of penetration and movement of fire blight Pathol. 10:178-182. Erwinia amylovora. (Abstr.) Phytopathology bacteria in apple leaf and stem tissue. 23. Sprague, R., and Covey, R. P. 1969. Fungous 74:825. Phytopathology 55:719-723. and bacterial pear diseases of eastern Washing- 30. van der Zwet, T., Bell, R. L., and Stroo, H. 18. Miller, T. D., and Schroth, M. N. 1972. ton. Wash. Agric. Exp. Stn. Circ. 498. F. 1982. Long distance dissemination of Erwinia Monitoring the epiphytic population of Erwinia 24. Thomson, S. V. 1986. The role of the stigma amylovora as resident bacteria in apparently amylovora on pear with a selective medium. in fire blight infections. Phytopathology 76:476- healthy pear budwood. (Abstr.) Phytopathology Phytopathology 62:1175-1182. 482. 72:711. 19. Plurad, S. B., Goodman, R. N., and Enns, W. 25. Thomson, S. V., Schroth, M. N., Reil, W. O., 31. van der Zwet, T., and Keil, H. L. 1972. R. 1967. Factors influencing the efficacy of and Moller, W. J. 1975. Incidence of Erwinia Importance of pear tissue injury to infection by Aphis pomi as a potential vector for Erwinia amylovora in Bartlett pear flowers of different Erwinia amylovora and control with strepto- amylovora. Phytopathology 57:1060-1063. ages. (Abstr.) Proc. Am. Phytopathol. Soc. 2:67. mycin. Can. J. Microbiol. 18:893-900. 20. Roberts, R. G., and Reymond, S. T. 1989. Evalu- 26. University of California. 1965. Do summer oil 32. van der Zwet, T., Thomson, S. V., Covey, R. ation of post-harvest treatments for eradication sprays favor fire blight development in pear P., and Bonn, W. G. 1986. Endophytic Erwinia of Erwinia amylovora from apple fruit. Crop fruit? Calif. Agric. Ext. Serv. Fruit Nut Grape amylovora not recovered from core tissues of Prot. 8:283-288. Dis. Newsl. (Jan.):2. apples from apparently healthy trees. (Abstr.) 21. Roberts, R. G., Reymond, S. T., and 27. Wright, T. R. 1948. Fire blight of Bartlett pears Phytopathology 76:1140. McLaughlin, R. J. 1989. Evaluation of mature in storage, Wenatchee, 1947. Plant Dis. Rep. 33. van der Zwet, T., and Van Buskirk, P. D. 1984. apple fruit from Washington State for the 32:58-61. Detection of endophytic and epiphytic Erwinia presence of Erwinia amylovora. Plant Dis. 28. van der Zwet, T. 1983. Occurrence of fire blight amylovora in various pear and apple tissues. 73:917-921. in commercial pear seedling rootstocks follow- Acta Hortic. 151:69-77. 22. Sholberg, P. L., Gaunce, A. P., and Owen, G. ing budding with symptomless scionwood. 34. van der Zwet, T., and Walter, J. C. 1985. R. 1988. Occurrence of Erwinia amylovora of (Abstr.) Phytopathology 73:969. Significance of apple tissue injury to artificial pome fruit in British Columbia in 1985 and its 29. van der Zwet, T. 1984. In vitro testing of various infection by Erwinia amylovora. (Abstr.) elimination from the apple surface. Can. J. Plant chemicals for bactericidal activity against Phytopathology 75:629. Detection of Aphanomyces euteiches in Field Soil from Northern Idaho by a Wet-Sieving/Baiting Technique JOHN M. KRAFT, Supervisory Research Plant Pathologist, Vegetable and Forage Crops Production, Agricultural Research Service, U.S. Department of Agriculture, Prosser, WA 99350-9687; JOANNA MARCINKOWSKA, Plant Pathologist, Department of Plant Pathology, Warsaw Agricultural University, Warsaw, Poland 02-766; and FRED J. MUEHLBAUER, Research Geneticist, Agricultural Research Service, U.S. Department of Agriculture, Pullman, WA 99164 (7,8,10). The two factors that complicate ABSTRACT the development of commercially accept- Kraft, J. M., Marcinkowska, J., and Muehlbauer, F. J. 1990. Detection of Aphanomyces euteiches able pea cultivars with resistance to A. in field soil from northern Idaho by a wet-sieving/baiting technique. Plant Dis. 74: 716-718. euteiches are the presence of strains or Aphanomyces euteiches was detected at three locations in northern Idaho by a wet-sieving races of the pathogen that vary in patho- technique. Five-day-old pea seedlings were used to bait the fungus from the sieved organic genicity and virulence (1,21), and the fact fraction from 10 soil samples per field. Inoculated seedlings in rolled germination paper were that A. euteiches often interacts with incubated and observed at weekly intervals for 3 wk. Almost all fields sampled in the Grangeville other root disease pathogens, which con- area were severely infested, and fields in the Kendrick area and two out of six fields in the fuses field disease screening trials (12). Genesee area were heavily infested with A. euteiches. This wet-sieving technique gave results Previous research has demonstrated similar to the greenhouse-pot technique currently used to determine Aphanomyces root rot that the severity of common root rot is potential and required no greenhouse space or special watering requirements. This technique directly related to population densities revealed that infective oospores of A. euteiches were as deep as 60 cm in the soil profile and of the pathogen and the macro- and were present in areas with poor drainage in fields with low inoculum levels overall. A hierarchical statistical analysis was used to separate differences in inoculum levels within areas and between microenvironment (4,14,15). Aphano- fields. myces euteiches was reported to be pre- dominant in the plowed layer of soil and favored by heavy clay soils with increased Common root rot of peas (Pisum disease first described in the United soil moisture retention compared to light sativum L.), caused by Aphanomyces States in 1925 (6,12). Since that time, sandy soils (12,14). The exception is the euteiches Drechs., is a very destructive the disease has been reported in England, occurrence of the disease in sandy soils Europe, Scandinavia, Australia, New under sprinkler irrigation (J. M. Kraft, Cooperative investigations of the ARS, USDA, and Zealand, and Japan, and wherever peas personalobservation). the Washington State University Agricultural Research Center, Prosser. Plant Pathology PPNS are grown in short rotations (5,6,12). No quantitative techniques have been 0060, College of Agriculture and Home Economics There are currently no control practices developed to directly determine the inoc- Research Center, Washington State University, that are economically feasible other than ulum density of A. euteiches under field Pullman. avoidance of heavily infested fields conditions. As a result, a greenhouse Mention of a trademark or proprietary product does (4,6,12,16,20). The use of dinitrophenol technique was developed that utilizes soil not constitute a guarantee or warranty of the product and dinitroaniline herbicides and samples from test fields. Good correla- by the USDA and does not imply its approval to cruciferous soil amendments has given tions are found between the severity of the exclusion of other products that may also be some control (5,17), however, these con- suitable. This article reports the results of research root rot on peas planted in soil samples only. trols are not sufficient when inoculum from test fields and the severity of root levels are high. No commercial varieties rot that develops in those fields (6,16,20). Accepted for publication 28 February 1990 (sub- are available that have resistance suffi- Mitchell et al (11) determined that pea mitted for electronic processing). cient enough to avoid economic loss in roots develop typical Aphanomyces root severely infested fields (6). Recently, rot symptoms when inoculated with This article is in the public domain and not copy- rightable. It may be freely reprinted with cus- advanced breeding lines that approach organic debris from infested fields. They tomary crediting of the source. The American commercial type and are resistant or tol- concluded that wet-sieving the organic Phytopathological Society, 1990. erant to A. euteiches have been released debris and inoculating pea seedlings with 716 Plant Disease/Vol. 74 No. 9 this debris could be used to determine three replications of five seedlings each cal, statistical analysis was conducted the relative infective inoculum level of with approximately 10 mm3 of debris on (3,18) followed with a mean comparison a given soil. However, no one has re- each seedling root. Inoculated seedlings with a Duncan's multiple range test. A ported use of the wet-sieving technique were sufficiently isolated to prevent hierarchical analysis is an analysis of to determine inoculum levels in field cross-contamination. Seedlings were variance of stratified data. In this soils. observed at 1, 2, and 3 wk. At each har- instance, geographic area was the first In 1985, A. euteiches was reported to vest date, seedlings were counted and strata, fields the second, and sites within be present at several locations in north- those that had Aphanomyces root rot fields the third. The data was analyzed ern Idaho (2), and it has continued to were removed. When more than one with the MSTAT (Integrated Microcom- be a serious problem in that area. How- pathogen was involved, infected tissue puter Program, Crop & Soil Sciences ever, no studies have been conducted to was placed on a semi-selective medium Department, Michigan State University, determine the distribution of A. euteiches (13) to isolate A. euteiches. Data were East Lansing) program. in northern Idaho. This study was con- recorded as the percent of seedlings that ducted to determine if the procedure of developed Aphanomyces root rot. RESULTS Mitchell et al could be used to detect Because of the inability to collect Aphanomyces euteiches was detected A. euteiches and to describe its distri- random samples from each field and to from all 17 fields sampled. Differences bution in northern Idaho. randomly sample fields or geographical in levels of A. euteiches between fields sites and because three measurements and geographic areas were evident when MATERIALS AND METHODS were made on each sample, a hierarchi- the procedure of Mitchell et al (1 1) was A field survey was initiated in Sep- tember 1988 to determine the extent and severity of Aphanomyces root rot in Table 1.