Typical Diseases of Greenhouse Tomatoes and Preventive Measures

1. Reasons for the occurrence of greenhouse tomato diseases

1.1 High temperature, high humidity, sealed environment

When the greenhouse environment is closed, it will cause high temperature and high humidity inside, which is conducive to the reproduction of pathogenic bacteria, resulting in serious moisture-loving diseases, such as tomato botrytis cinerea disease. In addition, it is easy to dew condensation if the temperature difference between day and night is large, and the humidity of the greenhouse can be increased twice. In winter, the ground temperature is low, which prolongs the infection period of the fungus and causes the root infection to be significantly aggravated.

1.2 Continuous cropping

Continuous cropping in the greenhouse provides suitable living and breeding conditions for the occurrence of soil-borne diseases, promotes the accumulation of pathogenic bacteria in the greenhouse soil, and leads to the occurrence of soil-borne diseases, such as fusarium wilt disease, sclerotinia disease, root knot nematode disease.

1.3 Extensive management and backward technology

When the insecticide, sterilization and disinfection are not thorough, and the tomato plant residues with pathogens piled up outside the greenhouse cannot be destroyed in time, the pathogenic bacteria will spread with the wind and cause diseases. At the same time, the use of pesticides is unreasonable, the identification of tomato diseases is inaccurate, and the right medicine is not prescribed, making the disease more difficult to control.

2. Common diseases of greenhouse tomatoes

2.1 Tomato early blight disease

The disease can occur at both the seedling and adult stages, mainly affecting leaves, stems, flowers and fruits. The leaves are dark brown spots at the initial stage, and later expand to round or oval gray-brown lesions，generally, the disease starts from the lower leaves and gradually expands upwards. The diseased spots on stems, petioles and fruit stalks are oblong, and the plant is easily broken from the diseased part, the plant dies when the diseased spots at the base of the stem circle around the stem. The disease of green fruit forms an oval dark brown lesion at the calyx or umbilicus, the lesion is sunken, the diseased fruit is easy to crack and turn red early.

2.2 Tomato botrytis cinerea disease

The disease can occur throughout the growth period of tomato, mainly in the flowering and fruiting stages, and can damage flowers, fruits, leaves and stems. The disease of leaves starts from the tip of the leaves, and water-soaked state light brown lesions appear, the lesions are V-shaped, and there are obvious gray mold layers on the surface. The fruit disease mainly occurs from the green fruit stage to the mature stage which infects the residual stigma or petals firstly, and then develops to the fruit surface and fruit stalk. The diseased pericarp is gray-white, water-soaked state, and softly rotted, in the middle and late stage of the disease, a gray villous mold layer appeared.

2.3 Tomato late blight disease

It mainly damage seedlings, leaves, stems and fruits. The seedlings are water-soaked dark green at the beginning of the disease, and the disease spots spread from the leaves to the main stems, making the stems thin and dark brown. When the humidity is high, a sparse white mold layer appears, and in severe cases, the whole plant scorches and dies. The disease mostly starts from the leaf tip or leaf margin, and initially forms dark green, water-soaked irregular lesions, which become brown after expansion. The lesions on the stems are dark-brown lesions, which can be putrefying after embracing the stems, constricting and shrinking after dehydration, and wilting the affected part, while the lower part often grows adventitious roots. Fruit disease mostly occurs in the green fruit stage, with oil-soaked light brown lesions at the initial stage, and dark brown after expansion. When the humidity is high, a small amount of white mold grows on the edges of the lesions. Severely afflicted fruit diseased parts appeared strip-shaped cracks, and oily droplets were leached.

2.4 Tomato fusarium wilt disease

It is a kind of vascular disease caused by the fusarium wilt fungus. It usually occurs during the flowering and fruiting period. In the early stage of the disease, only the lower leaves of the plant turn yellow, and then turn brown, wilt and dry up, but do not fall off. Symptoms sometimes appear only on one side of the stem, or a leaf is yellow on one side and normal on the other. Sectioning the stem, petioles and fruit stalks, it can be seen that the vascular bundles are brown. In humid conditions, pink mold develops at the base of the stem of the diseased plant. The disease progresses slowly, usually withering within 15-30 days. 

2.5 Tomato virus disease

It is caused by individual or mixed infection of various viruses, mainly tobacco mosaic virus (TMV) and cucumber mosaic virus (CMV). Symptoms such as chlorosis, yellowing, dwarfing and deformity will appear after tomato disease. There are mainly three types: mosaic type, striped type and fern type.

3. Prevention and control measures of diseases

Greenhouse tomato diseases have the characteristics of recurrence and uncertainty. It is necessary to well implement the prevention and control policy of "prevention first, comprehensive control", and coordinate the use of agricultural measures, physical measures, chemical measures and other measures for comprehensive control.

3.1 Agricultural measures

3.1.1 Select high-quality disease-resistant varieties, adjust crop layout, pay attention to variety mix when planting, avoid long-term contiguous planting of a single variety, and reasonably intercropping and crop rotation

3.1.2 Reasonable fertilization, balanced fertilization, and timely spraying of foliar fertilizer to avoid early weakening of tomato plants. The application of high calcium potassium magnesium and biological bacterial fertilizer can enhance the comprehensive disease resistance of tomato.

3.1.3 Optimize cultivation equipment, improve ventilation and light transmission conditions, and hang reflective films. Use a drip-free film to comprehensively reduce condensation and frosting. Irrigation under the film to reduce the humidity in the greenhouse. 

3.1.4 Clean up the field in time, remove the lower diseased old leaves, clean the diseased plant residues, and reduce the source of pests and diseases.

3.1.5 Film mulching, drip irrigation and scaffolding can significantly reduce the incidence of diseases and increase production.

3.2 Physical control

3.2.1 Dry the seeds for sterilization. Place the seeds in the sun on a sunny day and expose them to the sun for 2-3 days to kill the germs on the surface of the seeds and improve the germination potential of the seeds.

3.2.2 During the high temperature period, turn up the soil and dry the shed to achieve the purpose of disinfection through the solarization and soil ploughing in the high temperature weather during the summer idle period.

3.2.3 After the greenhouse is completed, cover the greenhouse with film on sunny days, and seal the greenhouse for 10 days. When the indoor temperature of the greenhouse reaches 60°C, the spores of the bacteria that survive in the room can be killed, reducing the probability of disease occurrence in the tomato growing period.

3.3 Chemical control

3.3.1 Symptomatic medication, scientific medication. It is necessary to accurately understand the types of greenhouse tomato diseases,  the active ingredients and application methods of pesticides, then choose appropriate pesticide formulations, and at the same time choose high-efficiency, low-toxicity, low-residue pesticides, and strive to achieve symptomatic medication and timely medication.

3.3.2 Actively promote biological pesticides. Alternating use of biological fungicides and chemical agents can not only reduce the use of chemical agents, reduce the risk of chemical residues exceeding the standard, but also overcome the problems of narrow control spectrum and poor control effect of biological pesticides, which can effectively control diseases and delay development of diseases.

3.3.3 Disinfect and sterilize seeds, soil and seedbeds.

1) Seed treatment: use 0.1% copper sulfate, 0.1% potassium permanganate or 300 times formalin for seed soaking treatment, or mix the seeds with 50% carbendazim WP according to the seed amount of 0.3%-0.4%.

2) Soil disinfection: Thoroughly disinfect the soil in the greenhouse before tomato sowing or planting to kill the surviving bacteria in the soil. According to the occurrence of past diseases, choose appropriate agents, such as 50% carbendazim WP of 22kg/hm2, 40% Pentachloronitrobenzene WP of 15kg/hm2, etc., mix the medicament with an appropriate amount of fine dry soil and stir evenly , sprinkled on the surface, and then plowed into the soil.

3) Disinfection of seedbed soil: Change new soil for the nursery bed, or treat the bed soil with soil disinfectant, which can reduce root damage and shorten the seedling period. For each 1m3 of seedbed soil can use 150mL of 50% phoxim EC and 100g of 50% carbendazim wettable powder to disinfect the seedbed soil, which can effectively prevent the occurrence of early blight, stem rot and other seedling diseases.