Fungicide cross resistance

Dimethomorph inhibits the formation of the fungal cell wall and is not cross-resistant to any known class of fungicides. 

Cohen, Y. Samoucha, Y. (1984) Cross-Resistance to Four Systemic Fungicides in Metalaxyl-Resistant Strains of Phytopthora infes-tans and Pseudoperonospora cubensis. Plant Disease 68, 137-139. 

All benzimidazole fungicides (benomyl, carbendazim, fuberidazole, thiabendazole) and thiophanate fungicides are cross-resistant. However, the reeently diseovered fungicide, zoxamide, which also inhibits the B-tubulin assembly, is not cross-resistant in oomycetes... 

The third important point illustrated in Figure 1 is that two populations that our records indicated had experienced little or no dicofol selection had unexpectedly high frequencies of dicofol resistance. These data points are from orchards that had zero or one dicofol treatment since 1982, yet they show greater than 20% dicofol-resistant individuals (<80% mortality). It is possible that these locations represent errors in pesticide use record-keeping. Alternatively, it is possible that these data reflect either effects of other pesticides (including fungicides), that may select for dicofol resistance, or immigration of resistant spider mites from nearby resistant orchard populations. Studies of cross-resistance of dicofol-resistant mites, currently underway, address this former possibility. 

In principle, one of the most effective means to cope with resistance of plant pathogens is the use of fungicides to which resistant strains show negatively correlated cross resistance. However, until now this strategy has seen limited use in practice, principally for lack of suitably effective compounds. Herein we discuss one of the first practically useful examples of negatively correlated cross resistance, and report on its genetic and molecular basis. 

Leroux and Gredt reported that benzimidazole-resistant strains of Botrytis cinerea and Penicillium expansum exhibited negatively correlated cross resistance to herbicidal N-phenylcarbamates such as barban, chlorpropham, and chlorbufam (10, 11, 12). Based on their observation, Sumitomo scientists evaluated many examples of N-phenylcarbamates to search for compounds with potent fungitoxicity and no phytotoxicity. This effort was eventually successful, leading to new fungicides such as MDPC ( , 21 ) and diethofencarb (14, 23, 24) (Figure 2). 

Cross resistance studies All of the edifenphos-resistant strains were tested for their cross-resistance to related and unrelated systemic and contact fungicides. The growth rate of both mutant and sensitive strains were compared and their values were estimated. The resistance level was expressed as Q value (ratio of EDjq of sensitive strain/ED5Q of mutant strains)(31). 

The continuous use of o-phenylphenol, biphenyl, thiabendazole, benomyl, and sec-butyl amine for 20 years to control postharvest decay of citrus fruits has resulted 1n a serious problem of fungicide-resistance in Peniclllium digitatum and P. italicum. Fungicide-resistant Perilc1111 um Isolates are cross-resistant to structurally-related compounds and, 1n addition, may be resistant to two or more unrelated compounds. Biotypes of P. digitatum resistant to imazalil have been found recently in 19 packinghouses throughout California. 

Virtually all benzimidazole-resistant isolates of Penicilllum are cross-resistant to all other benzimidazole fungicides (7,8,13,14). Furthermore, many of these isolates show resistance also to the unrelated postharvest fungicides, SOPP, biphenyl, sec-butylamine, and guazatine (13,14,17,18). Multi-resistant biotypes apparently emerged from a benzimidazole-resistant population that was subjected to the selection pressure of the unrelated fungicides. 

Considerable interest has developed in resistance management through the use of 1) compounds that are more active against fungicide-resistant biotypes than their sensitive counterparts (i.e., the resistant biotypes exhibit negatively-correlated cross resistance) and 2) compounds that interfere with the resistance mechanism (i.e., synergists). Both approaches to resistance control have been reviewed by De Waard (24,45). 

Cross-resistance (to existing fungicides with known resistance problems). 

Determine the resistance and cross-resistance potential of each new candidate with major fungicide groups. 

Continue to promote the alternation of metalaxyl with a non-cross-resistant Pythium fungicide. Ciba-Geigy held a turf symposium on December 4, 1987. 

The nearly identical mode of action of benzimidazoles, evidenced by the similarity of their fungitoxic range of action and of their structure, is also supported by the development of cross-resistance. Resistance developed rather rapidly to systemic fungicides has also been observed for benzimidazole derivatives. 

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