Tomato (Lycopersicon esculenton L.) is a vegetable belonging to the Solanaceae family, which includes potato (Solanum tuberosum), eggplant (Solanum melongena), and peppers (Capsicum sp.). The largest European tomato producers are Italy, Spain, Portugal, Greece, France and Poland. Tomato is originally from South and Central America and was brought to Europe by Spanish navigators and, presently, is an important component of the Mediterranean diet. Its intense red colour is the result of lycopene’s presence, which is a powerful antioxidant.
The use of high-tech greenhouses, which include cutting-edge technology to control the climatic conditions inside the greenhouses (temperature, humidity, ventilation, artificial lighting, shading, etc.), transforms the methods of producing plants and contributes to food security. Under protected conditions, tomato can be cultivated in soil or in a soilless system (hydroponic condition with a substrate and drip fertigation), as grafted or non-grafted plants.
If not appropriately managed, the greenhouses’ climatic conditions provide an ideal environment for the development of several foliar, stem and soil-borne plant diseases. A brief summary of the main tomato diseases that cause severe reductions in plant productivity and economic losses in greenhouses is presented below.
Late Blight (Phytophthora infestans (Mont.) de Bary)
Late blight disease is a highly devastating disease that can infect and destroy the leaves, petioles, stems and fruit of tomato plants. The undersides of leaf lesions or spots on the fruit can be covered with a velvet white to grey mycelium that consists of branched conidiophores that emerge through the stomata. The pathogen sporulates on the lesions under favourable environmental conditions, which are wet leaves for longer than 10-12 hours at cool/moderate temperatures (15-21°C), an environment quite common in greenhouse conditions. P. infestans can be dispersed by water splash or wind currents for up to several kilometres from the infected plant tissues via the asexual fruiting bodies, called sporangia. The pathogen overwinters in volunteer or abandoned tomato plant material in greenhouses or the surrounding area. Smart technologies can focus on monitoring and detection of disease in the greenhouse, for instance by remote sensing spectral imagery and automatic counting.
Early Blight (Alternaria solani Sorauer)
Early blight is an important disease of tomato plants in humid climates or in semiarid areas where adequate moisture permits disease development. The disease primarily affects leaves and stems, which can result in considerable defoliation, yield reduction and sunburn damage of the exposed fruits. As the disease develops, spots enlarge and can reach 8-10 mm in diameter, containing the characteristic concentric rings. The disease is favoured by mild (24-29°C), rainy weather, although the disease may also be quite active at higher temperatures. Conidia disperse via wind and splashing water. The pathogen can survive between crops in the soil on infected crop debris on volunteer plants and other Solanaceous hosts, and on infected tomato seeds. Smart technology can focus on the detection of Early blight disease in the greenhouse by using sensor systems, multispectral imaging and automatic counting.
Alternaria Stem Canker (Alternaria alternata (Fr.:Fr.) Keissl. f. sp. lycopersici)
The pathogen can infect all above ground parts of the tomato plant. Dark brown to black cankers, with concentric rings, form on the stems and may eventually girdle the stem, killing it or even the entire plant. Conidia are easily spread by the wind and require free moisture on the tomato plant to germinate. Overhead irrigation, rain, and heavy dew favour the development of the disease. The optimum temperature for disease development is 25°C. The entry of the fungus into the stem is facilitated by pruning cuts or other wounds; however, it can also infect healthy and uninjured plants. When susceptible varieties are growing it is advised not to use irrigation with overhead sprinklers and to rotate the crops.
White Mold (Sclerotinia sclerotiorum (Lib.) de Bary and Sclerotinia minor Jagger)
White mold is a common greenhouse disease that damages tomato throughout the world. The pathogens produce sclerotia on white mycelial mats 7-10 days after infection. Sclerotia fall on to the soil where they can survive for several years. When weather conditions are favourable, they germinate to produce apothecia which release ascospores causing a new infection. The infection occurs especially at high air humidity and moderate temperature. Growing the plants in substrate with drip irrigation is recommended.
Botrytis Gray Mold (Botrytis cinerea Pers.:Fr.)
Gray mold is a very common disease of Solanaceous crops that can be particularly damaging in greenhouse environments in the presence of high air humidity. The pathogen can cause damping-off, as well as blight of the flowers, fruits, stems, and foliage and is a major cause of post-harvest rot. The fungus sporulates profusely on the fruit calyx or in the centre of the fruit lesion, where the skin ruptures and appears as a grey, velvety or fuzzy mold.
Conidia are carried on to the host surface by wind or splashing rain drops. A high level of air humidity is necessary for prolific spore production and spores germinate and produce an infection when free moisture occurs on the plant surface. Optimum temperatures for infection are between 18-24°C, but infections can also result from direct contact with moist infested soil or plant debris. Under greenhouse conditions, effective management can be achieved by avoiding the conditions that favour gray mold development (high air humidity and cool temperatures), by adequate ventilation, careful handling of fruit to prevent wounding, and by removing inoculum sources through adequate plant sanitation. Detection of Gray mold before the appearance of visual symptoms can be done using a multispectral sensor and automatic counting.
Fusarium Wilt and Fusarium Crown and Root Rot (Fusarium spp.)
Fusarium wilt disease is a warm-weather disease caused by the fungus Fusarium oxysporum f. sp. lycopersici. It is a soil-inhabiting fungus that can survive for extended periods in the absence of the plant host, mainly in the form of thick-walled chlamydospores. After the infection of the host roots, the mycelium advances intercellularly through the root cortex until it reaches the xylem vessels to rapidly colonize the host. The management of Fusarium wilt in greenhouses is possible by prevention, using resistant cultivars, disease-free seedlings, new growth media for each crop, crop rotation with alternative crops, good sanitation practices and by maintaining optimum growing conditions.
Fusarium crown and root rot disease, caused by Fusarium oxysporum f. sp. radicis-lycopersici, is a brown discoloration of the root system, predominantly at the tip of main root, the base of the stem, and the vascular region of the central root. The discoloration does not extend beyond 10-30 cm from the soil surface and this feature helps to distinguish this disease from Fusarium wilt, in which the vascular browning can extend far into the upper stems. The infected plants must be carefully removed and destroyed. Effective control of the fungus may be obtained by soil disinfestation, crop rotation or the use of substrate, resistant cultivars, and by grafting on resistant rootstocks.
Pythium Damping-off (Pythium spp.)
Different Pythium spp. may attack tomato plants during their early stages of growth, causing seed rot, seedling damping-off or stem rot. Pre-emergence damping-off is the most common symptom. Pythium spp. are spread by sporangia, which release hundreds of zoospores. The fungi survive in the soil as saprophytes and are most favoured by wet soil conditions and cool temperatures (15-20°C). The irrigation practice in greenhouses contributes to the fast development and spread of Pythium spp. Plants should be placed on raised beds and in well-drained soils, and grown under optimal temperature, moisture, and nutritional conditions. They are seed and soil-borne pathogens where they form oospores and chlamydospores on decaying plant roots that can survive prolonged periods under adverse conditions, causing subsequent infections.
Efforts to design and implement effective plant health management programmes have evolved through an increased understanding of plant ecology and physiology and the interactions of factors causing adverse effects on plant health. The pressure on growers is exacerbated due to the restrictions imposed on the use of pesticides and the high demands of retailers and consumers in terms of quality. It is important to use innovative solutions to support the sustainability of production systems, quality and safety in horticultural crops.
Integrated Pest Management (IPM) is based on the elimination of the pathogen inoculum through high standards of hygiene (sterilizing soil or using soilless media, obtaining disease-free planting material, etc.), cultural practices to limit disease spread, and biological control. The use of molecular techniques improves the speed and accuracy of detection and identification of individual plant pathogens in laboratory diagnostics. The availability of disease-resistant cultivars also facilitates crop protection and reduced pesticide applications.
Effective plant health management depends on the availability of precise data regarding accurate systems for monitoring and modelling the population dynamics of pest insects and disease. With the installation of climatic stations, the producers can receive timely and accurate information about atmospheric parameters (e.g., relative humidity, temperature, precipitation, solar radiation, leaf wetness) and soil parameters (e.g., soil moisture and temperature). The next steps include the monitoring the data through the customized user interface of a web application and the development of the scientific models for crop protection, fertilization and irrigation.
Drones provide aerial images and allow visual monitoring of crops and equipment, and with a very detailed analysis, also help to estimate the productivity of the crops and to detect pests or diseases that allow the producer to apply pesticides with greater precision. The use of information obtained from NDVI maps (Normalized Difference Vegetation Index) allow growers to identify spatial or temporal variability in the health of the crops. The micro-robots are already on their way to the agricultural industry, and may be able to replace pollinating bees or to examine the roots of plants to prevent pest attacks and anticipate their management.
In a web conference “The future of agriculture: how technology is changing the way we grow food” (8 October 2020), Steve Hoffman stated that precision agriculture will soon be one of the most valuable industries in the world, and is expected to reach a market value of around $ 240 billion by 2050. He also referred to the importance of the role that technology will have in doubling food production, which will be needed to feed the expected 10 billion people on planet Earth in the coming years. SMART technologies will drive a more sustainable agriculture, diagnose/detect problems, monitor pest and disease infestations, and provide efficient advisory systems.