Submitted to: Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: May 1, 2008
Publication Date: June 1, 2008
Citation: Porter, L. 2008. Occurrence, aggressiveness, somatic growth and reproduction of phenylamide-resistant and -sensitive oomycete pathogens. Phytopathology. 98:S186. Technical Abstract: Pink rot of potato, caused by Phytophthora erythroseptica, is a major field and post-harvest problem in Idaho, particularly since 1998 when isolates resistant to the phenylamide fungicide metalaxyl-M 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 of 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 than those of MS isolates at 10, 15, 20 and 25 °C, and oospore production was between 6.8 and 20.5 times greater for MR than for MS isolates at the same temperatures. Colony growth in V8 broth at 18°C was greater for MR than for MS isolates. However, zoospore production at 18°C was greater for MS than for MR isolates, and zoospore production mm-1 of colony circumference was also greater for MS than for MR isolates, 14,191 and 9,959, respectively. Rate of growth through tuber tissue was significantly different for resistant and sensitive isolates at 3.3°C but not at 7.2, 10, 15, 20, and 25°C, although growth of resistant isolates was numerically greater at all temperatures. Sexual reproduction of MR isolates in nature may be greater than MS isolates, but MS isolates may be more asexually fit. Current research is addressing the wide-spread occurrence of phenylamide-resistant isolates of Pythium ultimum in the Pacific Northwest.