Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/1996
Publication Date: N/A
Interpretive Summary: Sudden death syndrome (SDS) of soybean produces symptoms on plants that include root rot, and interveinal yellowing of leaves which are the most noticeable symptoms. The causal agent, Fusarium solani, has been isolated mostly from roots, but not from leaves. It is proposed that a fungal toxin(s) may be moving from roots of plants to the leaves causing the leaf symptoms. We determined that a toxin was involved in producing leaf symptoms, and used tissue culture techniques to determine soybean calli were sensitive to the fungal toxin. Fungal isolates from SDS-infected plants were compared to other fungal isolates not from soybeans. Leaf yellowing occurred when the fungal culture filtrate was (without the fungus) was injected into soybean stems or when cut stems were immersed in the filtrate solution. Sensitivity of calli to the culture filtrate was correlated to SDS ratings of inoculated plants. Fungal isolates from hosts other than soybean did not cause SDS and their culture filtrates affected soybean calli less than that of soybean SDS-causing isolates. This suggests that the toxin(s) produced by F. solani isolates that causes SDS is involved in producing leaf symptoms associated with SDS. These results will be used to further explain the basis for symptom development of soybean leaves and how this toxin may be used in selecting resistant soybean types.
Technical Abstract: An isolate of Fusarium solani, that caused sudden death syndrome (SDS) of soybean (Glycine max), was grown in a semi-defined liquid medium. Leaf mottling and intervenial chlorosis occurred when the fungal culture filtrate was injected into soybean stems or when cut stems were immersed in the filtrate solution. The phytotoxicity of cell-free culture filtrates was determined with soybean calli derived from hypocotyls by measuring calli fresh weight and the absorbance of calli extracts at 330 nm after 5 days. Calli reactions of five soybean cultivars grown on culture medium amended with culture filtrate were compared to the reactions of the same five cultivars inoculated with fungus under greenhouse conditions. Sensitivity of calli to the culture filtrate was significantly (P = 0.05) positively correlated (r = 0.995) to SDS ratings of inoculated plants. Phytotoxicity and pathogenicity of F. solani isolates from different hosts were tested on soybean calli and on soybean plants, respectively. Isolates from hosts other than soybean did not cause SDS, although they decreased soybean root growth and their culture filtrates had significantly lower toxicity to soybean calli than that of soybean SDS-causing isolates. This suggests that the toxin(s) produced by F. solani isolates that cause SDS are involved in producing leaf symptoms associated with SDS.