Fusarium virguliforme Aoki, O'Donnell, Homma and Lattanzi (FV), a causal organism of sudden death syndrome (SDS), and Heterodera glycines Ichinohe, the soybean cyst nematode (SCN), are two of the most important soil-borne pathogens affecting soybean production in the USA. These pathogens often occur together in production fields and interact, which results in significant yield losses. In co-infected plants, SDS symptoms appear earlier and are more severe than in plants infected by FV alone. However, the mechanism of the SCN-FV interaction remains unclear. Fusarium virguliforme is able to infect soybean plants of different growth stages, but plants infected at younger stages develop more severe foliar SDS symptoms than plants infected later. The effect of SCN infection on age-related resistance to SDS is unknown.

A set of greenhouse experiments were conducted to investigate: 1) how the presence of SCN in soybean roots affects the FV infection process and SDS severity, and characterize the effect of water availability on the SCN-FV interaction in soybean roots, and 2) investigate if age-related resistance of older soybean plants to SDS is affected by SCN infection.

Foliar SDS severity was always greater in co-infected plants than in plants infected by FV alone in all experiments. The amount of available soil water affected foliar SDS severity. Plants grown under normal watering showed more foliar SDS than plants in reduced watering treatments. Foliar severity, expressed as the area under the disease progress curve (AUDPC), was greater in the SCN-susceptible cultivar (AG2403) than in the SCN-resistant cultivar (MACO24757).

No evidence was found that root rot severity was consistently affected by the SCN-FV interaction, but root rot decreased as the plant age at inoculation with FV increased. Root weight was reduced by infection with FV, but was not significantly affected by SCN or watering regimes. Root weight increased as the plant age at infection with FV increased. Cultivar AG2403 had greater root weight than cultivar MACO24757.

The number of SCN females was greater under reduced watering regime. SCN syncytia were found predominantly in the stele in the absence of FV, and in the cortex in the presence of FV. Mycelia of FV were found more frequently in the cortex when SCN was present, and more frequently in the stele under reduced watering.

Microscopic observations showed the majority of root tips were colonized by FV. These findings suggest the importance of lateral roots for FV colonization of soybean roots.