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Sudden death syndrome (SDS) caused by Fusarium virguliforme is among the most important diseases affecting soybean in the United States. The use of biological control agents (BCAs) such as Trichoderma spp. can be a valuable resource to suppress F. virguliforme populations. Therefore, this research focused on screening possible BCAs against F. virguliforme and evaluating mycoparasitism and the induction of systemic resistance as mechanisms underlying the antagonistic activity of selected BCAs against F. virguliforme. In total, 47 potential BCAs, including 41 Trichoderma isolates and 6 Mortierella isolates, were screened in a dual-plate assay. The most effective isolates belonged to the Trichoderma harzianum species and were able to inhibit F. virguliforme radial growth by up to 92%. Selected Trichoderma isolates were tested in the greenhouse and in a microplot study. They reduced root rot caused by F. virguliforme when the plants were coinoculated with the pathogen and the BCA. The tested BCA’s ability to reduce F. virguliforme growth may be related to several mechanisms of action, including mycoparasitism and induction of defense-related genes in plants, as revealed by monitoring the expression of defense-related genes in soybean. Our results highlight the potential of native Trichoderma isolates to inhibit F. virguliforme growth and reduce SDS severity, providing the basis for future implementation of biological control in soybean production. More efforts are needed to implement the use of these approaches in production fields, and to deepen the current knowledge on the biology of these highly antagonistic isolates.

Spread of Glomus caledonius and Glomus mosseae was monitored 3, 15 and 21 months after inoculation in the field. The occurrence of their characteristic spores or sporocarps was used to establish their presence. Thirteen weeks after inoculation, G. caledonius had spread an average of 7-5 cm and a maximum of 225 cm from the point of inoculation. After 15 months its spores were irregularly distributed all over the plot irrespective of the initial inoculation points and the fungus had bridged a gap of 4 5 m between inoculation points. After 21 months G. caledonius spores throughout the plot had multiplied about fivefold since the previous survey and many G. mosseae sporocarps were also present. Spores and sporocarp numbers were more similar to those in pot cultures than in normal field situations.Inoculation had residual effects within the plot on the growth of lucerne sown 12 months after inoculation and cut 3 months later.

SUMMARYThe effect of root density on the spread of vesicular-arbuscular mycorrhizal infection was measured using Trifolium repens (clover) and Festuca rubra (fescue). Seeds of both host species were sown in large troughs at three different distances apart, which resulted in different root densities that persisted for up to 15 weeks. For clover, colonization (i.e. total length of infected root) and linear spread were favoured by greater root densities, and at the greatest root density colonization seemed to proceed faster than root extension. In fescue, root density was positively related to linear spread up to about 23 cm root cm~^ soil, but greater root density was supraoptimal for linear spread but not for colonization. Although root density had important effects on the spread of infection, effects of plant species were much greater. Fescue had more total root (five times that of clover), but it supported less linear spread and less than half the total length of infected root.