Soilborne fungal pathogen V. dahliae infects a wide range of crop plants including cotton and considered as a major disease issue for many Australian cotton growing regions causing significant yield losses. This soilborne fungus shown to persist in the soil as microsclerotia for more than a decade. Recent evidence shows that V. dahliae can infect crops that are not generally considered susceptible but can buildup of the inoculum even when no apparent disease symptoms are seen. Also, decomposition of crop residues which releases microsclerotia and their degradation is considered as one of the pathways that reduces inoculum level during non-crop period. Hence, management of the disease requires an integrated approach that aims to reduce the inoculum load from the previously infected crop residues and that persisting in bulk soil.
In this study, we investigated the changes in V. dahliae inoculum levels associated with plant residues from cotton and other rotations crops using litter bag based residue experiments in field plots and glasshouse trials. Glasshouse grown plant residues of wheat, oats, lablab and barley inoculated with the D strain of V. dahliae and field infected cotton residues were used. At regular intervals, residues samples were destructively sampled and analysed for dry weights, V. dahliae levels (qPCR), composition of bacterial and fungal communities (group specific amplicon sequencing) and C and N concentrations.
Verticillium dahliae inoculum levels were significantly different with different crop and residue types including cotton and other rotation crops. Results from the glasshouse experiment indicated that a large reduction in the V. dahliae inoculum levels occurred during the first 4-8 weeks of the incubation study with all the residue types. Whereas, in the field experiment there was no significant change in the V. dahliae levels during the first 8 weeks of incubation and inoculum levels only responded following rainfall/irrigation events. The diversity of fungal communities associated with crop residues was quicker in the glasshouse experiment coinciding with the decrease in the V. dahliae inoculum level whereas in the field residues the increase was only observed following the rainfall/irrigation events. The diversity of bacterial communities associated with residues used in glasshouse experiment (T0 samples) showed significant differences between residue types. Microbial communities associated with and potentially involved in the decomposition of crop residues varied with crop and residue types and results suggest that higher soil fungal diversity result in faster decline in the V. dahliae inoculum level. Results suggest that the decrease in V. dahliae inoculum level during non-crop period seems to be largely influenced by soil moisture level, chemical composition (e.g., C:N ratio) and rate of residue decomposition and diversity or soil microbial communities.
Overall, the observation of greater than previously assumed for the V. dahliae abundance with crops that are not considered as impacted crops e.g., cereal crops, lablab etc, suggests that it is important to manage the pathogen inoculum level during the non-cotton periods of rotation.