Modern Planting Research: A Microbial Approach to Alleviating Peach Soil Replantation Syndrome
1. Brief introduction
Replanting syndrome, often referred to as replanting disease, is a global soil-related challenge induced in newly planted trees at old orchard sites, where repeated monocultures lead to stunted tree growth and reduced yields.
The etiology of RS is not fully understood, but the decline in orchard productivity due to RS is caused by a microbial complex of plant pathogens/competitors. Replanting symptoms are non-specific and affect multiple genera of fruit trees, often associated with pathogenic generalists such as root lesion nematodes and Fusarium spp.
Biotic factors such as microorganisms contribute to RS, which is supported by studies in which peach tree biomass is higher in high-pressure soils than in non-high-pressure soils.
Cover crops are a sustainable soil strategy where crops are grown to regenerate soil health, not harvested for economic value. Cover crops protect the soil, reduce water runoff, and increase soil organic matter content.
Since cover crops affect the chemical and physical properties of the soil, they also alter the biological properties of the rhizosphere, i.e., the narrow area of the soil where root-microbial associations occur.
Root-microbial associations within the rhizosphere may improve soil fertility and degrade toxic chemicals. In addition, beneficial associations in the rhizosphere can affect pathogen populations.
In the current study, alfalfa, fescue, corn, and tomatoes were tested as cover crops to alleviate RS symptoms in peaches. In addition, autoclaving is used to determine whether the benefits of soil disinfection can complement the benefits of cover crops.
Previous studies have focused on identifying plant-pathogenic instigators of RS recurrence, such as fungi, oomycetes, and nematodes. The scope of this study is to highlight sustainable agricultural techniques that promote peach health and to identify potential plant growth that promotes rhizobia for future inoculation studies.
These findings show some of the drawbacks of soil disinfection, using cover crops as a favorable soil regeneration strategy. In addition, the correlation between microbial taxa and RS mitigation in peaches was determined.
2. Materials and methods
RS soil for the experiment was obtained from a peach orchard study block in 2007 at the experimental orchard of the Colorado State University's Western Colorado Research Center in Orchard de Mesa, Colorado.
This peach orchard was built using Prunus persica "Cresthaven" scion with grafted peach "Lovell" rootstocks. The soil in this area is described as Billings silty clay soil. RS soil is transported to Colorado State University's horticulture center.
In the horticultural center, the soil is homogenized by means of a metal sieve. Samples of replanted soil were collected before and after autoclaving and stored at -80 °C for use as a control for soil bacterial group analysis.
The soil is placed in an autoclave bag and then set up at 121 °C for a 40-minute liquid cycle in a STERIS brand steam autoclave, which has been run three times. Between cycles, the bag carrying the soil is shaken to redistribute the soil and then returned to the autoclave for the second and third time.
Then, 4 L black plastic pots lined with Vigoro Weed Control Fabric Medium were placed on Vigoro 15.24 cm plastic plant dishes and filled with 2.1 kg of untreated RS soil or autoclaved RS soil.
3. Effects of soil disinfection on cover crop biomass
The Kruskal-Wallis test of above-ground biomass cover crops showed significant soil treatment, χ2 = 16.398. The biomass of all cover crops grown in high-pressure soils was higher than that of cover crops grown in non-high-pressure soils.
Corn, fescue and tomato biomass differed significantly between their respective autoclaved and non-autoclaved biomass. However, there was no significant difference in alfalfa crop biomass between autoclaving and non-autoclaving treatments.
Alfalfa biomass in non-high-pressure soils decreased by 34.6% compared to alfalfa grown in high-pressure soils. Corn grown in autoclaved soil has the highest biomass of all autoclaved and non-autoclaved cover crop treatments.
In this study, the biomass of tomato plants grown in untreated soil decreased by 85.8% compared to tomato plants grown in high-pressure soil. This supports the trend of soil disinfection to improve plant health.
4. Effects of cover crops and biomass incorporation on soil microbiota
Bulk soil bacterial groups that had been grown for 12 weeks in cover crops were analyzed. The perMANOVA test showed that crop type, autoclaved and non-autoclavable, and the interaction between the two factors were significant, with the CAP axis explaining a total of 37.9% variance for all samples.
The separation between high-pressure and non-high-pressure soils is evident along axis 1, explaining the 29.9% difference. For cover crops, autoclaving has a lower dispersion than non-autoclaved soil.
In high-pressure soil treatments, cover crop bulk soil microbiota overlap, while cover crop treatments play a greater role in shaping the microbiome in non-high-pressure soils.
Maize grown in autoclaved soil had the highest biomass after 12 weeks of growth, but its microbiome was not significantly separated from other cover crops. In bulk soils of cover crops, non-cover crop controls overlap with crop treatments in their respective soil treatments.
