Occurrence Patterns and Control Methods of Vegetable Seedling-Stage Diseases

Raising seedlings is a very important step in vegetable cultivation, and the saying "good seedlings mean half the harvest" is the best summary of raising strong seedlings. With the continuous development of protected vegetable cultivation and the continuous expansion of vegetable cultivation areas in the "natural greenhouse" of the south, seedling-raising methods and seasons are also constantly changing, and disease control during the process of raising strong seedlings has become one of the key techniques.
　　
1. Occurrence and damage characteristics of vegetable seedling-stage diseases:
At present, winter and spring seedling raising of solanaceous fruits, cucurbits, and legumes mainly provides strong seedlings for early spring cultivation under protection and open-field spring cultivation; summer and autumn seedling raising of solanaceous fruits, cucurbits, crucifers, and legumes provides strong seedlings for delayed autumn cultivation under protection, open-field autumn cultivation, and vegetable cultivation for the south-to-north vegetable transport program. During winter and spring vegetable seedling raising, the main diseases occurring at the seedling stage include damping-off, Rhizoctonia rot, gray mold, anthracnose, and root rotting; during summer and autumn vegetable seedling raising, the main diseases occurring at the seedling stage include viral disease, anthracnose, sclerotinia rot, and heat injury.
1.1 Damping-off: It mainly damages seedlings of solanaceous fruits, cucurbits, celery, lettuce, cabbage, onion, and other vegetables. It is a type of disease that frequently occurs during the period from winter to early spring seedling raising, and occurs more often during the period from when the seedlings have not yet emerged to before the true leaves have fully unfolded. At this time, the nutrients in the cotyledons of the seedlings have been exhausted, the new roots have not yet taken firm hold, the young stems have not yet become suberized, and the seedlings have the weakest disease resistance. During this period, cold waves, low temperatures, and continuous rainy weather, or sowing too densely and relatively high soil moisture, are unfavorable for seedling growth and can easily cause damping-off.
1.2 Rhizoctonia rot: It has a very wide host range, reaching more than 160 species according to incomplete statistics, including vegetables such as solanaceous fruits, cucurbits, legumes, crucifers, celery, and lettuce. It mostly occurs during the period from just after seedling emergence to before transplanting, and is especially more common in the middle and late seedling stages. More rainy weather and relatively high seedbed humidity can easily cause excessive seedling growth and aggravate the occurrence and spread of Rhizoctonia rot.
1.3 Gray mold: In recent years, it is a vegetable seedling-stage disease that develops rapidly and causes relatively serious damage during winter and spring seedling raising. It mainly damages vegetables such as solanaceous fruits, lettuce, celery, and legumes, and occurs more often in the middle and late stages of seedling growth. In seedbeds, it often develops from spots to patches, and when the temperature is relatively low and the humidity is relatively high, it can easily cause patches of seedlings to die, or even destroy the seedlings in an entire shed.
1.4 Anthracnose: In recent years, it has occurred relatively seriously at the seedling stage of vegetables such as chili pepper, sweet pepper, watermelon, melon, winter melon, and common bean. It can occur from the emergence of cotyledons to before transplanting. When temperature and humidity are suitable, the growing points of the seedlings are damaged, and the seedlings very easily die. Because there are many kinds of anthracnose pathogens harming vegetables, and the pathogens have a relatively wide adaptability to temperature, seedlings of many vegetables in both winter-spring and summer-autumn seasons are prone to anthracnose. Especially under the high-temperature and rainy climate conditions of summer and autumn, it is even more likely to produce symptoms of large areas of water-soaked lesions on leaves and acute defoliation from bottom to top, causing great harm to the seedlings.
1.5 Root rotting: Almost all winter and spring vegetable seedlings can suffer from root rotting. After emergence, the seedlings do not produce new roots for a long time, the outer skin of the young roots becomes rusty brown and gradually rots; stem and leaf growth is inhibited, leaves gradually turn yellow, and no new leaves are produced. Diseased seedlings can easily be pulled out of the soil, and in severe cases the diseased plants wilt and die. Seedling root rotting is a physiological disease, mainly caused by more snow, rain, or cloudy and rainy weather in winter and spring, severely insufficient light, low seedbed soil temperature, and high humidity, which block seedling respiration and reduce water absorption capacity.
1.6 Sclerotinia rot: It is an important disease to which many vegetable seedlings are extremely prone during summer and autumn vegetable seedling raising, especially the seedlings of chili (sweet) pepper, eggplant, legumes, and cucurbits in vegetable bases for the south-to-north vegetable transport program.
1.7 Viral disease: Viral disease occurs in almost all summer and autumn vegetable seedlings. Because the seed-borne virus rate of vegetables such as legumes and cucurbits is very high, seedlings from virus-carrying seeds show viral disease as soon as they emerge. In addition, insects such as aphids very easily transmit viruses, and some viruses are very easily spread through contact such as farming operations. Therefore, the incidence of viral disease on summer and autumn vegetable seedlings is very high.
1.8 Heat injury: It is a physiological disease often encountered by summer and autumn vegetable seedlings. It often occurs on seedlings raised using rain-shelter facilities, mainly because the film is not removed in time after rain clears and the temperature inside the shed exceeds 40°C, sometimes reaching above 50°C, causing scorching of seedling leaves or growing points that is difficult to recover from in a short time.
　　
2. Integrated control techniques for vegetable seedling-stage diseases:
Only by adopting integrated disease control measures can the occurrence and damage of seedling-stage diseases truly be controlled. At present, the following integrated measures are generally adopted.
2.1 Selection of seedbed site. The seedbed site should be selected as a disease-free plot with high terrain, smooth drainage, and loose, fertile soil. For winter and spring seedling raising, the seedbed is best located in a wind-sheltered, sunny place; for summer and autumn seedling raising and seedling raising in south-to-north vegetable transport bases, it is best to choose a plot that is easy to ventilate and dissipate heat.
2.2 Selection of seedbed soil and fertilizer. For bed soil, it is best to use disease-free new soil. Old bed soil or old garden soil is very likely to carry pathogens, so bed soil should usually be disinfected. At present, the most commonly used method is to apply 5 g of 50% carbendazim or 50% thiophanate-methyl wettable powder per square meter of seedbed, mix evenly with 5 kg of fine soil, first water the seedbed with bottom water before applying the chemical, watering thoroughly in one application. After the water infiltrates, take 1/3 of the fully mixed medicated soil and spread it on the bed surface. After sowing, cover the seeds with the remaining 2/3 of the medicated soil so that the seeds are sandwiched in the middle of the medicated soil. The control effect is obvious and the efficacy lasts a long time, but the amount of chemical must be strictly controlled and must not be excessive, otherwise phytotoxicity easily occurs. This method has a very good control effect on seedling-stage damping-off and Rhizoctonia rot. Organic fertilizer used for the seedbed is best used after full fermentation at high temperature, which can not only kill pathogens but also kill insect eggs and weed seeds.
2.3 Seed disinfection. ① Hot water seed soaking: This is the most commonly used method for seed disinfection treatment. Seeds of solanaceous fruits and cucurbits are generally soaked in 55°C warm water for 10-15 min, stirring continuously during this period. After the water temperature drops below 30°C, cucumber is soaked for another 2 h, tomato, chili pepper, and eggplant for 4 h, and winter melon for 0.5 h. When soaking seeds in warm water, the soaking temperature and time should be strictly controlled to avoid affecting seed germination. ② Dry heat treatment of seeds: Dry seeds of cucumber, watermelon, melon, or tomato treated with dry heat at 70°C for 3 d can kill pathogens on the seed surface and inside the seeds. ③ Chemical treatment of seeds: Soak seeds in 0.1% 60% carbendazim hydrochloride ultrafine powder liquid for 50-60 min, then remove, rinse clean, and germinate. This is beneficial for emergence and prevents seedling-stage sclerotinia rot and anthracnose; dressing seeds with 0.4% captan can prevent damping-off and Rhizoctonia rot; soaking tomato, chili pepper, sweet pepper, and watermelon seeds in 10% trisodium phosphate (Na3PO4) aqueous solution for 20-30 min and then rinsing clean with clear water can prevent viral disease.
2.4 Strengthen seedbed management. ① In winter and spring, electric heating seedbeds or factory-style plug rapid seedling raising are advocated; in summer and autumn, insect-proof, shade, and rain-shelter seedling raising are advocated, so as to minimize the impact of adverse weather during seedling raising and at the same time facilitate scientific management. ② Winter and spring seedling raising should adopt freeze-proof and heat-preservation measures, and covers should be removed and put on at the proper time. If the seedbed temperature is too high or too low, it can easily reduce the disease resistance of the seedlings. As long as the seedbed temperature permits, covers should be removed in time to increase light, thereby improving seedling disease resistance. ③ Summer and autumn seedling raising should adopt insect-proof, cooling, and rain-shelter measures; none of these three links can be omitted. If insect prevention is not timely, seedlings are prone to viral disease, and once seedlings are infected with virus, it is very difficult to cure them; if cooling measures are inadequate, seedlings grow poorly and are easily scorched; if rain-shelter measures cannot keep up, seedlings are very easily wounded by strong wind and heavy rain, and sometimes typhoons and rainstorms may wash the seedlings away. At the same time, attention should be paid to promptly removing the rainproof film after rain clears to prevent seedling scorching. ④ Strictly control seedbed moisture. Whether raising seedlings in winter-spring or summer-autumn, seedbed soil humidity and air humidity are the key factors determining the severity of seedling-stage diseases. Especially for winter and spring seedling raising, watering the seedbed is best done on sunny days from 10:00 to 12:00, and ventilation should be provided at noon after watering to reduce humidity. ⑤ Reducing soil humidity and increasing soil temperature are the fundamental measures to alleviate root rotting of winter and spring vegetable seedlings. ⑥ Strengthen seedbed inspection. When scattered diseased plants with viral disease, gray mold, sclerotinia rot, or Rhizoctonia rot are found, they should be removed in time. Reducing inoculum sources can also achieve the purpose of controlling disease spread.
2.5 Chemical control. In the early stage of disease, spray on a sunny morning with 15% hymexazol aqueous solution at 400 times dilution, or 40% Dacoling suspension concentrate at 500 times dilution, or 50% thiophanate-methyl wettable powder at 500 times dilution, or 60% carbendazim hydrochloride ultrafine powder at 600 times dilution, all of which have relatively good control effects on damping-off and anthracnose. Spraying with 58% Ridomil wettable powder at 800 times dilution, or 25% metalaxyl wettable powder at 600 times dilution, or 64% Sandofan M8 at 500 times dilution has a relatively good control effect on seedling-stage Rhizoctonia rot. When spraying, attention should be paid to spraying the stem base of the seedlings. For seedling-stage gray mold and sclerotinia rot, spraying with 50% procymidone wettable powder at 1000 times dilution, or 30% vinclozolin wettable powder at 2000 times dilution on a sunny morning has a relatively good effect. If continuous rainy weather occurs, 10% procymidone or vinclozolin smoke agent, or 15% procymidone-chlorothalonil compound smoke agent can be used at 250 g per mu of seedling shed (room, chamber), divided into 4-5 ignition points and burned, then kept closed for fumigation overnight. The control effect on gray mold and sclerotinia rot is better than spraying. Using 15% procymidone smoke agent at 250 g per mu has a relatively good control effect on damping-off and Rhizoctonia rot. Depending on weather conditions, alternating use of smoke agents and sprays is beneficial for improving control efficacy. Depending on the disease condition, applying the chemical continuously 2-3 times can control the spread of the disease. For summer and autumn seedling raising, attention to controlling aphids is a key measure to prevent the occurrence of viral disease. At present, the better aphid-control chemicals include 90% methomyl soluble powder at 2000 times dilution, 10% chlorfenapyr suspension concentrate at 2000 times dilution, 21% fenvalerate emulsion at 5000 times dilution, 10% imidacloprid wettable powder at 1500 times dilution, and 50% pirimicarb (pirimor) wettable powder at 1500 times dilution. For seedlings with mild viral disease, spraying 20% Virus A wettable powder at 500 times dilution, or 1.5% Zhibingling emulsion at 800 times dilution has a certain effect.