The threat of roya Asian of the soybean

It can produce losses of until a 80% in the yields; in Argentina not yet it is a problem, but already was detected it

PUERTO IGUAZU (Misiones). - "roya of the soybean already is for remaining and is very probable that it expands by all sides. It is a disease of high impact and more frightful than others ", said, in tone of alert, Daniel Ploper, fitopatólogo of the Experimental Station Agro-industrial Obispo Colombres, of the province of Tucumán.

For those who participated in the Pan-American Workshop of Roya of the Soybean, that BASF organized the week last in this city, the words that the technician pronounced in his dissertation indicate that Argentina must be put in guard as opposed to this disease and anticipate solutions if gets to scatter itself in the main regions where the oily one is cultivated.

In fact, this disease, that was reported for the first time in 1902 in Japan, has already been observed in the last campaign in lots of producers of the northeast of Currents and in tests that the INTA has in their experimental Blue Hill station, in Missions.

Nevertheless, so far, the situation of our country is quite different from which it happens in Paraguay from 2001, where for example the region of Pirapó became an endemic zone, or in Brazil, that after to have transformed into 2002 in the second South American country that presents/displays roya saw how it was scattered until 95% of the area seeded with soybean.

Also it is worth to consider what happens in Asia, with losses near 60% in the production if any strategy of control is not made on the culture, and what it happens in Africa, that habitually registers damages that hit in the 80 percent of renders.

With these antecedents, roya Asian of the soybean, so its denomination because pachyrhizi is caused by the Phakopsora species, appears more dangerous than other diseases as it stains frog eye, podredumbre carbonaceous, cancro of the stem and the syndrome of the sudden death, for example.

And this because the disease has several factors that justify their destructive power and, also, because the pathogen would find favorable conditions for its development before a possible dispersion in the most important sojeras zones of our country.

Roya is a disease of moderate temperatures, with a rank that oscillate between the 16 and the 28 degrees. The optimal values usually are located between the 19 and 24 degrees.

In addition to the temperature, the humidity is another condition that favors its presence. That is to say, it must have free water (dew) so that the surface of the leaf is wet and the esporas can germinate in a minimum time of approximately six hours.

Among other excellent characteristics it is necessary to mention that the pathogen is policíclico (with many generations) and that disperses by the wind to great distances.

It is not transmitted by the seed and, as he is biotrófico, it does not survive in the infected strubbles. But it can be left in plants guachas of soybean and more than 60 alternative hospedantes, like kudzu, trébol, poroto, caupi and sorts cajanus, among others.

On the other hand, it is a pathogen that has many races. For example, in Thailand they were gotten to detect 45, in Japan 18, China 4, Taiwan 3 and Australia two. Still it is not known how many races it has in Brazil and Paraguay.

Meanwhile, the initial symptoms of the disease of roya of the soybean can be observed in the inferior leaves of the plants that are next to flowering.

In this sense, first it is possible to notice a yellow decoloración in the beam of the inferior leaves and soon, when the infection managed to advance, the leaves become more yellowish and with injuries that are pronounced like small pustules of brown color. Although are more abundant in the leaves, the injuries also usually they possibly appear in pecíolos, stems and cases.

Beyond which roya affects to several processes related to the plant - for example, growth of the stem, abortion of cases and grain filling the most important problem of the disease is related to the anticipation of the maturity of the culture and its yield.

An example: cultures that must have rendered 3000 kilos soon lost 2000 of the attack of the disease. This offers a panorama of the magnitude of its gravity, since, if it has favorable conditions, in only three weeks it can advance from an initial infection to including 90%.

According to Ploper, if the plant is affected in an early state, the loss level will be greater. This is a variable that is related to the moment and to when appears the attack of the disease. The damage also usually is based on how the epidemic progresses. This way, it can find some limitante if the climatic conditions are not favorable.

Of Africa, one of the strategies to control the disease consists of making three applications of fungicidas if the attack is very severe. First one becomes in flowering, still without observing symptoms, and the following ones with an interval of three weeks.

On the other hand, as it indicated X.B. Yang, of the University of Iowa, the United States, in Asia already exists resistant varieties and normally it is accustomed, like in China, to make two applications of fungicidas.