Diseases

Current Status and Challenges

Disease control in tomatoes

Introduction

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.

Conclusions

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.

Pests

Current Status and Challenges

© JKI, Alexander Pfaff

Pest control in tomatoes

Introduction

Tomato (Lycopersicon esculenton L.) is a vegetable belonging to the Solanaceae family, such as potatoes (Solanum tuberosum), eggplants (Solanum melongena), and peppers (Capsicum sp.). The largest tomato producers in Europe 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, currently, is an important component of the Mediterranean diet. Its intense red color is the result of the presence of lycopene, which is a powerful antioxidant.

The use of high-tech greenhouses that 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 soil-less (hydroponic condition with a substrate and drip irrigation), as grafted or non-grafted.

If not appropriately managed, the greenhouses climatic conditions provide an ideal environment for the development of different Arthropod pests. The exclusion of natural enemies by the protected environment additionally hinders the natural regulation of pest outbreaks. When cultivation takes place in soil, root-knot nematodes can cause problems as well. A brief summary of the most common tomato pests in the greenhouses are presented below.

Tomato leaf-miner (Tuta absoluta Meyrick)

The tomato leaf-miner is a major pest in southern countries for a long time, but has become more widespread in recent years and has increasingly caused significant yield losses in central European greenhouse production. The larvae cause mines in leaves and bore holes in fruit. In case of high infestation, even the head of the plant may die. Pupation takes place in the soil or on the surface of a leaf, but also in mines. Adult moths are grey-brown with filiform antennae, alternating light or dark segments with black spots on the anterior wings. Adults are nocturnal and hide between leaves during the day.

Monitoring is implemented with special pheromone traps or black sticky traps, or a combination of a simple sticky trap with pheromone dispenser attached.

Mating disruption technique is an important tool to prevent population development of Tuta absoluta. Several dispensers are distributed throughout the greenhouse and emit constantly sexual pheromone for about 4-5 months depending on T and RH inside the greenhouse. Thereby, males are impeded from finding their mate and consequently no fertile eggs can be laid. Predatory bugs also feed on larvae of different miners. In those greenhouses where chemical treatments are low, a native Mediterranean parasitoid called Necremnus tutae, often appears and is effectively controlling T. absoluta populations, not only by larvae parasitation but also by host feeding. It is easy to detect be checking the T. absoluta mines with a simple thread counter and some eye training. In case of higher attacks, besides chemical products, there is also a granulovirus commercial product approved in some countries against Tuta absoluta. Successful control of this pest is only possible with a good IPM strategy and a proper combination of different techniques.

Leaf-miner flies (Agromyzidae like Liriomyza huidobrensis and L. bryoniae)

Liriomyza huidobrensis is native to South America, L. bryoniae may origin from South Europe, where it is also present in open fields. To date both flies are widely distributed in Europe and cause problems especially in greenhouse cultivation. Both Leaf-miner flies have a brought host spectrum. Further species occur in tomato cultivation.

The adult insects are about 1.5 – 2.5 cm in size and bright yellow and black in color. Adults are often present on leaves and first signs of attack are punctures on upper leaf sides, used by both sexes to feed on the plant sap. Only some of these punctures are used for egg deposition. Larvae feed on leaf tissue, causing the typical mines. Mines are narrower as compared to Tuta absoluta. The puparium is oval, golden to dark black-brown, sometimes black, about 1 mm wide and 2 mm long. The puparium may be attached to the lower leaf side, or may drop to the ground, or larvae drop to the ground and pupate in the soil or other structures available.

Adults can be monitored with yellow sticky traps. They can usually be controlled easily by biological control agents, such as Diglyphus iseaea or/and Dacnusa sibirica. Successful control can be determined by checking if new mines are present on younger plant parts after the introduction of beneficials.

Tomato russet mite (Aculops lycopersici Massee)

The tomato russet mite is a free-living gall mite that attacks various solanaceous plants. Severe damage occurs mainly in tomato cultivation. For many years, it has been an important pest in southern European countries, and in the USA, Australia, Brazil and Egypt as well. However, its importance in central Europe protected tomato production has increased in recent years.

With their < 0.2 mm size, early detection of tomato rust mite in the crop is particularly challenging. Even with stronger handheld magnifiers, detection is difficult. Initial symptoms on host plants include chlorotic discoloration of leaves and rusty brown discoloration of stems beginning at the base, and later wilting of entire leaves and cracked corking of young fruit. Symptoms result from the sucking activity of tomato rust mites on superficial cells. It is also typical that some plants are already severely damaged to the point of death, while in other greenhouse areas no symptoms are yet visible. The population growth of the tomato rust mite is greatly accelerated in plants under drought stress. At about 25°C the tomato rust mite has optimal conditions.

Particular attention should be paid in farms that cultivate tomatoes virtually year-round without a significant break in cultivation during the winter and without crop rotation including non-solanaceous plant families. In those productions, if tomato russet mite infestations occur in previous sets, A. lycopersici are likely to persist in the greenhouse infrastructure and colonize subsequent tomato sets early in the crop cycle, thus increasing its damage potential.

In the future, early detection could be accomplished with on-farm spectrometer measurements or hyperspectral imaging. Initial research results on this were promising. For russet mites’ control, the active ingredient abamectin has been proved to be effective in tomato cultivation. Its application is possible in combination with beneficials’ application, only in spot treatment in the greenhouse, so that the beneficials are not affected by the active ingredient. In Southern Europe, treatments with sulfur products are the standard. One way to improve treatment success could be to thin out or remove the leaf mass before treatment, but research on this is not yet complete. Beneficial insects available on the market, such as predatory mites, can at best be a supportive measure because of poor establishment on tomatoes and rapid reproduction rates of tomato rust mites. Predatory bugs do prey on tomato rust mites, but they are also not sufficient for effective control.

White-flies (Greenhouse white-fly Trialeurodes vaporariorum Westwood and Silver-leaf white-fly Bemisia tabaci Gennadius)

Both white-flies are major pests in protected tomato cultivation causing direct damage by sucking plant sap, but more importantly indirect damage by production of honeydew and thereby facilitating sooty mold growth, and virus transmission.

Adults, nymphs and pupae are mostly found on the lower leaf side. Adults fly up rapidly, especially under warm conditions, and are found more often on young leaves. Later nymphal stages are immobile, look scale like, and are found on older leaves. The species can be roughly distinguished in adult and pupal stages with a hand lense: the pupa of T. vaporariorum are whitish and have a few long hairs and a fringe of very short hairs around the upper edge. Pupae of B. tabaci have a few short hairs only, no spines around the rim, and appear yellowish. Adults of T. vaporariorum appear more moth-like with white wings held flat and roof-like over the body. In comparison, B. tabaci has a smaller body, is generally yellower and holds its wings at a steeper angle or tent-like.

Whitefly monitoring is commonly carried out using yellow sticky cards. For a meaningful monitoring, it is recommended to use one trap per 100 m². Traps should be hung just above the canopy of the crop as whiteflies are attracted to the young growth of the plants. Development on automated traps is ongoing and the first products are on the market, which will facilitate an exact monitoring in the near future.

Reduced susceptibility and resistance development to common insecticides makes chemical control difficult. However, successful control is often achieved by the early introduction of predatory bugs, depending on the region, i.e., Macrolophus pygmaeus in Central Europe or in Nesidiocoris tenuis Southern Europe. A combination with repeated introductions of Encarsia formosa in early season, before the predatory bugs are established, can be reasonable. In short crop cycles such as summer cultivation in low tech greenhouses in Central Europe, it is recommended to introduce only E. Formosa. Under warmer conditions, and especially if B. tabaci is present in the crop, the introduction of Eretmocerus eremicus instead of E. formosa is recommended.

Root-knot nematodes (Meloidogyne spp.)

When grown in soil, root-knot nematodes can cause severe damages, especially in tomato greenhouse production. Important species are Meloidogyne incognita, M. arenaria and M. javanica. The attack is visible at the roots by formation of root galls and a reduced root zone. Plant’s growth and vigor can be reduced by insufficient water and nutrient uptake by the roots. Higher attack can be expected, if host plants are cultivated over several years successively. However, due to the large host range of root-gall nematodes, it is not easy to implement a suitable crop rotation to prevent pest populations’ growth.

If the greenhouse is free of Meloidogyne spp., the best prevention is to keep strict hygienic measures such as disinfection of shoes at entrances, change of clothes and especially shoes before work start and usage of plastic shoe covers for visitors. In addition, extra care should be taken regarding the irrigation water, which should also be disinfected properly, e.g., by solarization, filtration, ozonation or others. Introduction of plant material from nurseries also poses a high risk of introduction, preferably material should be certified nematode free. Once the occurrence of the pest has been observed in the greenhouse, control is difficult. Dampen of the soil can be an effective measure, but is costly. Radiation can be a suitable measure in warmer regions, but requires a sufficient culture break. However, both physical methods are non-selective with regard to other soil living organisms. Using resistant cultivars as root stock and biological or chemical treatments are other possible measures.

Bugs (Brown marmorated stink bug Halyomorpha halys, southern green stink bug Nezara viridula)

Both bugs cause increasing problems in tomato cultivation during recent years. H. halys having invaded from East Asia, is a brown-grey bug and can be recognized by the black-white banded antennae, black and white mustered sides and the existence of 3 to 5 orange-yellow dots on the upper boarder of the Scutellum (triangle on the back). N. viridula is very variable in color but has 3 to 5 white points at the upper boarder of the Scutellum, which are at both sides bordered by a black color. Both bugs are between 12 and 17 mm in length. As both species can easily be confused with other bugs, a careful determination should take place. In vegetables, both species cause major problems in tomato, bell pepper, cucumber, zucchini and beans.

For monitoring of the H. halys, traps using aggregation pheromone are available, but particular care should be taken in order not to attract bugs that were not yet present into the greenhouse. As control of bugs is generally very challenging, insect-guard nets in front of ventilation openings and doors should be installed to protect the crop growing inside the greenhouses. Studies on Egg-parasitoids (parasitic wasps) are also being conducted. The samurai wasp, Trissolcus japonicus that has already been introduced in Switzerland as a Classical Biological Control approach, is also present in other European countries and is expected to be able to adapt in all climatic regions of its host, H. halys.

Aphids and caterpillars

Although they are not considered as major pests in tomato crop, severe outbreaks of different Aphid species such as Myzus persicae, Aphis fabae, Aulacorthum solani or Macrosiphum euphorbiae and caterpillars of different Lepidoptera, often of the family Noctuidae and Sphingidae, can occur. Plants should therefore be inspected regularly for the occurrence of these pests. Control can be performed by application of parasitoids and gall midges in case of aphids, and by application of BT-products in case of caterpillars.

Conclusions

Successful pest management is always a combination of preventive measures, adequate monitoring and the implementation of a timely control measure only if required. Following the rules of Integrated Pest Management (IPM), the best management is to avoid the pest entering the crop, which is very crucial in greenhouse production, and to avoid providing the pest a suitable environment to develop large populations. Resistant varieties, crop rotation and culture breaks are important measures in this regard.

However, to ensure that no pests cause problems, adequate monitoring is required. Although monitoring has traditionally been a time-consuming, work intensive and expensive practice it can be considered as a very significant tool in the implementation of integrated pest management. Unfortunately, until today, it has not been applied to its optimum in agricultural practices. However, much development with regard to technical innovations in trap techniques, automated counts of pests, pheromone lures and sensors for plant damage detection have been recorded during the last years and aim to facilitate modern pest monitoring. In combination with smart decision support tools, this development will consist an important addition to the grower’s tool box in pest management systems.

No matter if a chemical product or a biological control agent must be applied in a crop, a suitable and effective distribution in the crop must be assured. Application techniques ranging from spot sprayers over beneficial blowers and UV-systems up to drones, which even may detect and physically eliminate pest insects, enter the market or are under development.

In conclusion, the grower is the most important decision maker and manual work cannot be replaced – but help is on its way.

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