Minimizing the Selection Pressure of Site-Specific Fungicides Towards Phakopsora pachyrhizi in Mato Grosso State: A Review

Abstract

The determining cause of selection pression that has resulted in the increased development of resistance of Phakopsora pachyrhizi to fungicides used in Mato Grosso, Brazil, is the use of site-specific fungicides (DMIs, QoIs and SDHIs) solo or in double and triple mixtures. These fungicides have selected mutants resulting in cross and multiple resistance to P. pachyrhizi. The other secundary selection factors are: (a) fungus with high reproductive potential; (b) three sprayingns/area/season; (c) annually sprayed area of more than 10 million hectares; (d) fungus under site-specific selection during 20 years totaling 200 applications/area. Reports indicate that resistance develops only for site-specific, having resulted in eight mutations in Brazil involved with sensitivity reduction to P. pachyrhizi. In contrast, for multisites (chlorothalonil, mancozeb, copper oxychloride) there was found no report on the resistance development in rust-causing fungi and for general fungi to copper fungicides. It is not necessary to withdraw site-specifics from the market, nor the synthesis of molecules with new mode of action, but to avoid their use alone or in their mixtures. In this sense, it is more efficient to market them only in combination with multissites. An example is the worlwide success achieved in the control of oomycetes in potatoes, tomatoes and vines with metalaxyl + mancozebe. This review clearly shows the problem, site-specific mode of action fungicides solo, on resistance development to P. pachyrhizi and the solution, site-specific + mode of action multissites, sprayed in all soybean grown area and in all applications. Preventing and delaying adaptation to fungicide resistance in the pathogen is the main objective of disease management.