Interactions Between SCN and Other Diseases

Several diseases may affect soybean at the same time. These plants are infected with SCN, and also have symptoms of seedling blight (red coloration), charcoal rot (gray lesions on stem), and potassium deficiency (yellowed leaf edges). Diagnosis of SCN in such a situation requires professional help.

 Soybean plants growing in SCN infested fields in southern states are often infected with other parasitic nematodes (root-knot, lesion, sting, or lance) or fungi that cause diseases such as stem canker, red crown rot, charcoal rot, Rhizoctonia root rot, and Fusarium wilt. In the northern soybean-producing states, other nematodes (such as lesion), or fungal diseases such as Phytophthora root rot, charcoal rot, Fusarium wilt, brown stem rot, and white mold may be present in SCN-infested fields.

 In addition, SCN infections may suppress nodule formation and nitrogen fixation by the beneficial Bradyrhizobium bacteria.

 Most studies involving SCN and one or more pathogenic organisms have shown that yield loss is additive, that is, the yield loss from two or more pathogens is equal to the sum of the yield loss due to each pathogen alone. For example, yield loss on a susceptible variety grown in soil infested with both SCN and root-knot nematodes was additive. Yield loss in a SCN-resistant, root- knotsusceptible variety was due to root-knot alone and vice versa. Similar results have been obtained with These plants are Phytophthora root blight (red rot. Other studies on the effects of SCN infections on insect damage by the corn earworm, soybean looper, or three- cornered leafhopper and various weed infestation levels have shown also that yield loss is additive among these pests. There are reports that SCN-resistant varieties are more tolerant of sudden death syndrome (SDS); however, SDS severity is greater on SCN susceptible varieties grown in SCN infested fields. Synergistic (greater than additive) yield losses have been reported for SCN in combinations with Fusarium wilt and red crown rot.

 SCN does not "break" resistance to other pathogens. Thus, nematologists, pathologists, and breeders have combined efforts to develop soybean varieties with resistance to several pathogens. Important examples in northern varieties are those with resistance to both SCN and Phytophthora root rot, while several southern varieties are resistant to both SCN and root-knot nematodes. Information on specific resistance cannot be included here but is available from local Cooperative Extension Service offices.

• one pathogen may enhance infection by other pathogens, 
• one pathogen may suppress infection by other pathogens, or 

multiple infections may produce greater stress and yield loss in the plant compared with infections by either pathogen alone.