Location of Pathogen Inoculum in the Potting Substrate Influences Damage by Globisporangium ultimum, Fusarium culmorum and Rhizoctonia solani and Effectiveness of Control Agents in Maize Seedlings

The aim of this study was to determine the impact of the location of the pathogen inoculum
on damage caused by Globisporangium (syn. Pythium) ultimum, Fusarium culmorum and Rhizoctonia
solani in pot tests with maize. For this purpose, pathogen inoculum was added to potting substrate,
and the resulting mix was used to fill the whole pot volume, the upper half, or the lower half of
pots. The remaining volume was filled with non-inoculated substrate. In a second experimental
approach, maize seeds were germinated in non-inoculated potting substrate and the seedlings were
transferred to inoculated substrate. The seeds were untreated, treated with the chemical thiram,
or treated with a bacterial or a fungal biocontrol agent. With each of the pathogens, the damage
to the developing maize seedlings was the strongest when the seeds germinated in the inoculated
potting substrate. When only the roots were in contact with the inoculum, there was limited damage
by R. solani and F. culmorum, and no damage by G. ultimum. This implies that in experiments with
artificial inoculation, the seeds should always be in immediate contact with the inoculum if a strong
pathogenic effect is desired. Conversely, seed treatments must, in the first place, be able to protect
the spermosphere, while the requirement to protect the roots at a distance from the seed seems to
depend on the pathogen.