Edifenphos resistance

Edifenphos Resistance in Pyricularia oryzae and Drechslera oryzae... 

Fungal strains Pyricularia oryzae and Drechslera oryzae (parent), sensitive to edifenphos were maintained on potato dextrose agar (PDA). The edifenphos-resistant mutants were maintained on edifenphos-amended PDA. For the present study, uniformaly 8 and 10-day-old cultures of IK oryzae and . oryzae, respectively, were used throughout the study. 

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). 

Rate of efflux of electrolytes The results in Table X clearly indicates a significant reduction in efflux of electrolytes from all the edifenphos resistant mutants of . oryzae and IK oryzae throughout the observation period. 

Macromolecular synthesis The macromolecular contents increased in all of the mutants. The Total DNA content (Table II) was observed to be higher in the edifenphos resistant mutants. UV-induced . oryzae mutants showed only 20% increase of DNA over sensitive strain. Similarly D oryzae mutants also showed an increase in the DNA content. Among the phospholipids the increase in phosphatidylcholine was significant in the resistant mutants. 

Electrophoretic protein pattern The electrophoretic pattern of protein (Table III) revealed extra bands in two of the edifenphos-resistant mutants (POLR-1 and POLR-4) of P. oryzae. POLR-2 did not show any extra band. In D. oryzae there was no difference in the number of protein bands but the intensity was greater in all the edifenphos-resistant mutants. 

Analysis of edifenphos-resistant mutant and sensitive strains of P. oryzae and D. oryzae (Fig.4, 5a and5b)showed a single plasmid DNA band in the agarose gel electrophoresis. Further, the... 

However, isoprothiolane (Figure 4.8) is cross-resistant to iprobenfos and edifenphos, suggesting a common mode of action. 

The phosphoro-thiolate compounds such as pyrazophos interfere with phoqjholipid biosynthesis [66], Pyrazophos is active against several powdery mildew species by being converted to 2-hydroxy-5-methyl-6-ethoxycarbonylpyrazolo (l-5-a)-pyrimidine (PP). However, pathogen species such as U. nuda were incapable of metabolising pyrazophos and thus insensitive. In addition, resistant mutants of P. oryzae were unable to metabolise pyrazophos [67] and were cross-resistant to other molecules of the same class of flmgicides such as iprobenfos (IBP) and edifenphos. No information on genes in this metabolic pathway is available. 

Edifenphos (EDDP) is effectively used for the control of both P. oryzae and JD. oryzae in India. Level of resistance found in the field of EDDP is usually rather low and therefore it is difficult to detect and measure the EDDP- resistance. Since fungicidal activity and fungal resistance are easier to evaluate in vitro, the present investigation employed im vitro methods to determine the probability level, mechanism and molecular basis of resistance and control measures to break resistance in p. oryzae and D. oryzae to EDDP. 

Various cellular contents analysed,showed significant increase in all the macromolecular contents viz. DNA, RNA Protein, lipids and phospholipids of the EDDP-resistant mutants of P. oryzae and IK oryzae. The efflux of electrolytes was reduced in the resistant mutants when compared to control. In interpreting the results, it should be realized that the phospholipids, the target site of inhibition for edifenphos seem to be unaffected in the mutants as evidenced from the higher values of phosphatidylcholine compared sensitive strain. These data indicate that the mechanism of resistance to edifenphos is not related to the target site of edifenphos. De waard and Van Nistelrooy(38) also reported that the mechanism of resistance in J . oryzae to pyrazophos (PP) was not related to the change of target site of PP. It seems probable that resistance in P oryzae and IK oryzae to edifenphos... 

IK oryzae to edifenphos was presumed to be pleiotropic mutation. Because, such a mutation may not only cause resistance to unrelated compounds (62). Such pleiotropic mutation has been reported for resistance to triarimol in Botrytis cinerea (63) and imazalil in Aspergillus nidulans (64). All of the mutants of oryzae were sensitive to Ziram and IK oryzae to Mancozeb though number of chemicals to which each mutant showed varied sensitivity. 

Cross resistance in an organism to two different chemicals cannot be assumed without evidence that sensitivity to both chemicals is controlled by the same gene (62), but it does influence the selection of alternate chemicals for immediate control of the pathogen in the field. For a country like India, information on alternate chemicals to overcome disease control failure is very important as the farmers cannot afford yield losses at any cost. The present observation on cross resistance to unrelated compounds have been confirmed by many workers (55, 65-70). It is, therefore, concluded that, to overcome disease control failure of p. oryzae due to EDDP-resistance, Ziram may be used as an alternate chemical. In IK oryzae Mancozeb is recommended as an alternate chemical to break edifenphos... 

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