A 4-year study (1997 to 2000) was conducted to determine the sensitivities of the potato tuber rot pathogens, Phytophthora erythroseptica and Pythium ultimum, to mefenoxam (metalaxyl). A total of 2,277 tubers showing symptoms of “water rot” were collected from 16 states and 2 Canadian provinces. From these, 849 isolates of P. erythroseptica and 213 isolates of P. ultimum were obtained, and 805 and 190 isolates, respectively, were tested for their ability to grow on V8 medium amended at 0.01 to 100 μg/ml with fungicide. Isolates ranged widely in their responses to mefenoxam. The presence of resistant isolates (EC50 > 100 μg ml-1) of P. erythroseptica in the potato producing areas of Maine was confirmed. The presence of P. erythroseptica isolates in Idaho and Minnesota resistant to mefenoxam is reported for the first time. The proportion of P. erythroseptica isolates resistant to mefenoxam varied from 2.9 to 36.2% between 1997 and 2000. The proportion of resistant P. ultimum isolates represented only a small proportion of the isolates tested (3.7%). A single resistant P. ultimum isolate was recovered from Washington, whereas most of the resistant isolates obtained (5 of 7) were collected in Minnesota during the final year of the study. This is the first report of resistance in P. ultimum pathogenic to potato tubers. These observations suggest that pink rot and leak could become significant problems in the future, particularly in those areas where resistance has been detected. Our results have implications for the effective management of water rot. Monitoring the sensitivity of the pathogen population to mefenoxam in all production areas should be considered and integrated as a part of the overall disease management strategy.

The agar diffusion procedure for quantifying pectolytic enzyme activity punched in the agarose with a # 1 cork borer. After incubation at 37 C for 17 was modified to optimize assay sensitivity and simplify its implementation. hr, the gel was developed with 10 ml of 0.05% ruthenium red for 30 min, and The revised assay was run in 100 X 50 mm petri plates containing 20 ml of the diameter of the clear zone of activity was measured microscopically. 1% agarose (Type II), ammonium oxalate (0.5%), and sodium azide (0.2%) Polygalacturonase equivalents as low as 2.3 X 10-4 units were detected. The in phosphate buffer (0.2 M, pH 5.3) with polygalacturonic acid (0.0 1%) as modified assay required less sample and reduced the problem of gel the substrate. Samples (35 yl) were pipetted into 4.1 mm diameter wells dehydration associated with the standard assay.

Tubers of 34 potato cultivars were examined for their susceptibility to infection by zoospores of Phytophthora erythroseptica and mycelia of Pythium ultimum. Incidence of infected tubers (%) and penetration of rot (mm) were the parameters used to determine the susceptibility of each cultivar. Tubers of cv. Atlantic appeared to have some resistance to infection and colonization by Phytophthora erythroseptica. Cvs. Russet Norkotah and Snowden were the most susceptible to infection by P. erythroseptica. Snowden was found to be highly susceptible to P. erythroseptica, but it was the most resistant to Pythium ultimum. Cvs. FL-1625 and FL-1867 also were less susceptible to P. ultimum than the other cultivars. Cvs. Superior, Itasca, and Dark Red Norland were the most susceptible to P. ultimum. Cultivar susceptibility should be considered when making disease management decisions, particularly in fields where these soilborne diseases are a recurring problem.

Data supplementing a previously published survey of North American isolates of Phytophthora erythroseptica and Pythium ultimum demonstrated that the proportion of the populations sensitive to mefenoxam remains high, 79.6 and 96.9% with EC50 sensitivities ranging from <0.01 to 0.9 µg ml-1 and <0.01 to 0.8 µg ml-1, respectively. Mefenoxam should provide control of these pathogens in most potato production areas. Factors affecting the development of pink rot and leak in potato tubers and the efficacy of mefenoxam to control these diseases with different etiologies were examined. Results confirmed that P. erythroseptica is capable of directly infecting potato tubers causing pink rot, whereas Pythium ultimum requires a wound to infect and cause leak. Mefenoxam was applied to replicated field plots as a single in-furrow application at planting, as an in-furrow application at planting followed by an additional sidedress application 3 weeks after planting, as a single foliar application when tubers were 7 to 8 mm in diameter, and as two foliar applications when the tubers were 7 to 8 mm in diameter and 14 days later. The recommended label rate plus two additional lower application rates were used with each method. For tubers challenge-inoculated after harvest, mefenoxam was found to be more effective in controlling pink rot relative to leak over all application methods. The greatest level of pink rot control (89%) was attained with the in-furrow at planting and sidedress application. All rates tested provided similar levels of control with this application method, but this method provided only a modest level of leak control (35%), and leak was not controlled by foliar applications of mefenoxam at any rate tested. In contrast, the foliar applications of mefenoxam resulted in 10 to 50% control of pink rot. Since the isolates of both pathogens were highly sensitive to me-fenoxam, disease-specific control was attributed to differences in disease etiology. Therefore, the use of mefenoxam to control pink rot in the field and storage appears to be well founded.

Tubers from plants treated with in-furrow and foliar applications of mefenoxam were inoculated with eight isolates of Phytophthora erythroseptica having varying levels of sensitivity to the fungicide. Two isolates with effective concentration causing 50% reduction of mycelial growth (EC50) values of 0.02 and 0.04 μg ml-1 were categorized as being mefenoxam sensitive. Isolates with EC50 values >1.0 μg ml-1 were designated as insensitive to mefenoxam and were grouped two each into low intermediate (EC50 = 1.1 and 5.3 μg ml-1), high intermediate (EC50 = 26 and 74 μg ml-1), and resistant (EC50 ≥ 100 μg ml-1). The biological significance of these isolates was examined by quantifying disease control. P. erythroseptica isolates classified in the resistant group infected a significantly greater proportion of untreated tubers than isolates in any other group. Mefenoxam reduced infection frequency of sensitive isolates by as much as 37%. Mefenoxam did not provide disease control of any isolate possessing insensitivity to the fungicide, with the greatest decrease in control observed with the low intermediate group. Aggressiveness indices, representing tuber infection frequency and depth of penetration, were calculated for untreated and mefenoxam-treated tubers. According to these indices, both isolates classified in the resistant group and high intermediate isolate 252–4 were more aggressive than sensitive isolates in the absence of mefenoxam pressure, and significantly so in the presence of mefenoxam. These results suggest that pink rot may become more severe in fields known to contain P. erythroseptica populations with mefenoxam EC50 values >1.0 μg ml-1 if the fungicide is applied. These factors should be considered when developing strategies to manage pink rot and mefenoxam-resistant populations of P. erythroseptica.