2021, Vol. 4, Issue 1, Part A

author(s)
Laércio Zambolim, Fernando C Juliatti and Wanderlei Guerra

abstract
Asian soybean rust (ASR) caused by Phakopsora pachyrhizi is the most important disease of the crop. The pathogen is highly aggressive under field conditions when the environmental conditions favor the disease development. The disease was first described in Japan, in 1902 and in 1914, it had already spread to several countries in Southeast Asia.). The first report in South America was made in Brazil in 1979 in wild soybeans and later in 2001 in the soythern area of the country in comercial plantations. P. pachyrhizi can naturally infect a wide range of plant species, including 41 species in 17 genera of the Fabaceae family. The symptoms of the disease can be seen on the abaxial and eventually on the adaxial surface of the lesions. In susceptible varieties the number of uredia per lesion varies from four to eight and the latent period around seven days. P. pachyrhizi requires more than 8 hours a day of continuous leaf wetness, and the optimal temperature for maximum germination around 22oC. There is no varieties with complete resistance to the disease available to planting. The main method of control of the disease is the application of fungicides. The main groups of fungicides to control the disease belongs to the demetilation inhibitores (DMI’s), quinone outside inhibitor (QoI’s) and carboxamides (SDHI). But with the continuous spraying with DMI’s and QoI’s alone to control ASR resistant mutants of P. pac HIhyrhizi multiplied in the population over the country. The use of mixture of DMI’S with QoI’s in the beginning proportioned resonable controle of the disease. But few years latter P. pachyrhizi acquired resistance to the mixture of triazol with stobolurins. Then it was introduced the carboxamide group (SDHI) to use in mixture with triazol with stobolurins. The triple mixture was then recommended for more a few years giving good results. Finally researchers decided to incorporate multisite fungicides in a mixture with triazol, strobilurin and or carboxamide to minimize the probability to build up resistant mutants in the population of P. pachyrhizi. The addition of multisite fungicides in a mixture with site specific is very important to reinforce the fight against fungal resistance. In conclusion to cope with the vulnerability of P. pachyrhizi to site specific fungicides the strategy has to involve integration of measures such as sanitary vacuum, rotation of a mixture of different biochemical mechanisms of action plus multisite, avoid sequential and curative applications, use of early cultivars and sowing at the beginning of the recommended season, eliminate voluntary soybean plants from the field, use of fungicides in the onset of preventively at least once before the planting lines closed, sowing at the beginning of the recommended season, and use of cultivars with resistance gene (s) to reduce the number of spraying. The present overview discuss how difficult is to cope with the resistance of P. pachyrhizi to site specific fungicides.