Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/14/1998
Publication Date: N/A
Citation: Interpretive Summary: Sudden death syndrome (SDS) of soybean is caused by the soilborne fungus Fusarium solani. Disease symptoms include mottling, interveinal chlorosis and necrosis on the upper leaves at flowering and also root rot, crown rot, vascular discoloration of stems, defoliation and pod abortion. Identification of soybean cultivars and lines with low SDS foliar disease severity is one of the most promising control tactics. Toxins produced by the fungus appear to be responsible for causing the foliar symptoms. The objectives of this report were to compare resistant and susceptible soybeans: by monitoring root infection, root and stem colonization, and disease severity on foliage; and by evaluating cut soybean seedlings immersed in culture filtrate of F. solani SDS isolates. There was little difference between resistant and susceptible soybean lines in root infection, root and stem colonization, but there were differences in foliar disease severity. Foliar disease developed when seedlings were either inoculated with the fungus or when cut seedlings were immersed in culture filtrate. Our results indicate that SDS resistance in soybeans may not be due to fungal infection and colonization differences of roots, but due to the interaction of the fungal toxin and soybean genotype. This research is the first report to characterize the resistance response in these soybean plant introductions that potentially represent new sources of resistance to SDS. These results will provide soybean researchers with new sources of resistance and potentially a different technique to screen genotypes for resistance by using cut seedlings immersed in fungal culture filtrate.
Technical Abstract: Four soybean plant introductions, PI 520.733, PI 567.374, PI 567.650B, PI 567.659 and one cv., Great Lakes 3202, ranged from resistant to susceptible when inoculated with four Fusarium solani isolates that cause sudden death syndrome in soybean. Foliar disease severity was greatest on PI 567.659, followed by cv. Great Lakes 3202, PI 520.733, PI 567.650B and PI 567.374. Several experiments using isolate Mont-1 were conducted to determine: if root exudates from the five entries affected mycelial growth of F. solani; if the entries differed in disease development and root colonization after inoculation with F. solani; and if the entries differed in symptom development when cut seedlings were immersed in a culture filtrate of F. solani. Mycelial growth exposed to root exudates and root infection on the five entries did not significantly (P = 0.05) differ. Foliar disease progress curves increases were greater for PI 567.659 and Great Lakes 3202, than for PI 567.374. The AUDPC of PI 567.374 was the lowest and differed significantly (P = 0.01) from AUDPCs of Great Lakes 3202 and PI 567.659. Interveinal chlorosis symptoms appeared on leaves of cut seedlings of each entry 2 days after immersion in the culture filtrate. The AUDPC for cut seedlings of PI 567.374 was 109 and was significantly (P < 0.0001) lower than the AUDPCs of the other entries that had values ranging from 193 to 410. There was a significant (P = 0.05) correlation (r = +0.94) between AUDPC values of the five entries inoculated with the fungus and the cut seedling test using the culture filtrate. Soybean cut seedlings immersed in F. solani culture filtrate may be used to test toxicity of isolates and to evaluate resistance.