Apple replant disease is a complex soil syndrome that occurs when the same fields are repeatedly utilized for apple orchard cultivation. It can be caused by various pathogens, and Fusarium solani is the main pathogen. Fusarium solani disrupts the structure and function of the orchard soil ecosystem and inhibits the growth and development of apple trees, significantly impacting the quality and yield of apples. In this study, we conducted a transcriptome comparison between uninoculated apple saplings and those inoculated with F. solani. The differentially expressed genes were mainly enriched in processes such as response to symbiotic fungus. Plant defensins are antimicrobial peptides, but their roles during F. solani infection remain unclear. We performed a genome-wide identification of apple defensin genes and identified 25 genes with the conserved motif of eight cysteine residues. In wild- type apple rootstock inoculated with F. solani, the root surface cells experienced severe damage, and showed significant differences in the total root length, total root projection area, root tips, root forks, and total root surface area compared to the control group. qRT-PCR analysis revealed that MdDEF3 and MdDEF25 were triggered in response to F. solani infection in apples. Subcellular localization showed specific expression of the MdDEF3-YFP and MdDEF25-YFP proteins on the cell membrane. Overexpressing the MdDEF25-YFP fusion gene enhanced resistance against F. solani in apple, providing a new strategy for the future prevention and biological control of apple replant disease.

The root-lesion nematode, Pratylenchus penetrans, is a ubiquitous parasite of roots of temperate fruit trees. It affects early growth of trees replanted into former orchard sites where populations have built up and may contribute to decline complexes of older trees. Most British Columbia, Canada, apple acreage is planted with M.9 rootstock, but growers are increasingly considering Geneva-series rootstocks such as G.41 and G.935. Among these rootstocks, responses to P. penetrans, specifically, are poorly known. To compare the resistance and tolerance to P. penetrans of G.41, G.935, and M.9 rootstocks ('Ambrosia' scion), a field microplot experiment was established in spring of 2020 at the Summerland Research and Development Centre. The experimental design was a two by three factorial combination of: P. penetrans inoculation (+/-) and rootstock (G.41, G.935, and M.9), with 20 replicate microplots of each of the six treatment combinations arranged in a randomized complete block design. The P. penetrans inoculum was 5,400 nematodes per microplot (54 P. penetrans liter-1 soil), which is below commonly accepted damage thresholds. Though P. penetrans population densities were lower for the G.41 rootstock by the end of the 2021 growing season, the effects of P. penetrans were similar among rootstocks. In the establishment year (2020), P. penetrans caused significant reductions in aboveground growth. In 2021, shoot growth and root weight were reduced by P. penetrans. The nematode also reduced rates of leaf gas exchange and stem water potential. These data suggest that while G.41 and G.935 may have other horticultural benefits over M.9, they are equally susceptible to P. penetrans at the early stages of tree growth.