Autors: Jesus G. Töfoli; Ricardo J. Domingues; Josiane T. Ferrari, LDFH, CPDSV, Instituto Biológico

E-mail: jesus.tofoli@sp.gov.br

Main host: Lettuce (Lactuca sativa L.)

Causal agent: Fusarium oxysporum Schlechtend.: Fr. f. sp. lactucae Matuo & Motohashi

Etiology: The disease is caused by the fungus Fusarium oxysporum Schlechtend.: Fr. f. sp. lactucae Matuo & Motohashi belonging to Kingdom Fungi, Division Ascomycota, Class Sordariomycetes, Order Hypocreales, Family Nectriaceae. It has vigorous mycelium, which can vary in color, has septate hyphae and produces curved, fusiform, septate or not septate macro and microconidia. The disease can be caused by 4 races of the pathogen (races 1, 2, 3 and 4), being identified in Brazil, races 1 and 3. The occurrence of Fusariosis is more frequent in the summer, when periods with high temperatures and humidity prevail. Acid compact soil with low oxygenation also favors the disease. Several of the cultivated types of lettuce are susceptible to the disease. In addition to lettuce, fusariosis also affects another Asteraceae — Valerianella locusta, also known as lamb´s lettuce. It is also known that the pathogen can colonize the radicle system of tomato, melon, watermelon and cotton plants without, however, causing symptoms. The fungus penetrates the roots through natural openings or injuries and colonizes the vascular system of plants, making it difficult to absorb water and nutrients. The obstruction of the xylem causes, over time, wilting, atrophy and death of the plants. In the absence of susceptible hosts or adverse conditions, it can remain viable in the infested area for long periods through resistance structures called chlamydospores or, even, associated with crop residues or organic matter. Studies have shown that the pathogen can remain viable in the soil for periods of up to 8 years. The use of infected seeds is considered the most important way of spreading the disease. The introduction of the fungus in healthy areas is done by planting infected seedlings. In the field, transmission of the disease is mainly due to the use of infested agricultural implements and tools, irrigation water, rain and the movement of people and vehicles.

Symptoms: Yellowing of the basal leaves, generalized atrophy, no head formation, brown or black stripes in the vascular system, brownish or reddish cortex, progressive wilting, significant reduction of the radicle system and death of plants are characteristic symptoms of the disease. In the field, fusariosis can affect plants at random or occur in patches. In some situations it can occur associated with other diseases such as bottom rot (Rhizoctonia solani) and black root rot (Thielaviopsis basicola).

Economic importance: Fusariosis or Fusarium wilt represents one of the most recent threats to the lettuce crop in Brazil. The disease significantly affects plant development, productivity and quality, and can cause losses of over 70%.

Distribuition: Occurs in most countries in which lettuce is grown.

Management: For the management of fusariosis, it is recommended the adoption of integrated measures that aim to avoid its introduction in areas that are free of it, reduce the inoculum potential or hinder its dissemination.

- Avoid planting in areas with a recent history of the disease.

- Correct preparation of the soil, avoiding the formation of compacted areas in the cultivation areas. These cause the accumulation of moisture and low oxygenation in the surface layers of the soil, favoring the disease.

- Choose resistant/tolerant cultivars. Among the materials available on the market we can mention: Valentina (raça 1), Marisa (raça 1), Naide (raça 1), Astorga (raça 1), Tendita (raça 1), Jonction (raça 1) among others.

- Use healthy, fungicide-treated seeds. For the production of seedlings, use substrate free of pathogens, avoid deep sowing and excessive watering. Trays must be disinfected with 10% chlorine for 30 minutes. - Plant healthy seedlings in the field. Obtain seedlings from nurseries committed to quality and health.

 - Correct the pH of the soil (6.5) and fertilize in a balanced way. Excess nitrogen fertilization can favor the disease. The incorporation of green manures in the soil promotes the increase of organic matter and favors the development of a beneficial microflora that, when competing for food and space, can reduce the population of the pathogen in the soil. Furthermore, the decomposition of green manure releases carbon dioxide which reduces the competitive capacity of various fungi.

- Irrigate moderately with good quality water.

- Perform crop rotation for three to five years in order to reduce the inoculum in affected areas. - Eliminate and destroy diseased plants, including stems and roots.

- Eliminate crop residues that could serve as a substrate for the pathogen to survive in the area. - The use of solarization associated with biological control agents (Trichoderma harzianum) can significantly reduce disease in the field.

- Wash and disinfest tools, implements, vehicles, shoes and boots with a 2% sodium hypochlorite solution.

References:

CABRAL, C.S.; REIS, A. Screening of lettuce accessions for resistance to Fusarium oxysporum f. sp. lactucae race 1. Tropical Plant Pathology, Viçosa, MG, v.38, n.4, p. 275-281, 2013.

CLAERBOUT et al. First Report of Fusarium oxysporum f. sp. lactucae Race 4 on Lettuce in Belgium. In: https://doi.org/10.1094/PDIS-10-17-1627-PDN.

FRIAS, A. G. Caracterização de isolados de Fusarium oxysporum f.sp lactucae obtidos de campos de produção comercial no Estado de São P aulo e avaliação de genótipos de alface. 2014. ix, 45 f. Dissertação (mestrado) - Universidade Estadual Paulista, Faculdade de Ciências Agronômicas de Botucatu, 2014. http://hdl.handle.net/11449/110957.

KRAUSE-SAKATE, R. et al. (Ed.) Manual de Fitopatologia: volume 2: Doenças das Plantas Cultivadas. Ouro Fino: Ceres, 2016. p.33-40. TÖFOLI, J.G; DOMINGUES, R.J. Doenças causadas por fungos. Aspectos fitossanitários da alface. Boletim Técnico. Instituto Biológico. São Paulo, v. 29, p. 28 – 46, 2017, 126p

Keywords: Lettuce, Lactuca sativa, Fusarium oxysporum f. sp. lactucae

Suggested Citation:

TÖFOLI, J. G.; DOMINGUES, R.J.; FERRARI, J. T. Lettuce (Lactuca sativa L.): Fusarium wilt: Fusarium oxysporum Schlechtend.: Fr. f. sp. lactucae Matuo & Motohashi. In: Instituto Biológico. Guia de Sanidade Vegetal. Available at: http://www.sica.bio.br/guiabiologico/busca_culturas_resultado_ok.php?Id=214&Vlt=3. Accessed on: month day, year