In vitro somatic growth and reproduction of phenylamide- resistant and -sensitive isolates of Phytophthora erythroseptica from infected potato tubers in Idaho
Porter. L. D Miller. J. S Nolte. P Price. W. J
Plant Pathology ; 2007 [Vol.56] Pages:492-499
Abstract
Pink rot of potato, most commonly caused by Phytophthora erythroseptica, is a major field and post-harvest problem in southern Idaho, USA, particularly since 1998 when isolates resistant to the phenylamide fungicide metalaxyl-M (mefenoxam) were detected. Isolates of P. erythroseptica were collected from infected tubers in 2001 and 2002 from six Idaho counties and tested for resistance to metalaxyl-M on amended agar. Metalaxyl-M resistant (MR) and metalaxyl-M-sensitive (MS) isolates were identified in six counties; 160 isolates were highly resistant, seven moderately resistant and 57 sensitive to metalaxyl-M with mean EC50 values of 182, 23 and 0·5 mg L-1 ai metalaxyl-M, respectively. Mycelial growth rates and oospore production in agar were assessed for 20 MS and 20 MR isolates at 10, 15, 20, 25 and 30°C. Growth rates of MR isolates were between 2·5 and 3·1 times greater (P < 0·05) than those of MS isolates at 10, 15, 20 and 25°C, and oospore production was between 6·8 and 20·5 times greater (P < 0·0001) for MR than for MS isolates at the same temperatures. Colony growth in V8 broth at 18°C was greater for MR than MS isolates (P < 0·0032). However, zoospore production at 18°C was greater for MS than for MR isolates (P < 0·0109), and zoospore production mM-1 of colony circumference was also greater for MS than for MR isolates, 14 191 and 9959, respectively (P = 0·0109).Sexual reproduction of MR isolates in nature may be greater than MS isolates, but MS isolates may be more asexually fit based on the fitness parameters studied.
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