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Interrelationships Between Meloidogyne hapla and Phytophthora megasperma f. sp. medicaginis in Seedling Damping-off and Root Infection of Alfalfa. F. A. Gray, Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071; G. D. Griffin(2), D. A. Johnson(3), J. W. Eckert(4), and J. E. Kazimir(5). (2)(3)U.S. Department of Agriculture, Agricultural Research Service, Forage and Range Research Laboratory, Utah State University, Logan 84322-6300; (4)Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071. Phytopathology 80:228-232. Accepted for publication 31 August 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/Phyto-80-228.

Interrelationships between Phytophthora megasperma f. sp. medicaginis and Meloidogyne hapla were studied in three alfalfa cultivars: Nevada Synthetic XX, resistant to P. m. medicaginis and highly resistant to M. hapla; Apollo II, resistant to P. m. medicaginis and susceptible to M. hapla; Deseret, susceptible to P. m. medicaginis and M. hapla. Post-emergence damping-off, attributed to P. m. medicaginis, was suppressed when both pathogens were applied in combination as compared with application of P. m. medicaginis alone at planting. Percent stand loss (percent of control) at 2 wk was 33 and 26, 45 and 21, and 87 and 35 with P. m. medicaginis alone and in combination with M. hapla for Apollo II, Nevada Synthetic XX, and Deseret, respectively. Phytophthora root rot was increased in surviving plants at 7 wk in the combination treatment as compared with the treatment of P. m. medicaginis alone. P. m. medicaginis with M. hapla resulted in less root galling and nematode reproduction in all three cultivars. Inoculation with both pathogens 7 wk after planting resulted in a 59% increase in infection of roots of Deseret by P. m. medicaginis, and an 87 and 13% increase in infection of nitrogen-fixing nodules of Rhizobium meliloti in Deseret and Apollo II, respectively, 6 mo after inoculation. The percentage of nodules per root system was less in the presence of M. hapla in Deseret (67%) and Apollo II (68%) and was less in Deseret (23%) in the presence of P. m. medicaginis, as compared with the uninoculated control. The benefit of dual resistance in preventing loss from Phytophthora root rot was shown in Nevada Synthetic XX, which had no detectable infection of nodules by P. m. medicaginis and only slight root infection when P. m. medicaginis was applied alone or in combination with M. hapla (11.3 and 10.3% infection, respectively).

Additional keywords: interaction, Medicago sativa, Northern root-knot nematode, reproduction.