Management of late blight and downy mildew in crops in Brazil

The causal agents of white rust (Albugo) and mildews (Plasmopara, Peronospora, Peronosclerospora, Pseudoperonospora, Sclerophthora, Basidiophora and Bremia) are consideredobligate parasites, and the causal agents of late blight, gummosis, rot, canker, wilting and fire (Phytophthora) and tipping over, rot of the roots, neck and stem (Pythium),facultative parasites. All of these species belong to the Chromista Kingdom, class of Oomycetes. Although they are always grouped with diseases caused by fungi, these species are not true fungi, as they have hyphae without septa and a cell wall with β-glucans, cellulose and no chitin.

This group of phytopathogens has asexual reproductive structures, which are sporangia, which, through cytoplasmic cleavage, give rise to zoospores and the sexual reproductive structures oogonium (female gametangium) and antheridium (male gametangium), responsible, in most cases, for survival of the oomycete in the soil or seed.

Fungi are responsible for around 65% of plant diseases and are part of a large group of organisms belonging to three kingdoms of living beings: in addition to cromixed,also  are in the Kingdoms Protozoa (hernia  of crucifers/Plamodiophora; powdery potato scab/ Spongospora) and Kingdom Fungi (zygomycetes: Rhizopus; Ascomycetes: Erysiphe, Sclerotinia, Guignardia, Venturia; Basidiomycetes: Moniliophtora, Puccinia, Hemileia, Phakosopra, Usitilago: Deuteromycetes or Mitosporic or Anamorphic or Imperfect: Rhizoctonia, Oidium, Fusarium, Colletotrichum, Phomopsis, Alternaria).

Like other fungi and other agents that cause plant diseases (bacteria, viruses, nematodes and phytoplasmas), oomycetes cause diseases when they encounter susceptible hosts and favorable environments. Generally in conditions of high humidity/rain and mild/low temperatures, they infect, colonize and reproduce in plants, interfering with various physiological processes and causing characteristic symptoms.

The most important disease caused by oomycetes is potato and tomato late blight, caused by Phytophthora infestans.  This disease was responsible for the deaths of two million people and one million emigrants in Ireland around 1845, due to the 80% reduction in potato production, the population's staple food at that time.

More than 50% of cultivated plant species can be affected by oomycetes; Among the main ones are cotton, rice, cocoa, sugarcane, beans, tobacco, corn, rubber trees, sorghum, avocado, pineapple, cashew, citrus, apple, papaya, strawberry, vine, lettuce, onion, crucifers, cucurbits, ginger, ornamentals and solanaceae.

To manage plant diseases, several measures can be used that are part of biological, chemical, genetic, cultural, physical and integrated methods. To control diseases caused by oomycetes, the most commonly used measure is the application of fungicides, which belong to groups, in general, different from the products normally used to control ascomycetes, basidiomycetes and deuteromycetes (these fungi have ergosterol in their cell membrane; oomycetes have little ergosterol) and use of cultivars with genetic resistance to oomycetes.

In addition, it is possible to adopt several cultural measures such as a cultivation location without fog or waterlogged soil, aeration, pruning, healthy propagation material, elimination of diseased tissues and alternative hosts, crop rotation with non-host species, good drainage, nutrition balanced, avoiding injuries, balanced soil pH, loosening the soil and shallow sowing all contribute to reducing the amount of these diseases. The use of biological control agents, such as application of Trichoderma e Gliocladium, and physical measures such as solarization and storage at low temperatures, can also contribute to the management of diseases caused by oomycetes.

Among fungicides, immobile/protective and eradicant products such as cuprics, mancozeb, chlorothalonil, thiram, metiram, Captan, propineb, fluazinam, folpet, famoxodine, zoxamide, fentin, dithiamine and mandipropamide can be used. Systemic fungicides/curatives specialized in oomycetes can also be used, such as cymoxamil, metalaxyl-M, fosetyl, benalaxyl, dimethomorph, iprovalicarb, propamicarb, azoxystrobin, acibenzolar-S-methyl and others. These products, as they are site-specific, must be used with discretion to reduce the possibility of the emergence of strains resistant to oomycetes. Currently, 169 commercial products (single or double mixtures) are registered in Brazil, comprising 33 active ingredients from 25 different chemical groups, to control 21 species of oomycetes in more than 40 species of hosts.