Management measures against Fusarium wilt in tomatoes

Fusarium wilt is a disease of great importance in tomato, which can make cultivation unfeasible in certain regions or times of the year. The disease is caused by soil fungus Fusarium oxysporum f.sp. lycopersici, which has different physiological races, of which at least three have already been identified in Brazil, according to their ability to infect and cause disease in a series of differentiating cultivars. Races 1 and 2 predominate in most areas of table tomato production and processing, while race 3 is more restricted, having been confirmed to cause epidemics in table tomato production areas in the states of Espírito Santo, Rio de Janeiro , Bahia and Minas Gerais. In recent decades, the disease was not of great importance in the crop, as the cultivars planted showed effective resistance against the prevailing races of the pathogen (1 and 2). However, with the appearance of race 3 in the production areas of Espírito Santo, the disease has once again become part of the main phytosanitary problems in tomato farming, although it has not yet been reported in the regions where tomatoes are grown for industrial processing in Brazil.

The disease occurs at any time or stage of tomato development, most frequently in adult plants from the flowering and fruiting stages onwards. In nurseries, symptoms of the disease can be observed in seedlings in the form of lightening of leaf veins and curving of petioles (epinasty). However, these symptoms have been attributed, in most cases, to race 3 of the pathogen. The first symptoms of the disease are observed individually in some plants or in small plants, with the yellowing of older leaves, which gradually wither and show marginal or total necrosis of the limb, accompanied by wilting of the upper leaves in the hottest hours of the day, due to to partial impairment of the plant's vascular system due to the presence of the pathogen in the xylem vessels. As the disease progresses, the yellowing increases upwards until it also affects the youngest leaves. In this condition, the fruits do not develop, ripen when they are still small or fall prematurely. It is common for wilting or yellowing to appear only on one side of the plant or leaf.

When the stem of plants with visible symptoms is cut longitudinally, a characteristic brown coloration is observed in the xylem region, more intense at the base of the stem, while the pith does not show any discoloration. It is important to highlight that, in tomato, this symptom is striking, but not exclusive to the attack of F. oxysporum f.sp. lycopersici. Tomato plants infected by Verticillium dahliae also present vascular necrosis, but not as intense as F. oxysporum f.sp. lycopersici. In the roots, reduced growth or atrophy is initially observed, but over time they can rot. Sick plants have reduced growth and may die after their vascular system is completely compromised by the pathogen.

The pathogen can be spread to new cultivation areas through contaminated seed lots, seedlings, contaminated agricultural machinery and tools, or through runoff from infested crops located above new cultivation areas. In the field, the disease is spread through soil movement and runoff of rainwater and irrigation, and can be seen by the increase in the size of the reboleiras after successive cycles of cultivation.

When present in the cultivation area, the fungus can survive in the soil and in crop residues from one growing season to another, or longer periods in the form of a resistance structure, known as a chlamydospore, which allows the pathogen to survive in a viable form for up to eight years in the soil, even in the absence of the host. In addition to chlamydospores, numerous spores of the fungus are produced in contaminated crop residues, known as macroconidia and microconidia, which, like the fungus' hyphae, are responsible for plant infection.

Once on the plant, the hyphae and/or the germ tube emitted by the fungus spores penetrate directly through natural openings in the plant roots, formed by the emission of lateral roots, or in wounds caused by the friction of the roots with the soil, insects, nematodes or by mechanical damage caused by cultural treatments. After penetration, the fungal hyphae grow through the root cortex intercellularly and reach the xylem vessels. The mycelium then develops inside the vessels, colonizing the cells, producing spores (microconidia) and promoting the systemic distribution of the fungus throughout the plant, through the ascending sap current. As a result of this colonization, the plant accumulates gels, gums and tyloses in the vessels as a defense strategy, which results in the obstruction of the xylem vessels, making it difficult for the upper part of the plant to absorb water and nutrients. As a consequence, the plant does not absorb water and nutrients necessary for its production or survival, resulting in its death.

The development of the pathogen is favored by temperatures between 21°C and 33°C, an optimum of 28°C, and high soil humidity. Plants grown in acidic, poor and calcium-deficient soils tend to be more affected, as well as those grown in soil with low nitrogen and phosphorus and high potassium content. Soils with high nematode infestation also increase the severity of the disease in some cases, due to injuries caused to the roots, which serve as a gateway for the pathogen.

The control measures adopted for Fusarium wilt are preventive, since, after soil infestation, it is impossible to eradicate the pathogen. The planting of cultivars resistant to physiological races 1 and 2 of F. oxysporum f.sp. lycopersici has been adopted by most producers, as there are currently a significant number of resistant cultivars available on the market. On the other hand, there is great concern regarding race 3, given the limited number of resistant cultivars or rootstocks available commercially.

Many table tomato producers in the country use resistant tomato rootstocks as an alternative to control diseases caused by soil-borne pathogens in crops grown in a protected environment. In this sense, research has been conducted with the aim of obtaining cultivars and rootstocks resistant to these pathogens, since chemical control is not effective and economically viable for managing the disease.

The use of healthy seeds and seedlings and planting in areas free from the pathogen are, of course, recommended. Cultural practices such as soil solarization and rotation with non-host crops (grasses) for at least five years, although they contribute to reducing the pathogen population in the soil, are expensive and have limited efficiency, due to the persistence of the fungus in the soil for years. Other cultural measures, such as soil liming, to increase the pH to 6,5 to 6,8, balanced fertilization and the use of organic compounds, with the aim of increasing the antagonistic microflora, are recommended as complementary measures.

Identification and correct diagnosis of the disease are essential for adequate management. It is important that the producer knows the history of the area and always adopts preventive measures to prevent contamination of new areas. Once the area is contaminated, the producer must rotate crops with non-host species, preferably grasses, to reduce the pathogen population and assist in the recovery of the areas. Finally, it is worth highlighting that prevention and control measures must be adopted in an integrated manner, which will lead the producer to achieve safer and more profitable production.

Ricardo Borges Pereira and Alexandre Augusto de Morais, Embrapa Hortaliças

Growing Vegetables and Fruits September 2020

With each new edition, Cultivar Hortaliças e Frutas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in vegetable and fruit cultivation, the Magazine helps farmers in the search for management solutions that increase their profitability. 

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