root cutting. The key to successful grafting is to cultivate strong seedlings and keep them moist and warm for 3 ~ 5 days after grafting.

D. soil treatment. We should choose leek land and non-Gua Tian soil for seedling raising, add fertilizer to make nutrient soil, and use nutrient pots to protect seedlings. The field soil should be deeply ridged, and the acid soil can be applied with slaked lime or sprayed with lime water. Before sowing the field with a history of watermelon Fusarium wilt, spray the melon ditch with pentachloronitrobenzene, carbendazim and dixone bactericides or smear medicinal soil in the sowing hole for soil disinfection.

E. strengthen fertilizer and water management. Base fertilizer should be fully decomposed; Do not use garbage and feces of sick and disabled people, increase the application amount of organic fertilizer and phosphorus and potassium fertilizer, do not apply nitrogen fertilizer unilaterally, and implement formula fertilization. The suitable ratio of nitrogen, phosphorus and potassium during watermelon growth is 2∶ 1∶ 1.50. Spraying micro-fertilizer on leaves can enhance the disease resistance of plants. Water less at seedling stage, and use drip irrigation according to seedling condition at growing stage. It is forbidden to flood irrigation and string irrigation, and the accumulated water in the field should be discharged in time. Top dressing should not damage the roots, and agricultural operations such as pruning and pressing vines should not cause too many wounds to reduce the invasion of germs in the soil.

F. chemical control. After planting, root irrigation with chemicals is combined with root irrigation water. At the initial stage of the disease, the root can be irrigated with 600 times solution of 70% thiophanate-methyl wettable powder or 2000 times solution of 15% triadimefon wettable powder, every 5-6 days 1 time, 2-3 times in total, and each plant has 250 ml of liquid medicine. At the early stage of the disease, Dixon and flour can be mixed into paste according to the ratio of 1∶20, and applied to the base of the diseased stem, which also has certain disease prevention effect.

(3) Control of watermelon anthracnose.

Watermelon anthracnose is a common fungal disease, which can occur at seedling stage and adult stage. Watermelon is not only serious in autumn shed, but also developed in spring shed and sunlight greenhouse, which can reduce production by 10% ~ 20% when it is serious. Sick melons develop continuously during storage and transportation, leading to rotten melons.

(1) harmful symptoms. The disease can occur from seedling stage to harvest stage of watermelon. At seedling stage, round or nearly round lesions were mainly produced on cotyledons, which were yellow-brown, with rings in the center and yellow halo at the edges. Petiole or melon vine disease is a spindle-shaped or nearly spindle-shaped lesion at first, and then the lesion is concave with dark brown color and many small black spots (that is, pathogenic conidia). After the lesion surrounded the melon vine for a week, the whole watermelon plant died. Leaves are susceptible to diseases, with round or spindle-shaped waterlogging spots at the initial stage, sometimes wheel marks, and then the diseased spots turn dark brown, which is easy to break when dry, and sticky substances appear on the surface of the diseased spots when humidity is high. At the early stage of fruit susceptibility, the diseased spots sank, and then the diseased spots turned dark brown and easily broken. When the humidity is high, sticky things appear on the surface, and even watermelons rot in severe cases.

② Pathogen. Colletotrichum gloeosporioides belongs to fungi subfamily.

③ Occurrence regularity. The disease parasitizes on soil disease residues through hyphae or pseudosclerotia, and produces conidiophore and conidia to infect watermelon when conditions are suitable. Conidia are produced after watermelon disease, which are spread by wind, rain and irrigation water and repeatedly infected. It is easy to get sick when the temperature is 20 ~ 24℃ and the air relative humidity is 90% ~ 95%, but it is not easy to get sick when the temperature is higher than 28℃ and the humidity is less than 54%. Low terrain, poor drainage, or too much nitrogen fertilizer, poor ventilation, but also easy to get sick.

④ Prevention methods.

A, selecting disease-resistant varieties and sterilizing the seeds. First, select disease-resistant varieties, and then disinfect the seeds. Seeds for production can be sterilized by soaking in warm water at 55℃ for 65438 05 minutes. Pesticide treatment: the seeds are soaked in 40% formaldehyde or glacial acetic acid for 30 minutes, respectively 100 times, cleaned with clear water, and then sown, or soaked in 500 times 50% carbendazim wettable powder for 60 minutes, or soaked in 500 times 50% mancozeb aqueous solution 1 hour, and then germinated with clear water or dried in the air for direct seeding.

B. cultivate and prevent diseases. Apply sufficient decomposed farmyard manure and increase phosphorus and potassium fertilizer. Do a good job of ventilation and dehumidification, prevent the shed film from dripping and dew from condensing on the leaves, and inhibit the occurrence of diseases. When pulling out seedlings, remove diseased leaves and clean up the fields.

C. chemical control. When picking diseased leaves at the early stage of the disease, it can be sprayed in time, 500 times of 50% carbendazim wettable powder, 600 times of 75% chlorothalonil wettable powder, 50% benomyl wettable powder 1500 times, 500 times of 50% anthrax thiram wettable powder, 600 times of 50% thiophanate methyl wettable powder or 70% Dyson. Spray every 7 ~ 10 days 1 time for 3 ~ 4 times continuously. In the nature reserve, 5% carbofuran dust agent or 5% chlorothalonil dust remover can also be used, and the dosage is per mu 1 kg each time.

(4) Powdery mildew of watermelon.

Watermelon powdery mildew, commonly known as white hair, is an important disease of watermelon in protected fields and a common disease of watermelon. Usually, in the middle and late growth stage, the disease is more serious, resulting in dry leaves and even early seedling pulling.

(1) harmful symptoms. Powdery mildew mainly harms watermelon leaves, followed by stems and petioles, and generally does not harm melon strips. At the early stage of the disease, small white powder spots grow on the front or back of the leaves, which gradually expand and become thick and contiguous. In severe cases, the whole leaves are covered with white powder, then turn gray, and the leaves are yellow and fragile, but they do not fall off. Generally speaking, diseased leaves spread from bottom to top. The lesions on petiole and stem are similar to those on leaves, but the amount of white powder is less. In the late growth stage, sometimes yellow-brown spots appear on the lesion, and then turn black, which is the closed envelope of bacteria.

② Pathogen. It is caused by fungal infection of Ascomycenae.

③ regularity of diseases. The pathogen overwinters in the soil as the remains of diseased plants, or in the living body in the greenhouse. With the temperature rising in May and June of the following year, germs spread through airflow and rain, and landed on the host to infect diseases. This pathogen needs a wide range of humidity. When the weather is dry, the swelling pressure of host epidermal cells decreases, which is beneficial to the invasion of germs and often aggravates the disease. In rainy and humid weather, the spores of bacteria often rupture due to excessive water absorption, which reduces bacterial infection. Extensive cultivation and management, insufficient fertilization, or partial application of nitrogen fertilizer, excessive watering, excessive plant growth, dense branches and leaves, poor ventilation and insufficient illumination are all conducive to the occurrence and harm of powdery mildew.

④ Prevention methods.

A. select disease-resistant varieties. Different watermelon varieties have different resistance to powdery mildew. General varieties resistant to downy mildew are also resistant to powdery mildew.

B. fumigation and disinfection of protected areas. 2 ~ 3 days before planting, the shed is sealed, and 250g of sulfur powder and 500g of sawdust are evenly mixed every 100 m3. Put it in a small flowerpot, put it in several places, ignite fumigation 1 night disinfection and sterilization. However, sulfur fumigation should not be used to control powdery mildew in watermelon growth period to prevent phytotoxicity.

C. strengthen cultivation management. Apply base fertilizer, increase the application of phosphorus and potassium fertilizer in protected areas, and apply appropriate topdressing in the middle and late growth stages to prevent plants from growing white and premature aging. Don't water on cloudy days, let out more air on sunny days, reduce the humidity, keep the appropriate temperature, prevent the sultry microclimate and control the occurrence of diseases.

D. chemical control. A. Smoke the shed with 45% chlorothalonil smoke agent at the early stage of onset. B fog method: 200 times of 2% wuyiencin or 2% antifungal agent (nongkang 120), 15% triadimefon wettable powder 1500 times, or 20% triadimefon emulsifiable concentrate 1500 ~, etc. can be used at the initial stage of onset. Or 800 times of 50% thiophanate methyl wettable powder, or 600 times of 75% chlorothalonil wettable powder, or 300 times of 50% polysulfide suspension, or 600 times of 20% mirex suspension, or 800 times of 20% mirex suspension, or 400 times of 20% mirex suspension. Spray/kloc-0 every 7 ~ 10 days, 2 ~ 3 times depending on the development of the disease.

(5) Watermelon leaf blight.

(1) disease symptoms. This disease damages the leaves, petioles, vines and young fruits of watermelon. At the early stage of the disease, needle-like transparent or translucent spots are produced on the back of the leaves, which are round and slightly concave. A few days later, the lesion expanded into a flake, the color of the lesion was brown or dark brown, the edge of the leaf rolled up and began to fade, and the top of the melon vine shrank and tilted. Then the disease developed rapidly, and the whole watermelon became ill. The leaves dried up from bottom to top, and even the watermelon died.

(2) The occurrence and epidemic law of diseases. From June 1 1 to March of the following year, the disease occurs more frequently, and it occurs faster when the climate is dry, the temperature is high, and the watermelon is planted with organic fertilizer or other nutrients. According to field investigation, the peak period of watermelon disease is from flowering to fruiting.

③ Prevention methods.

A. Increase the application amount of organic fertilizer to ensure the healthy growth of watermelon in the later period.

B) Grasp the regulation degree of fertilizer and water at the flowering stage of watermelon, so as to avoid leaf blight caused by insufficient nutrient supply of watermelon due to excessive control of water and fertilizer. Chemical prevention and treatment: At the initial stage of the disease, 25% Kairun 1800 times solution or 24% Shengpu 600 times solution were used in turn for 2-3 times, and/kloc-0 was applied every 4-6 days. Or spraying 75% haosujing 1200 times and 12% myclobutanil 4000 times on the leaves of watermelon fruit at the later stage of expansion. The key time of applying pesticide is on the eve of watermelon flowering and watermelon swelling.

(6) Watermelon and cotton rot.

(1) disease symptoms. Diseases at seedling stage cause sudden collapse, and fruits are mainly damaged at fruiting stage. Watermelon clay comes on first, the affected area is brown at first, soaked in water, and soon becomes soft, then the whole watermelon becomes brown and soft rot.

② Regularity of occurrence. The average temperature is 22 ~ 28℃, and rainy days or high humidity are conducive to the occurrence and spread of the disease.

③ Prevention methods.

A. use antibiotic bacteria to inhibit pathogenic bacteria. For example, spraying 600 times of "5406" No.3 agent in Gua Tian will rapidly increase the number of antibiotic bacteria in the soil, which will inhibit the growth of pathogenic bacteria after taking advantage of it, thus achieving the purpose of disease prevention.

B, adopting high ridge cultivation. Avoid flooding, drain water in time after heavy rain, and pad melons when necessary.

C) Spraying 300 times 14% copper complex aqueous solution or 500 times 10 copper succinate wettable powder at the initial stage of the disease, and continuously controlling for 2-3 times every10 day.

(7) Watermelon white silk disease.

(1) harmful symptoms. At the early stage of the disease, the plant wilted at noon, the leaves turned yellow, and the whole stem base died within a few days, and the plant withered completely. The affected part showed dark green spots at first, then expanded and slightly depressed. In the later stage, the lesion can be enlarged to 1 ~ 2 cm, the lesion is nearly round, and the affected part produces white cotton wool, which covers the stem surface in a fan shape. In the white cotton wool, there are light brown to dark brown sclerotia the size of mustard seeds. Fungi can also invade the fruit that comes into contact with the soil, causing the fruit to rot, and a large number of molds and sclerotia grow on it.

② Regularity of occurrence. Bacteria develop well under the conditions of high temperature, high humidity and sufficient air, so there are more diseases in loose sandy loam.

③ Prevention methods. Carry out rotation and deep ploughing, clear Gua Tian diseased plants in time, and disinfect the soil. For flood and drought disasters 1 year or more, implement crop rotation and deep tillage to avoid direct contact between fruits and ground. 0.5 kg of quintozene and 15 ~ 25 kg of fine dry yellow mud were mixed with pharmaceutical soil and applied to the ground of stem base, twice every two weeks 1 time. It is also possible to spray 800 ~ 1000 times of 50% mancozeb aqueous solution on the plant base and its surrounding soil, and the control effect is good.

(8) Brown rot of watermelon.

① symptoms. The stem produces dark green spots of oil stains, which rot and wither above the affected area and die. Irregular oily dark green to grayish green spots are produced when leaves are damaged, and both young fruits and mature fruits can be damaged. At first, oily dark green small round spots were produced, and then quickly expanded into dark brown irregular spots and rotted.

② Regularity of occurrence. In the rainy season, acid soil with poor drainage is prone to epidemic.

③ Prevention methods. At the initial stage of the disease, the same amount of Bordeaux solution 1% or mancozeb 600-800 times solution, thiophanate-methyl 50% 1000 times solution and chlorothalonil 70% 500-800 times solution were sprayed. Every 10 day 1 time, * * * spray 2 ~ 3 times.

(9) Watermelon virus disease.

Watermelon virus disease is a disease that infects the whole reproductive system of watermelon.

(1) harmful symptoms. The virus can reach anywhere except the growing point. At seedling stage, diseased cotyledons turn yellow and wither, young leaves are dark green and light green flowers, and diseased leaves shrink and deform to varying degrees. The diseased new leaves of adult plants are yellow-green, the diseased leaves are small and shriveled, and the leaves become thicker. In severe cases, the leaves will turn over and over. The internode of stem is shortened and the stem is deformed. In severe cases, the leaves of diseased plants withered, dark green and light green spots appeared on the surface of melons, the surface was uneven and the melons were deformed. The seriously ill plants are clustered with small leaves and do not produce melons, and finally atrophy and die.

② Prevention methods.

A, selecting and planting disease-resistant varieties or disease-resistant varieties, and sterilizing the seeds before sowing.

B. strengthen cultivation management. First of all, we should cultivate strong seedlings and apply more phosphorus and potassium fertilizer to improve the disease resistance of plants; Secondly, we should pay attention to the hygiene of field work and wash our hands with soap in time after contact with germs.

C do a good job in the prevention of diseases, especially aphids, especially at the seedling stage. When aphids occur in the field, 10% imidacloprid wettable powder can be used for control.

D. chemical control. At the beginning of the disease, 500 times of 20% virus A wettable powder or 600 times of 20% virus ling wettable powder can be sprayed. If 800 times of plant growth promoters such as Baoliwang or Xiaoyedi are mixed into the above liquid medicine, the inhibition effect will be better.

(10) Meloidogyne disease of watermelon.

(1) harmful symptoms. It only occurs in roots, and lateral roots and branch roots are the most susceptible. Tumors or nodules of different sizes are produced in the affected parts of the roots. In internal section, we can see that there are white tiny pear-shaped females in the diseased tissues. Nodules or the upper end of nodules often produce tiny new roots, which are invaded by nematodes and become swollen like nodules. The overground parts of seriously ill plants are weak and short, and seem to be short of water and fertilizer, with weak leaves and no or less melons.

② Regularity of occurrence. Overwintering in root nodes with second instar larvae or eggs of the current year. When the environment is suitable in the second year, the overwintering eggs hatch into larvae, and the overwintering larvae continue to develop, mostly invading from tender roots, stimulating the proliferation of host cells and forming nodules, in which the larvae develop. Neutral sandy loam is seriously ill.

③ Prevention methods. Implement two to three years of crop rotation, or paddy-upland crop rotation. Reasonable irrigation and fertilization can delay the symptoms of diseased plants and reduce losses. If there are root-knot nematodes in the seedbed, soil disinfection should be carried out 2-3 weeks before sowing, or the seedbed should be treated with chlorfenapyr and mequat.