Effect of mulching, compost and rootstock genotype on white root rot of apple

White root rot of apple, caused by Rosellinia necatrix, causes tree death and is becoming an increasing concern in the South African industry. The objectives of this project were to investigate whether the Geneva rootstocks are more tolerant to the pathogen than the industry standard rootstocks under glasshouse conditions, as well as whether mulch and compost applications could potentially promote or reduce disease development. Limited to no information is available on these subjects.

Thirteen clonal rootstock seedlings (M.9, G.222, G.41, G.202, G.757, M.7, G.210, G.969, M.793, MM.109, G.228, G.778 and G.890) were artificially inoculated with R. necatrix at two inoculum dosages under glasshouse conditions. Area under disease progress curve and days until plant death data were used to assess rootstock susceptibility. The effect of mulch and compost-plus-mulch applications on WRR severity was assessed using a glasshouse apple seedling bioassay conducted with soil from three apple orchards that received long-term applications. Laboratory bioassays were evaluated for pathogen detection.

All the rootstocks were infected by the pathogen and died at trial termination. The Geneva® dwarfing rootstocks G.202 and G.757 were the most tolerant among the evaluated rootstocks.

Long-term applied organic amendments had variable effects on WRR severity based on the soil to which it was applied. Organic amendments either significantly suppressed WRR (compost-plus-mulch), significantly increased WRR severity (compost-plus-mulch and mulch only treatments) or did not affect WRR severity (compost-plus-mulch and mulch only). Soil organic matter (SOM) was not indicative of disease suppression.

A reliable bioassay could not be developed to detect the pathogen in soil or in mulches.

Due to the destructive nature of WRR, the marginal WRR tolerance identified in the two dwarfing Geneva® rootstocks will likely be insufficient to manage the disease under orchard conditions.

Organic amendments affected WRR severity in an orchard soil-dependent manner, explaining why reports vary regarding the effect of organic amendments and SOM on WRR severity.

New breeding efforts should be initiated to identify WRR resistant rootstocks since this is the only feasible solution for curbing losses caused by the disease.

Due to the positive effect of compost and mulch applications in apple production systems, the use of these sustainable management practices should continue. This is because there was no no consistent trend that WRR will be increased by these practices.