Alleviation of Nematode-Mediated Apple Replant Disease by Pre-Cultivation of Tagetes

Exogenous melatonin alleviates apple replant disease by regulating rhizosphere soil microbial community structure and nitrogen metabolism

Apple replant disease (ARD) is a common soil-borne disease afflicting apple plants. Melatonin is a broad-spectrum oxygen scavenger that plays a key role in alleviating stress-induced damage in plants. In this study, we aimed to determine whether adding melatonin to replant soil can promote plant growth by improving the rhizosphere soil environment and nitrogen metabolism. In replant soil, chlorophyll synthesis was blocked, reactive oxygen species (ROS) accumulated in large quantities, and membrane lipid peroxidation was aggravated; this eventually resulted in slow plant growth. However, the application of 200 μM exogenous melatonin enhanced the tolerance of plants to ARD by up-regulating the expression of antioxidant enzyme-related genes and increasing ROS scavenging enzyme activity. Exogenous melatonin also increased the absorption and utilization of ¹⁵N by increasing the expression of nitrogen absorption genes and the activity of nitrogen metabolism enzymes. Exogenous melatonin enhanced the soil microbial environment by promoting soil enzyme activity and bacterial richness and decreasing the abundance of several harmful fungi in rhizosphere soil. Mantel test results showed that soil properties (except for AP) and growth indexes were positively correlated with the rate of ¹⁵N absorption and utilization. Spearman correlation analysis showed that the above factors were closely related to the richness and diversity of bacteria and fungi, indicating that the composition of microbial communities might play a key role in mediating change in the soil environment and thus affect nutrient absorption and growth. These findings provide new insights into how melatonin enhances ARD tolerance.

Toward a holistic view of orchard ecosystem dynamics: A comprehensive review of the multiple factors governing development or suppression of apple replant disease

Replant diseases are a common occurrence in perennial cropping systems. In apple, progress toward the development of a universally effective disease management strategy, beyond the use of broad-spectrum soil fumigants, is impeded by inconsistencies in defining replant disease etiology. A preponderance of evidence attributes apple replant disease to plant-induced changes in the soil microbiome including the proliferation of soilborne plant pathogens. Findings from alternative studies suggest that the contribution of abiotic factors, such as the accumulation of phenolic detritus from previous orchard plantings, may play a part as well. Engineering of the resident soil microbiome using resource-based strategies is demonstrating potential to limit activity of replant pathogens and improve productivity in newly established orchards. An understanding of factors promoting the assembly of a disease-suppressive soil microbiome along with consideration of host factors that confer disease tolerance or resistance is imperative to the developing a more holistic view of orchard ecosystem dynamics. Here, we review the literature concerning the transition of orchard soil from a healthy state to a replant disease-conducive state. Included in the scope of this review are studies on the influence of soil type and geography on the apple replant pathogen complex. Furthermore, several tolerance and innate resistance mechanisms that have been described in apple to date, including the role of root chemistry/exudates are discussed. Finally, the interplay between apple rootstock genotype and key resource-based strategies which have been shown to “reshape” the plant holobiont in favor of a more prophylactic or disease-suppressive state is highlighted.

Nematode-Microbe Complexes in Soils Replanted with Apple

Apple replant disease is a severe problem in orchards and tree nurseries. Evidence for the involvement of a nematode–microbe disease complex was reported. To search for this complex, plots with a history of apple replanting, and control plots cultivated for the first time with apple were sampled in two fields in two years. Shoot weight drastically decreased with each replanting. Amplicon sequencing of the nematode community and co-extracted fungal and bacterial communities revealed significant differences between replanted and control plots. Free-living nematodes of the genera Aphelenchus and Cephalenchus and an unidentified Dorylaimida were associated with replanted plots, as indicated by linear discriminant analysis effect size. Among the co-extracted fungi and bacteria, Mortierella and Methylotenera were most indicative of replanting. Some genera, mostly Rhabditis, Streptomyces and a fungus belonging to the Chaetomiaceae indicated healthy control plots. Isolating and investigating the putative disease complexes will help to understand and alleviate stress-induced root damage of apple in replanted soil.