Management of downy mildew in lettuce

Mildew, caused by bremia lactucae, represents one of the biggest threats to lettuce cultivation in humid periods and mild temperatures. At first, the disease manifests itself through light green or yellow leaf spots, moist, angular and of variable size. As they evolve, they become necrotic, brown and present a white growth on the lower surface of the lesions, made up of sporangia and sporangiophores of the causal agent. In favorable periods, the disease can cause intense defoliation, compromising productivity and quality of production.

The oomycete B. lactucae belongs to the kingdom Stramenopila and is characterized by being an obligate parasite that has a cell wall with cellulose and beta glucans, coenocytic mycelium, branched hyphae, sporangiophores with dilated ends, rounded sporangia, flagellated zoospores and can form resistance structures called oospores. The disease can be caused by several races, which makes it difficult to obtain resistant cultivars. It is noteworthy that pathogen populations are dynamic and the emergence of new breeds is expected, requiring that breeding programs always incorporate new resistance genes so that resistance can be maintained or expanded.

The disease is favored by periods of fine rain, dew and fog and by mild to low temperatures, ranging from 12ºC to 20°C. The action of winds associated with water splashes from rain and irrigation, as well as the presence of free water on the surface of plants, favors its dissemination throughout the crop and rapid colonization of tissues, respectively. The disease occurs most frequently during the seedling production phases and after the crop is closed in the field.

Downy mildew management

It is recommended that you pay close attention when choosing healthy seedlings. Avoid planting and producing seedlings in areas subject to moisture accumulation and limited air circulation.

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 at these times. Seed companies currently offer cultivars with resistance to different races of the pathogen (Table 1). In general, resistance to downy mildew is expressed through a reduction in the number and size of lesions and a decrease in the potential for sporulation.

In critical periods of 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.

It is beneficial not to plant densely, especially in times favorable to the disease, mainly to increase air circulation between the plants, thus avoiding the formation of a microclimate favorable to the disease.

Balanced fertilization

Avoid excess nitrogen fertilization, as 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 fungicidal action on oomycetes and stimulating the production of phytoalexins (compounds produced by the plant capable of reducing or inhibiting infection).

It is necessary to pay attention to the 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 humidity. It is important to 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 must 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), safety interval, product storage, packaging disposal, etc.

To avoid the occurrence of resistance of B. lactucae Regarding fungicides, it is recommended that specific fungicides be used alternately or formulated with contact products, that the repetitive use of products with the same mechanism of action should be avoided and that curative applications should not be made in situations of high disease pressure.

Among the active ingredients registered for the control of lettuce downy mildew in Brazil, products based on mandipropamide, fenamidone, dimetomorph, fluopicolide, propamocarb, bentiavalicarb and ciazofamide stand out (http://agrofit.agricultura.gov.br/agrofit_cons/ principal_agrofit_cons).

The lettuce

Originally from the Eastern Mediterranean Sea, lettuce (Lactuca sativa) is one of the most cultivated and consumed vegetables in the world. In Brazil, its cultivation is mainly concentrated in the South and Southeast regions and production serves everything from traditional to more differentiated markets, 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. The nutritional 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.

Jesus G. Töfoli, Ricardo J. Domingues and Josiane T. Ferrari, Secretariat of Agriculture and Supply of São Paulo, APTA - Instituto Biológico

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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