Examining the Sustainability of Copper Use for Disease Management and Horticultural Benefit in Tart Cherry Systems

Examining the Sustainability of Copper Use for Disease Management and Horticultural Benefit in Tart Cherry Systems

Summary

Foliar copper fungicide spray programs were shown to provide excellent control of cherry leaf spot, the most important fungal disease limiting tart cherry production. These results were incorporated into Extension programming for tart cherry growers in Michigan. Soil copper levels in Michigan orchards seemed to stabilize after initial increases following transitions to increased copper use as fungicides. Copper-resistant bacteria were isolated from soils, shown to accumulate copper in laboratory studies and to persist in soil following inoculation in field experiments. Two bacterial strains were analyzed for copper-accumulating ability in soil remediation.

Objectives/Performance Targets

The following objectives were addressed in this proposal:
1. To incorporate foliar copper sprays into tart cherry management programs on commercial and organic farms in Michigan and to evaluate its efficacy on key diseases including CLS and to investigate copper’s potential to reduce soft fruit at harvest.
2. To assess the potential of copper-hyperaccumulating plants in removing copper from agricultural soils in greenhouse experiments and at on-farm sites.
3. To educate tart cherry growers about the utility of copper in their management programs and about phytoremediation.
4. To increase the adoption of these practices in the long-term in an effort to promote agricultural sustainability.

Accomplishments/Milestones

Objective 1: The experiments utilizing foliar copper sprays for fungal disease control of tart cherry were completed in 2008, and for all years of the project, copper provided consistent and excellent efficacy against CLS. We did not observe phytotoxicity from copper use in any year, even with multiple applications per season. Additionally, for three years, we evaluated the effects of copper to minimize soft fruit, a problem that can result in losses to growers and processors conservatively estimated at $6.3M per year (J. Nugent, personal communication). To alleviate the frequency of soft fruit, we examined copper to reduce the incidence of this problem. In 2006 and 2008, we found that one application of copper resulted in orchards with slightly less soft fruit (3.72%) than orchards that received no copper (5.5%). No copper was applied in these orchards in 2007 due to high temperatures. At this time, we cannot conclude if minimal applications of copper will reduce the amount of soft fruit; however, increasing the number of sprays per season may result in firmer fruit.

Objective 2: We initiated a laboratory trial to determine the value of certain bacteria as copper accumulators from orchard soil in 2008 and field studies are continuing this season (2009). From an initial 120 bacterial isolates, 10 highly copper-resistant isolates were recovered and tested for long-term persistence in soil and overall sustainability in orchard systems. Marked strains with spontaneous rifampicin resistance were used in order to be able to track each isolate. Suspensions of the antibiotic resistant isolates were inoculated into sterile soil into containers in the laboratory, and soil samples were taken immediately following inoculation and after 2, 7, and 14 days. Dilution platings were made on rifampicin-containing media to measure the bacteria concentrations in the soil over time. Further sampling was done after 3 or 4 weeks for the most promising isolates. From these experiments, two isolates (GT8.1 and GT10.1) were selected to be tested in the field this summer. Isolates were placed into organic orchards on 15 June and 18 June. 2009

Many copper-resistant bacteria turn blue when grown on copper-amended growth medium (also blue in color) in petri dishes in the lab. When copper-resistant bacteria absorb copper from the medium, the bacteria become progressively richer in color as the medium loses its blue hue. Based on these color changes and evaluations in the laboratory, we hypothesized that copper-resistant bacteria would bind copper present at elevated levels in orchard soils, similar to their behavior in the petri dish. Although the copper would not be ‘removed’ from soils using this strategy, the heavy metal will be bound to the bacteria and thus unavailable for uptake by cherry roots. Hence, if copper was to increase in soil with repeated use, roots would not be exposed to damaging levels of copper exposure.

The current field experiment is being conducted in two tart cherry blocks in an organically managed block in Northport, MI and another conventional plot at the NWMHRS. Eight 12.5 cm2 plots at each location were inoculated with the bacteria, 4 plots per isolate, by pouring a concentrated cell suspension over the soil surface. Three plots of each isolate at each location were weeded prior to inoculation and will be maintained free of plant material, the fourth plot will remain under the weed management of the particular block. Soil sampling and dilution plating to determine bacteria concentration is done immediately following inoculation and 7 and 14 days post-inoculation. If the inoculated bacteria survive in the plots, bacteria will continue to be measured 1, 2, and 3 months post-inoculation. Pre-inoculation soil samples were used to determine levels of native bacteria in the orchards and will also be submitted for copper analysis. At the conclusion of the field trial, soil from each test plot will be submitted for copper analysis to determine if the inoculated bacteria affected copper levels. A sample of uninoculated soil from each orchard will be submitted for copper analysis as a control.

We also monitored soil copper levels in orchards that we had sampled previously in 2006 and 2007. The results illustrated that copper levels had actually decreased in two of the orchards (A and B) even though multiple copper sprays were applied in 2008. Since copper has been used for a period of years in these orchards now, we wonder if the bacterial flora in the soils of these orchards has been modified to select for increased copper resistance, and if these resistant native bacteria are already functioning to reduce the bioavailable copper load in these soils. This is an exciting prospect for the long-term sustainability of copper use for disease management programs in tart cherry.

Objectives 3 and 4: A total of 17 Extensions presentations were given throughout Michigan by PI’s Sundin and Rothwell between 2008 and 2009 to educate growers about the use of copper to control cherry leaf spot in tart cherry orchards. In addition, two newsletter articles were written in the MSU IPM Fruit CAT Alert, an online and written publication which, in an average week, reaches over 800 people.

Presentations:
1. Fungicide Spray Programs for Tart Cherry Growers, Spring IPM Kick-off, NW Michigan, Hort. Research Station, Traverse City, MI, 4-9-08.
2. Disease control update for 2008. Spring Tree Fruit meeting, Grand Rapids, MI, 4-17-08.
3. Cherry IPM update. Leelanau County grower meeting, 5-14-08.
4. Cherry IPM update. Old Mission Peninsula grower meeting, 5-14-08.
5. Cherry IPM update. Benzie-Manistee Counties grower meeting, 5-27-08.
6. Tart cherry disease trial results, 2008. Northwest Michigan Horticultural Research Station Open House, 8-21-08.
7. Continuing to Investigate Copper Use for Cherry Leaf Spot Control,
NWMHRS Annual Open House, 8-21-08.
8. Disease control highlights, 2008. Tree Fruit Organic Field Day, Flint, MI, 11-11-08.
9. Stone fruit disease control. Great Lakes Fruit and Vegetable EXPO, Grand Rapids, MI, 12-08-08.
10. Cherry leaf spot: 2008 trial results. Northwest Michigan Orchard and Vineyard show, Traverse City, MI, 1-21-09.
11. Tree fruit disease update for 2009. Southwest Michigan Hort Days, Benton Harbor, MI, 2-04-09.
12. Disease control update, 2009. West Central Spring Tree Fruit meeting, Hart, MI, 3-16-09.
13. Cherry Leaf Spot: 2008 Field Trial Results, Northwest Michigan Orchard and Vineyard Show, Traverse City, MI, 1-21-09.
14. Cherry IPM update. Leelanau County grower meeting, 5-20-09.
15. Cherry IPM update. Old Mission Peninsula grower meeting, 5-20-09.
16. Cherry IPM update. Antrim County grower meeting, 6-9-09.
17. Cherry IPM update. Benzie-Manistee Counties grower meeting, 6-9-09.

Newsletter articles:

1. Sundin, G.W., and N.L. Rothwell. 2008. Managing cherry leaf spot in poor spraying weather. MSU IPM CAT Alert 6-24-08 issue.
2. Sundin, G.W., N. Rothwell, and E. Lizotte. 2009. Fungicide cover spray considerations for cherry leaf spot control. MSU IPM Fruit CAT Alert, 6-09-09 issue.

Impacts and Contributions/Outcomes

Copper use in northwest Michigan for cherry leaf spot (CLS) control has increased by 10% in a three-year period based on increased copper sales in the region (M. Evans, personal communication). In 2009, we documented more calls, orchard visits, and questions at IPM Updates regarding copper use than in all years of the project (>45). We hypothesize that this interest was driven by cool growing conditions as well as multiple years of communication by MSU Extension personnel regarding the successful incorporation of copper into fungicide spray programs. Additionally, the information has spread to Wisconsin and Ontario tart cherry growers, who are now using copper in their orchards. The increase in copper use may also be influenced by our studies that show minimal phytotoxicity, even under 90 degree F conditions with no lime as a safener. Additionally newer formulations of copper have built-in safening components, and growers have a higher comfort level with these products.

Despite the change of direction of the phytoremediation portion of the project, we feel the copper-accumulating bacteria show tremendous potential to bind copper in the soil. Our 2009 preliminary data in the field show that bacteria survive and reproduce in soil amended with copper. We hope to continue these field trials and expand this concept in future studies. The use of copper-resistant bacteria for bioremediation, although at its initial stages of field testing, offers promise for increasing the utility of a critical fungicide and for orchard sustainability through maintaining soil health. If the bioremediating bacteria do indeed bind copper and minimize cherry roots’ exposure to elevated copper levels in the soil, this concept will provide a sustainable method of managing cherry diseases. This concept can also be applied to other crops.

Collaborators: