New strains3 of types A1, but especially A2, were introduced in particular in Europe at the beginning of the Eighties. Until the end of the 1970s, the A1 mating type was predominant in many production areas (the US-1 line is A1 type), except in Mexico where both A1 and A2 types coexisted with a ratio of 50 / 50%. It is a heterothallic chromist, its sexual reproduction requires the presence of strains belonging to two complementary compatibility groups: A1 and A2. They gradually supplanted the US-1 line.ĭuring this period, the sexuality of P. Some of them have also been found in other countries of the American continent, but also in Europe and in many countries of the United States. Thus, several strains2 were thus detected progressively in the United States from the end of the 1970s. Subsequently, the increase in international trade from the end of the 20th century would be at the origin of one or more other migrations of much more aggressive strains, again from Mexico where the genetic diversity of P. Indeed, the epidemics occurring on this crop were often of low intensity, and the mildew gave the impression of have almost disappeared on this nightshade. It should be remembered that this line, mainly present in France until the 1990s, did not seem particularly aggressive on tomatoes during the 1980s. After having colonized the European continent, the mildew is said to have spread to all the potato and tomato-producing countries in the world. 1 from moving to the United States, and subsequently to Europe, probably via contaminated potato seeds. A first migration would have allowed the US- line. The spread of this disease, from its area of origin (the Andean zone1, long contested to the detriment of Mexico), would have taken place in several stages from the 1840s. Its first epidemics in Europe, in the 1840s, caused dramatic famines and the deaths of over a million people (most notably in Ireland), resulting in mass emigration of Irish people. infestans, is a disease that has marked human history. Potato and tomato late blight, caused by P. Let us recall that in France, the diversity of strains isolated from tomato is greater than that observed from potato strains the ratio of A2 strains is also greater there. Its incidence is lower in crops under cover. Since the 1990s and the arrival of A2 strains in particular, its impact on tomatoes has sharply increased it is now frequent and feared by all field tomato growers and amateur gardeners, especially during the summer production period. infestans finds particularly conditions favorable for its development. In France, a country with a temperate climate par excellence, P. It is often a constant threat and requires preventive fungicide treatments. As its Anglo-Saxon name "late blight" suggests, it is rather late in season. infestans manifests itself especially in production areas that experience prolonged periods of humidity (rains, sprinkler irrigation, fog, dew, etc.) and good weather. This chromist mainly affects tomatoes grown in the open field, but it can be observed in shelters with poorly controlled climates. It is not uncommon in this case to observe completely devastated crops in which the number of marketable fruits is very limited. It is particularly formidable and destructive in regions with a humid climate, and its damage can be very important during rapid epidemics that are poorly controlled. English names: downy mildew, late blightĭowny mildew is rampant in many production areas around the world. synonymie : Botrytis infestans Mont., (1845) Peronospora infestans (Mont.) de Bary, (1863) classification : Chromista, Oomycota, Oomycetes, Peronosporomycetidae, Peronosporales, Peronosporaceae
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