Growth and disease response of soybeans from early maturity groups to ozone and Fusarium oxysporum

The single and combined effects of ozone (O 3 ) and Fusarium oxysporum on growth and disease expression of soybean genotypes differing in foliar sensitivity to O 3 were studied in the greenhouse. O 3 had no effect on root and hypocotyl rot severity of PI 153·283 (O 3 -sensitive, S) or PI 189·907 (O 3 -tolerant, T) maturity group I soybean lines. Plants of both genotypes infected with F. oxysporum and exposed to O 3 had greater reductions in relative growth rate (RGR), net assimilation rate (NAR), and had more stippled leaves per plant than Fusarium -free plants exposed to O 3 . O 3 alone had a greater impact on shoot dry weight, RGR, and NAR of PI 153·283 (S) than of PI 189·907 (T). O 3 alone reduced shoot and root dry weights primarily through a depression in NAR and less through reduced leaf area. F. oxysporum alone reduced root dry weight at 35 days; however, infected plants responded with increases in root dry weight from 49 to 63 days. Similarly, F. oxysporum alone lowered early RGR but subsequent RGR decline was less rapid while NAR remained high, particularly during later sampling intervals. Infection by F. oxysporum that causes root and hypocotyl rot increased soybean sensitivity to O 3 by prolonging active vegetative growth.