VIEW ARTICLE

Ecology and Epidemiology

Inoculum Density, Pathogenicity, and Interactions of Soybean Root-Infecting Fungi. L. Zambolim, Former graduate student, Department of Plant Pathology, University of Florida, Gainesville 32611, Present address of senior author: Departmento de Fitopatologia, Universidade Federal de Viçosa, Viçosa, Minas Gerais 36570, Brazil; N. C. Schenck(2), and D. J. Mitchell(3). (2)(3)Professors, respectively, Department of Plant Pathology, University of Florida, Gainesville 32611. Phytopathology 73:1398-1402. Accepted for publication 20 April 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1398.

In autoclaved and nonautoclaved soil (91 ppm P and pH 6.0), the percentages of infected roots and diseased plants increased with increasing levels of inoculum of Macrophomina phaseolina and Rhizoctonia solani. Fifty percent of the seedlings were infected (ED₅₀) by M. phaseolina at approximately 0.8 and 3.8 x 10⁴ sclerotia per kilogram of autoclaved and nonautoclaved soil, respectively. For R. solani the ED₅₀s were 0.1 and 0.2 x 10⁴ sclerotia per kilogram of autoclaved and nonautoclaved soil, respectively, M. phaseolina and R. solani or Fusarium solani at known inoculum densities in autoclaved soil were studied for their effect on soybean plants in a greenhouse at 28- 35 C. Within 25 days after planting, M. phaseolina had significantly reduced root weight; R. solani and F. solani had significantly reduced root weight, shoot weight, and plant height compared to the uninoculated control plants. The combination of M. phaseolina at 40 x 10⁴ sclerotia per kilogram of soil with either R. solani at 0.1 x 10⁴ sclerotia or F. solani at 300 x 10⁴ chlamydospores per kilogram of soil significantly reduced all three parameters when compared to the control or to treatments with M. phaseolina alone. The percentage of roots infected with M. phaseolina significantly decreased when combined with R. solani.