Improving Peach Disease Management

The research team has published a third research report at the sponsor’s website: https://projects.sare.org/project-reports/ls22-366. Here are some highlights:

Objective 1: Develop sustainable spray programs with biopesticides to reduce bacterial diseases.

Bacterial spot control: In the field trial of the 2023 growing season, a biopesticide treatment and copper significantly reduced the bacterial spot severity on peach leaves but copper caused significantly more damage (phytotoxicity) on leaves than bacterial spot. The best performing biopesticide is being tested in three commercial peach orchards in the 2024 growing season. The spring canker data from the 2023 dormant season trials are being collected.

Bacterial canker control: The canker data from the 2023 dormant season trials are being collected.

Objective 2: Assess sustainable soil management practices and their integration with spray programs on reducing bacterial diseases in research orchards.

Similar to the results obtained in 2022, the annual topical application of mulch with chicken manure in orchard #2 (known to have yearly natural bacterial spot infection) resulted in lower bacterial spot incidence and severity in leaves when compared with the trees that received mulch only and control (bare soil). Although there was no statistical difference in the bacterial spot incidence and severity on fruit between the three treatments, we observed numerically lower bacterial spot incidence on fruit of the trees that received mulch with chicken manure compared to the fruit of the trees that received mulch only and control treatments.

There was low pressure of bacterial spot in the other two orchards despite the higher strength/doses of Xap inoculum compared to the strength of Xap inoculum applied in 2022. There was no statistical difference in the bacterial spot incidence and severity in leaves and fruit between treatments. However, numerically lower bacterial spot incidence and severity were observed in leaves and fruit of the trees treated with compost incorporation when compared to the control, suggesting that the pre-plant incorporation of compost could improve the overall quality of the marketable fruits and orchards productivity.

Objective 3: Evaluate the independent spray programs and soil practices and their integration on tree health/performance, soil health, and management of other diseases and pests

Regarding the orchards’ horticultural performance, young trees receiving pre-plant incorporation of compost had a larger trunk cross-sectional area (TCSA) compared to the control treatment. A similar response trend was also observed in the matured tree orchards treated with annual topical application of mulch with and without chicken manure when compared to the trees treated with growers’ standard fertilizers. However, there was no significant difference between matured trees under topical application of mulch with or without chicken manure and young trees under pre-plant compost incorporation. This result suggested that the observed increase in TCSA either in matured trees or on young trees can be correlated to the observed numerical increase in the yield and productivity of the trees treated with compost and trees treated with or without chicken manure compared to the control treatment. Mature trees receiving mulch and/or mulch with chicken manure or young trees receiving compost produced similar yields and had a similar nutrient status while reducing or eliminating synthetic fertilizer applications. There have not been differences in other physiological parameters such as gas exchange. No other diseases and pests were observed in the experimental orchards.

Soil amendment did not change soil pH, but increased soil electrical conductivities in the mulch trials only. Similar amendment impacts were observed for soil NH4+ concentrations, enzyme activities, and active carbon at depths of 0-15 and 15-30 cm. Higher organic nitrogen mineralization potentials were found in orchard #2 when the soils were amended with mulch. However, no impacts on microbial respiration were observed. The results indicated that 1) soil amendments can improve soil nutrient availability and organic carbon content, which however depends on the sources of the amendments, 2) such improvement can reach soils at 30 cm, and 3) increased amendment rates may not produce proportional responses. A saturation effect may exist.    

Guido Schnabel, Hehe Wang, Juan Carlos Melgar, Ayodeji Idowu, Junaed Ahmed, Rongzhong Ye, and Michael Vassalos

Clemson University

I think we can all agree that peaches produced in commercial peach orchards in South Carolina are special. They are sold locally at farm stands and many retail stores along the East Coast. You may not know that 10-40 percent of all harvested fruit does not meet retail standards and is often discarded. This includes fruit with spots caused by the bacterium Xanthomonas arboricola pv. pruni, commonly known as the causal agent of bacterial spot or bacteriosis (Fig. 1).

Hehe Wang (plant bacteriologist) located at the EDISTO Research and Education Center in Blackville, SC is interested in suppressing bacterial spot in commercial peach orchards. Bacterial spot management has relied on season-long chemical spray programs with copper and oxytetracycline (an antibiotic). However, oxytetracycline resistance and copper tolerance have recently been found in the bacterial pathogen populations in South Carolina peach orchards. Wang has been working with Guido Schnabel (plant pathologist), Juan Carlos Melgar (pomologist), Rongzhong Ye (soil scientist), and Michael Vassalos (agricultural economist) to assess biological pesticide sprays as alternative and more sustainable options for the spray programs and specific soil treatments for their ability to suppress bacterial spot and increase the percentage of fruit that meet industry standards. They hypothesize that healthier soils lead to healthier trees and less disease.  Therefore, the goal of this project, funded by the USDA Southern SARE program, was to strengthen the sustainability of both conventional and organic peach production systems in the Southeast. This work is being done in collaboration with peach producers.

Three years into the study, the team can share some interesting findings. Graduate student Junaed and his advisor Wang investigated biological spray programs on spring canker management. Spring cankers are the most important inoculum sources for leaf and fruitlet infections. Applications of biological Bacillus subtilis (Theia, Certis Biologicals) during fall leaf drop consistently reduced spring canker incidence and severity up to 70% based on two years of study. Copper (Kocide 3000) and other biological products also reduced spring canker, but the efficacy was lower than Theia.

Theia applications during the growing season also reduced bacterial spot, but the effect was not as prominent compared to the dormant application mentioned above. Its application every 7-10 days reduced bacterial spot without causing phytotoxicity on leaves. Among all chemical and biological treatments, copper was still the most effective in reducing bacterial spot. Still, its continuous application caused phytotoxicity, which could impact fruit yield and quality for late-season cultivars. Perhaps Theia could be an alternative option to copper and be integrated into future spray programs to reduce phytotoxicity and improve disease management.

In addition to spray applications, graduate student Idowu and his advisor Melgar looked at soil management practices to make trees healthier and less susceptible to bacterial spot. Incorporating soil amendments such as composted food waste or mulch before and after planting reduced bacterial spot incidence and severity. Soil amendments reduced the conventional fertilizer needed between 50% and 100%. Depending on the timing and amendment volume applied, the trees may not need to be fertilized for several years. Soil amendments reduced pathogenic nematodes that can harm peach roots, and increased soil water holding capacity, nutrient cycling and availability, organic carbon, microbial biomass, and microbial activities, while maintaining or improving tree water and nutrient uptake, tree yield, fruit quality and whole tree physiological functions. The extent of these improvements depends mainly on the sources of the amendments, soil types, and management history. The study also indicated that more is not always better. Adding too much organic material can saturate the soil without benefitting the tree.  

The project team has generated two videos describing some of the newest outcomes.

-soil amendments for tree health video

-soil amendments for peach disease reduction video

The new project report is published and available online https://projects.sare.org/project-reports/ls22-366/