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

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

Main host: Lactuca sativa (lettuce)

Introduction

Native to the eastern Mediterranean Sea, lettuce (Lactuca sativa) is one of the most cultivated and consumed vegetables in the world. In Brazil, its cultivation is concentrated mainly in the South and Southeast regions and production serves from traditional markets to the most differentiated ones, such as: fast food, minimally processed products and haute cuisine.

The high popularity of lettuce is mainly due to its easy cultivation and its versatility and culinary characteristics such as: crunchiness, pleasant flavor, different colors, textures, shapes and sizes. Nutritious properties such as: low calories and significant presence of vitamins (A, E, C, B1, B2 and B3) and mineral salts (calcium, magnesium and potassium) in addition to natural sedative action also favor its consumption.

Etiology

Bremia lactucae is a biotrophic microorganism belonging to the Kingdom Chromista, Phylum Oomycota, Class Peronosporea, Order Peronosporales, Family Peronosporaceae. The sporangiophores have 4 to 6 dichotomous branches and dimensions ranging from 430 - 990 x 7 - 16 µm, with glass-shaped dilated ends (apophysis), each containing 4 to 5 sterigmata where the sporangia are formed. The sporangiophores are thin, long with a color that varies from white to dark brown and emerge in the injured tissue through the stomata. The sporangia are hyaline, spherical to ovoid in shape and papillate (12-31 x 11-27.5 μm).

The disease can be caused by several races of B. lactucae, which makes it difficult to obtain resistant cultivars, and makes it necessary to constantly reevaluate cultivars, depending on the predominant races of the pathogen in each region. In Europe, 36 races of the pathogen have been identified, while in Brazil, six races have been detected in the states of São Paulo and Minas Gerais. It should be noted that pathogen populations are dynamic, with the appearance or introduction of new races being expected.

This fact requires that new cultivars always present new resistance genes so that resistance is not broken. Currently, the species Lactuca serriola, Lactuca virosa and Lactuca saligna have been the most used sources in breeding programs aimed at resistance to downy mildew. In Brazil, in addition to lettuce (B. lactucae), the disease is found in cornflower (Centaurea cyanus L.), everlasting flower (Helichrysum bracteatum L.), common sowthistle (Sonchus oleraceus L.), spiny sowthistle (Sonchus asper L.) and artichoke (Cynara scolymus L.). Downy mildew finds optimal conditions for its development in periods of high humidity (greater than 90%) and temperatures ranging from 12 to 20°C. Cloudy, rainy or heavily dew days are highly favorable to the disease. Once present in the crop, the sporangia are quickly dispersed by the action of winds and water splashes from dew, rain and irrigation. Infection occurs in just 3 or 4 hours after contact with susceptible plants, with the first symptoms observed from 72 hours after inoculation. Dense plantings and excess nitrogen fertilization can favor the disease by promoting the accumulation of moisture in the foliage and generating more tender and susceptible tissues, respectively. Contaminated seeds, diseased seedlings and alternative hosts are the main sources of inoculum of the disease.

Symptoms

The disease can occur at any stage of the crop, affecting from newly emerged seedlings to adult plants. In seedlings, the disease is seen initially on cotyledonary leaves and then on the first true leaves. These yellow quickly, dry out and die prematurely. In developing or adult plants, the first symptoms of the disease are manifested on the outer leaves through light green or yellow leaf spots, wet and of variable size. These have an angular shape, being delimited by the veins of the leaves and as they evolve they become necrotic, brown and have a white growth on the underside of the lesions, consisting of sporangia and sporangiophores of the causal agent. In open field or hydroponic systems, the disease can significantly reduce the leaf area of plants, seriously compromising yield and production quality.

Economic importance

Downy mildew, caused by Bremia lactucae, represents one of the greatest threats to lettuce cultivation in humid periods with mild temperatures.

Distribution

Cosmopolitan

Management

Among the measures recommended for the management of downy mildew, the following stand out:

- Planting healthy seedlings.

- Avoid planting and producing seedlings in areas subject to moisture accumulation and with limited air circulation.

- Planting tolerant or resistant cultivars.

Considering the high destructive power of the disease in winter and spring, the adoption of cultivars with levels of resistance is important to enable production in these periods. Among the downy mildew resistant and/or tolerant cultivars available in the Brazilian market, the following stand out:

- Group Americana - Raider Plus, Mayumi, Maysah, Madras, Silvana, Rubette, Laís. Kazan, Callore, Pedrola, Ludmila, Rafaela, Bruma, Escarcha, Icebela.

- Group Crespa - Gizele, Malice, Inaiá, Bruna, Lirice, Paola, Melissa, Locarno, Valentina, MultiBlond, MultiGreen, Batuka, Bataille, Naide, Isadora, Excite, Jonction, Jade, Loreane, Filó, Brida.

 - Group Roxa - Scarlet, Red Star, Pira Roxa, Gourmandine, Bocado, Grenadine, Belíssima, Maíra, Redflax, Rosabela, Mirela, Carmin, Milamil, Bellagon, Carmoli, Barlach.

 - Group Mimosa - Imperial, Imperial Roxa, Querido, Angélica.

 - Group Batávia - Joaquina, Cacimba.

 - Group Frisées - Atalaia, Itaúna, Desirade.

 - Group Romana - Romana Bonnie, Astorga (mini), Tendita, Salvius.

 - Group Lisa - Ofélia, Luara, Letícia, Marcela, Inês, Melissa, Larissa, Natalia, Fortaleza.

 - Group Multifolhas - Multiblond, Multigreen, RedFlash, Cousteau.

(Source: Seed company catalogues. Consultation: August 2020).

- In periods critical to the disease, reduce watering and avoid it in the late afternoon. The use of localized irrigation can reduce the disease by reducing its spread and preventing the accumulation of free water on the surface of the leaves.

- Avoiding dense planting, especially in times favorable to the disease, mainly aims to increase air circulation between the plants, thus avoiding the formation of a microclimate favorable to the disease.

- Balanced fertilization.

Avoid excess nitrogen fertilization, since tender tissues favor infection. Adequate levels of phosphorus, calcium, potassium and silicon can reduce the disease. Registered as fertilizers, phosphites have systemic properties and are characterized by stimulating plant growth, having a fungicidal action on oomycetes and stimulating the production of phytoalexins (compounds produced by the plant capable of reducing or inhibiting infection).

- Correct management of invasive plants. In infested areas, moisture dispersion is slower, which may favor the disease.

- In a protected environment and hydroponic cultivation, promote air circulation through the correct handling of curtains and the use of fans in order to dissipate moisture.

- Eliminate and destroy remaining plants and post-harvest discards.

- Preventive application of registered fungicides.

In areas with a history of the disease, the use of fungicides should be preventive and carried out within integrated production programs. The producer must follow all the manufacturer´s recommendations regarding dose, volume, interval and number of applications, use of personal protective equipment (PPE), withdrawal period, product storage, packaging disposal, etc. To avoid the occurrence of B. lactucae resistance to fungicides, it is recommended that specific fungicides be used alternately or formulated with contact products; to avoid the repetitive use of products with the same mechanism of action; and that curative applications are not made in situations of high disease pressure. Among the active ingredients registered for the control of downy mildew of lettuce in Brazil, products based on mandipropamid, fenamidone, dimethomorph, fluopicolide, propamocarb, benthiavalicarb and cyazofamid stand out (http://agrofit.agricultura.gov.br/agrofit_cons/ principal_agrofit_cons).

References

KOIKE, S.T.; GLADDERS, P.; PAULUS, A.O. Vegetable Diseases: a colour handbook. St. Paul: APS. 2007. 448 p.

KRAUSE SAKATE, R. et al. (Ed.) Manual de fitopatologia: doenças das plantas cultivadas. 5. ed. São Paulo: Agronômica Ceres 2: 2016, p.33-40.

LOPES, C.A.; QUEZADO-DUVAL, A.M.; REIS, A. Doenças da alface. Brasília: Embrapa Hortaliças. 2010. 68p.

TOFOLI, J.G.; DOMINGUES, R.J. Manejo e métodos de controle de doenças fúngicas da alface. PROSAF. Instituto Biológico. http://www.biologico.sp.gov.br/uploads/files/pdf/prosaf/apostilas/doencas_alface.pdf

TÖFOLI, J.G.; DOMINGUES, R.J. Doenças causadas por fungos. Aspectos Fitossanitários da Cultura da Alface. Boletim Técnico - Instituto Biológico, nº 29, p. 28-26, 2017.

Keywords

Lettuce, Lactuca sativa, Downy mildew

Suggested Citation:

TÖFOLI, J. G.; DOMINGUES, R.J.; FERRARI, J. T. lettuce (Lactuca sativa): Downy mildew (Bremia lactucae Regel.). In: Instituto Biológico. Guia de Sanidade Vegetal. Available at: http://www.sica.bio.br/guiabiologico/busca_culturas_resultado_ing_ok.php?Id=13. Accessed on: month day, year