Department of Agronomy, The Pennsylvania State University, University Park, PA 16802, USA

Background and objectives
The effect of light and CO2 on plant growth is mediated by photosynthesis and there is evidence that light and CO2 may similarly influence response to infection by fungal vascular pathogens. Photosynthesis is reduced in susceptible alfalfa (Medicago sativa) but not in resistant alfalfa infected with Verticillium albo-atrum, the pathogen causing verticillium wilt [1]. When net photosynthesis is reduced by growing resistant alfalfa clones under 70% shade, resistance to V. albo-atrum is lost [2] - evidence that carbon assimilation is involved in the expression of resistance to this pathogen. The alfalfa clones used in those experiments had polygenic (quantitative) resistance to V. albo-atrum,leaving unresolved the question of whether light and CO2 affect the expression of oligogenic (major gene) resistance.

Materials and methods
Shading experiments with 100% and 20% ambient light were used to determine whether polygenic and oligogenic resistances respond similarly to reduced photosynthesis. Alfalfa clones are genetically heterogeneous, a confounding factor that was eliminated by using two experimental approaches. Commercial alfalfa cultivars have polygenic resistance to V. albo-atrum and oligogenic resistance to Fusarium oxysporum f. sp. medicaginis. The first approach used these vascular wilt pathogens to invoke the expression of either polygenic or oligogenic resistance within alfalfa clones, thus avoiding the problem of genetic heterogeneity among clones. The second approach addressed the problem of fungal genetic heterogeneity by using V. albo-atrum as the pathogen and comparing clones with polygenic or oligogenic resistance to that pathogen. In both approaches, clones were inoculated by soaking the roots in spore suspensions of either V. albo-atrum or F. oxysporum f. sp. medicaginis for 15 min. The experiments were conducted as split-plot randomized complete blocks with light as the main-plot treatment and a factorial arrangement of clones and vascular wilt pathogens as the subplot treatments. Disease rating, plant height, and leaf and stem dry weights were measured during weeks three and five of the 5-week experiments. The experiments were repeated.

Results and conclusions
When clonal heterogeneity was controlled by using the two vascular wilt fungi to examine polygenic and oligogenic resistances within alfalfa clones, significant pathogen-light interactions were noted for all variables. Polygenic and oligogenic resistances did not respond similarly to reduced light and subsequent reduced net photosynthesis. Within alfalfa clones, plants inoculated with V. albo-atrum and expressing polygenic resistance had suppressed growth and more symptoms under 20% light compared with 100% light, whereas plants expressing oligogenic resistance to F. oxysporum f. sp. medicaginis did not differ from the uninoculated control plants. When fungal heterogeneity was controlled by using V. albo-atrum to inoculate clones with either polygenic or oligogenic resistance to verticillium wilt, there were significant pathogen-clone-light interactions for height, stem dry weight and aerial biomass. Clones with polygenic resistance to V. albo-atrum had significantly greater growth suppression under 20% light than under 100% light compared with the uninoculated control plants. In contrast, the clone expressing oligogenic resistance did not differ from the ininoculated control plants. These experiments confirmed the sensitivity of polygenic resistance to light and demonstrated that oligogenic resistance in alfalfa to vascular wilt fungi is not sensitive to light levels.

1. Pennypacker BW, Knievel DP, Leath KT, Pell EJ, Hill RR, 1990. Phytopathology 80, 1300-1306.
2. Pennypacker BW, Knievel DP, Risius ML, Leath KT, 1994. Phytopathology 84, 1350-1358.