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Healthy vegetables at the cost of multi-fungicide-resistant pathogens
Margaret McGrath: Cornell University
<div>A commonly recommended and implemented resistance management strategy is applying in alternation fungicides with different modes of action (different chemical groups based on FRAC code). Premise is that cross resistance occurs among related but not unrelated fungicides, because mode of resistance usually is the result of changes in different target-sites, and also that pathogen strains with resistance to multiple chemistries are unlikely to develop or be fit to survive. Unfortunately, resistance has developed to multiple chemistries in single strains of several pathogens causing key diseases in vegetables, including powdery mildew of cucurbits (FRAC Code 1, 7, 11, and 13), gummy stem blight of cucurbits (1, 7, and 11), early blight of potato (7 and 11), Fusarium dry rot of potato (1 and 12), and likely downy mildew of cucurbits (4 and 11 plus 28 and/or 43). Stepwise accumulation of resistance to fungicides with successive exposure is the assumed reason. When the first fungicides at risk for resistance development were registered there was not other chemistry to use in alternation except multi-site contact fungicides, which typically are less effective. However, recent cases of resistance (cucurbit powdery mildew pathogen developing resistance to FRAC Code 13 fungicide in addition to Codes 1, 7, and 11) occurred when there were ample chemistries to alternate with (3, U6, and U8). Premixes can contribute to the problem when resistance has developed to one ingredient.</div>

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