Potato and tomato late blight

Late blight, due to its aggressiveness, is one of the most devastating planting diseases in the history of humanity, which caused, between 1845 and 1850, an 80% reduction in potato production in Ireland. As it was the population's basic food, around 2 million people died of hunger and 1 million emigrated. The causal agent was only identified in 1861. Currently, it occurs in all regions of the world where potatoes and tomatoes are grown.

In Brazil, it is one of the main crop diseases, due to the frequency of occurrence and damage caused. Also called downy mildew, mela, blight or mufa, it is caused by the “oomycete” fungus Phytophthora infestans. Damage can be severe, reaching 100%. The pathogen survives in crop residues, host plants and oospores. It is spread mainly by rain, agricultural implements, irrigation water and wind, through sporangia and zoospores. Temperature between 15 and 25°C, relative humidity greater than 90% and more than 10-12 hours of leaf wetness are necessary conditions for infection.

Symptoms can appear at all stages of plant development and are more evident in the aerial part. Initially, small, irregular, dark green lesions appear on the leaves and leaflets; later they become larger, almost black, with a burning appearance and a soggy halo. On the lower surface of the leaves, whitish growth may appear, which are structures for the fungus to multiply. Lesions may also be present on the stems and stems, such as brown lesions, which may result in girdling and death of the upper part. On potato tubers and tomato fruits, the lesions are of the hard rot type, dark brown in color, deep and with an irregular surface; There may be a whitish growth on the surface, made up of pathogen structures. The disease can cause plant death.

Late blight management is carried out mainly through the application of fungicides. Integrated management involves choosing the location and time of cultivation, avoiding lowlands and poorly drained areas, where the environment is favorable to the disease. The plant population must be adequate, providing ventilation for the plantation. Fertilization must be balanced, propagation materials must be healthy and it is important to rotate crops and destroy diseased plants. In both potatoes and tomatoes, there are no cultivars with a high level of resistance, however there are almost 200 commercial fungicides registered for the control of late blight in potatoes and tomatoes. Among these, specific, immunizing/curative and systemic products based on cymoxanil, metalaxyl-M, benalaxyl, mandipropamide, dimethomorph, propamocarb and isopropyl bentiavalicarb and non-specific, protective and non-systemic products such as mancozeb, chlorothalonil, fluazinam, propineb stand out. and cuprics. It is important to alternate or mix fungicides, with different mechanisms of action, to reduce the possibility of the emergence of resistant strains of Phytophthora infestans.

José Otávio Menten, President of the Sustainable Agro Scientific Council (CCAS), Agronomist and Senior Professor at ESALQ/USP