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Resistance

Correlation Between Root and Stem Reactions of Alfalfa to Phytophthora megasperma f. sp. medicaginis. J. A. G. Irwin, Departments of Plant Pathology and Agronomy, University of Wisconsin-Madison, WI 53706; D. P. Maxwell(2), and R. R. Smith(3). (2)(3)Agricultural Research, Science and Education Administration, U. S. Department of Agriculture, Madison, WI 53706. Phytopathology 70:987-990. Accepted for publication 6 April 1980. 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, 1980. DOI: 10.1094/Phyto-70-987.

The mature root and stem reactions to Phytophthora megasperma f. sp. medicaginis (Pmm) of 58 S₀ alfalfa plants from diverse sources were compared. Plants with highly susceptible roots always had highly susceptible stems, and plants with highly resistant roots never had highly susceptible stems. However, some plants with susceptible roots had resistant stems. The correlation between mature root and stem reaction to Pmm was highly significant (r = 0.82) for the 58 S₀ plants tested. Isolations of Pmm from stems showed that its colonization was restricted in resistant stems compared to susceptible stems. Both the roots and stems of S₁ plants from a highly susceptible and a highly resistant parent plant were all susceptible and resistant, respectively. The correlation between the mature root and stem reaction to Pmm on 44 F₁ plants of the cross of a resistant and a susceptible plant was highly significant (r = 0.59). When the stem inoculation method was used, no evidence was obtained for the presence of pathogenic specialization within North American and Australian isolates of Pmm. The main advantage of the stem inoculation method is that final disease development is evident 4 days after inoculation. However, only highly susceptible individuals can be reliably detected.