Biological Control and Re-curing of Sweet Potato Roots as Alternatives for Managing Rhizopus Soft Rot

Progress report for GS19-200

Project Type: Graduate Student
Funds awarded in 2019: $16,120.00
Projected End Date: 02/28/2022
Grant Recipient: Louisiana State University
Region: Southern
State: Louisiana
Graduate Student:
Major Professor:
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Project Information

Summary:

Sweetpotato storage is limited by several postharvest diseases, of which, Rhizopus soft rot (RSR), caused by the wound-dependent fungus Rhizopus stolonifer is most devastating. Roots are normally cured following harvest to heal wounds, thereby minimizing the risk of RSR infection in storage. During packing, however, sweetpotato roots are treated with the fungicide dicloran (Botran ®). Increasing concerns about pesticide residue on the sweetpotato roots pose the need for alternative ways of managing RSR. Even though the potential of using biological control or re-curing roots has been demonstrated by a few authors, there is a need to research further to determine the optimal application and determine if combining these methods will improve the level of control. The objectives of this two-year project were, therefore, to evaluate the effectiveness of integrating improved resistance to RSR with the biological control product, P. syringae strain ESC-10 (Bio-Save 10LP), applied either as an overhead spray or as a dip to sweetpotato roots; and re-curing sweetpotato roots on the incidence of RSR in comparison to the standard dicloran. These practices were evaluated individually and in combination. Five predominant commercial cultivars Beauregard, Bayou Belle, Bellevue, Orleans, and Covington were used. At 120 and 145 days in year 1 or 130 and 150 days after harvest in year 2, sweetpotato roots were artificially wounded and inoculated, and evaluated for RSR incidence up to 14 days. The expected outcome is the development of an integrated pest management (IPM) strategy for managing RSR using more sustainable methods, to meet the needs of all markets.

Project Objectives:
  • To evaluate the effectiveness of the biological control product, Pseudomonas syringae strain ESC-10 (Bio-Save 10LP) on the roots of five sweetpotato cultivars with varying resistance at the recommended label rate and applied either as a spray or as a dip.
  • Assess the effects of re-curing roots for 24 hours after wounding, five sweetpotato cultivars with varying resistance, inflicted with one wound type (bruise) on the incidence of RSR.
  • Assess the combined effects of the biological control product Bio-Save 10LP, applied to sweetpotato roots of varying RSR resistance as a dip and spray, followed by re-curing 24 hours on the incidence of RSR.

Cooperators

Click linked name(s) to expand
  • Dr. Don Labonte (Researcher)
  • Dr. Arthur Villordon (Researcher)
  • Dr. Guy .B. Padgett (Researcher)
  • Dr. Niranjan Baisakh (Researcher)
  • Dr. Tara Smith (Educator)

Research

Materials and methods:

Year 1 (2019/2020)

Five cultivars with different levels of resistance to RSR were selected, based on predominance among sweetpotato growers and/or packers. These include Beauregard, Bayou Belle, Bellevue, Covington and Orleans. Sweetpotato slips of all 5 cultivars were transplanted into the field on May 29th, 2019 and grown according to standard practices at the Burden Center in Baton Rouge (Figure 1). The sweetpotato roots were harvested on September 12th, 2019 (Figure 2), after which they were subjected to curing conditions (85oF and >85% RH) for 5 days and then placed in storage (60oF and >85%RH). At 120 and 145 days after harvest, roots were sampled for inoculation and treatment application. This selection was based on a previous study that showed sweetpotato roots were most susceptible to RSR at 100-150 days after harvest (Holmes and Stange, 2002). At each sampling time, roots of each cultivar were inoculated and randomly assigned to the following treatments (Table 1).

Table 1. Treatment table

Treatment

Application rate

Dicloran (Botran ®) dip

1.2g/ml water

BioSave 10LP dip

150g/9gal water

BioSave 10LP spray

150g/9gal water

Recure 24hours

BioSave 10LP dip + Recure 24h

150g/9gal water

BioSave 10LP spray + Recure 24h

150g/9gal water

Non- treated control

The dicloran was already available in the lab, so was not purchased. The Bio-Save 10 LP was donated by Jet Harvest Solutions, Longwood, FL. The curing room utilized in the study was located at the Burden Center in Baton Rouge, where all experiments were also conducted. Due to limitations with the numbers of roots for all cultivars caused by end rots (Figure 3) and generally low yields, a few modifications were made to the methods to ensure there were at least enough for the two sampling times. Consequently, only one wound type (bruise) was used with no non-wounded control and at most 10 roots were used per treatment for each cultivar. Additionally, only one re-curing period (24 hours) was used, based on the most effective for managing RSR determined in Sweany et al (2020).

  • 120-day inoculations

These were performed between 9th January – 21st January 2020. Rhizopus stolonifer isolate 92-Rs-2 was grown on Potato Dextrose Agar (PDA) for 6 days prior to inoculations at 28oC to promote sporulation.  A day prior to inoculations and treatment application, a total of 1050 roots (10roots/cultivar * 7 treatments * 3 replications * 5 cultivars) were washed using tap water and air dried (Figure 4). The roots were left in the open in readiness for the next day, which was a mistake as rats fed on several roots which had to be replaced the next day. On the day of inoculation, a 6-day-old culture of R. stolonifer (Figure 5) was used to prepare a spore suspension for inoculations as described by Edmunds and Holmes (2009). Spores were dislodged from the surface of the culture plate by adding 0.01% Triton-X solution, and scraping the surface using a flame-sterilized scalpel. The resulting spore suspension was poured through four layers of sterile cheesecloth in a funnel. The final spore concentration was adjusted to 106 sporangiospores/ml based on hematocytometer counts. A total volume of 877 ml was prepared to inoculate 1050 sweetpotato roots. The sweetpotato roots were wounded with a rubber band impelled wooden dowel (which inflicts a bruise wound) (Figure 6). The roots were then inoculated by brushing the sporangiospore suspension over the wounded surface using a foam paintbrush. 

The sweetpotato roots were left to air dry before applying the treatments. While the spore suspension dried out, the mixtures of dicloran (Figure 7) and Bio-Save (Figure 8) were prepared, by mixing 18 ml in 15 L water and dissolving 67 g in 15 L water, respectively. When the spore suspension had dried out, roots were either placed directly in storage (non-treated controls), submerged for 30s in the dicloran (dicloran dip), submerged for 30s in Bio-Save (Bio-Save dip), placed immediately in the curing room for 24 hours (re-curing), submerged for 30s in Bio-Save and re-cured for 24 hours (Bio-Save dip + re-cure), sprayed with Bio-Save using a backpack sprayer (Bio-Save spray) or sprayed with Bio-Save then re-cured for 24 hours (Bio-Save spray + re-cure). For the spray treatments, roots were placed on baskets and a 2-gal capacity backpack sprayer was used, spraying twice over all the roots by turning the roots to ensure coverage of the entire root (Figure 9). All roots except for those that received the re-curing treatment were placed under storage conditions (60oF) immediately following treatment application. All roots that were re-cured were moved to storage conditions after 24 hours in the curing room.  All roots were then evaluated for RSR incidence 12 days after inoculation, by counting how many roots out of the total had soft rot. 

  • 145-day inoculations

These were performed between 3rd February – 14th February, 2020. This time, roots were washed with tap water three days prior to inoculations and treatment application, then air dried. Due to limited root availability caused by increasing end rots, the number of roots per cultivar between treatments and replication was reduced, with 10 roots for Covington, Bayou Belle and Orleans, 9 roots for Beauregard and 8 roots for Orleans. The root size was also more variable, since there were not many roots to choose from to obtain uniform roots. After washing, the roots were placed back in the storage room until inoculation day. A 6-day-old culture of R. stolonifer was used to prepare a spore suspension for inoculations as described above. The final spore concentration was adjusted to 106 sporangiospores/ml based on hematocytometer counts. A total volume of 1260 ml (more than needed) was prepared to inoculate 987 sweetpotato roots. Wounding, inoculation and application of treatments was performed as described above, except the Bio-Save spray treatment was modified to improve the coverage on roots. Four rounds of spray were applied, two on each side of the root and all the mix in the 2-gal backpack sprayer was used. All roots were incubated in storage conditions (60o F) and evaluated for RSR incidence at 11 days after inoculation, by counting how many roots out of the total had soft rot (Figure 10). 

  • Experimental design and statistical analysis

The roots were arranged on baskets in a completely randomized design with 3 replications. The total number of sweetpotato roots with RSR was recorded at 12 day and 11 days, for the 120- and 145-day sampling, respectively. The root counts were converted to percent, by dividing the roots with RSR by the total roots inoculated. The data were analyzed as full models for main effects treatment and cultivar using generalized linear mixed models (PROC GLIMMIX) due to the binary nature of the outcome (rot or no rot). Both main effects were highly significant at both 120 and 145 days after harvest. The mean percent rot was determined for each treatment and for each cultivar using PROC UNIVARIATE, and these were used to generate bar graphs to show differences among treatments. Mean separation among treatments was based on Fisher’s least significant differences (LSD) among the odds of RSR, at a critical level of P < 0.05. All analysis was performed in SAS 9.4 (SAS Institute, Cary, NC). 

Literature cited

Edmunds, B.A. and Holmes, G.J., 2009. Evaluation of alternative decay control products for control of postharvest Rhizopus soft rot of sweetpotatoes. Plant Health Progress, 10, p.26.

Holmes, G.J. and Stange, R.R., 2002. Influence of wound type and storage duration on susceptibility of sweetpotatoes to Rhizopus soft rot. Plant disease, 86, pp.345-348.

Sweany, R.R., Picha, D.H. and Clark, C.A., 2020. Hot‐water baths, biologicals and re‐curing effects on Rhizopus soft rot during sweetpotato packing. Plant Pathology, 69, pp.284-293.

 

Year 2 (2020/2021)

Five cultivars representing different levels of resistance to RSR were selected, based on predominance among sweetpotato growers and/or packers. These include Beauregard, Bayou Belle, Bellevue, Covington, and Orleans. Sweetpotato slips of all 5 cultivars were transplanted into the field and grown according to standard practices at the Sweet Potato Research Center in Chase, LA in 2020. The sweetpotato roots were harvested and subjected to recommended curing conditions (85oF and >85% RH) and then placed in storage (60oF and >85%RH). The roots were transported to the Burden Center in Baton Rouge on December 14, 2020 and placed in storage until ready for use. At 130 and 150 days after harvest, roots were taken out of storage for inoculation and treatment application. The selection times were based on a previous study that showed sweetpotato roots were most susceptible to RSR at 100-150 days after harvest (Holmes and Stange, 2002). At each sampling time, roots of each cultivar were inoculated with an R. stolonifer spore suspension and randomly assigned to the following treatments (Table 1).

Table 1. Treatment table

Treatment

Application rate

Dicloran (Botran ®) dip

1.2g/ml water

Bio-Save 10LP dip

150g/9gal water

Bio-Save 10LP spray

150g/9gal water

Recure 24hours

Bio-Save 10LP dip + Recure 24h

150g/9gal water

Bio-Save 10LP spray + Recure 24h

150g/9gal water

Non- treated control

The dicloran was already available in the lab, thus was not purchased. The Bio-Save 10 LP was once again donated by Jet Harvest Solutions, Longwood, FL. The curing room used for the recure treatment was located at the Burden Center in Baton Rouge, where all experiments were also conducted. For the first inoculation at 130 days, two wound types (scrape and bruise) were used, but for the second inoculation at 150 days only one wound type (bruise) was used due to reduced root numbers for all cultivars. Additionally, at both sampling times, no non-wounded control was used due to insufficient root numbers, and only one re-curing period (24 hours) was used, based on the most effective for managing RSR determined in Sweany et al (2020).

  • 130-day inoculations

These were performed between 22nd January – 5th February 2021. A Rhizopus stolonifer isolate 92-Rs-2 was grown on Potato Dextrose Agar (PDA) for 7 days in an incubator maintained at 28oC to promote sporulation, for inoculations.  A day prior to inoculations and treatment application, a total of 2100 roots (10 roots/cultivar * 7 treatments * 5 cultivars * 2 wound types * 3 replications) were washed using tap water and air-dried. Roots were placed on plastic racks, each of which contained 20 roots of a cultivar for a given replication within a treatment (10 with the scrape wound and 10 with the bruise wound). The washed roots were placed back in storage ready for use the following day. On the day of inoculation, 7-day-old cultures of R. stolonifer (Figure 1) were used to prepare a spore suspension (Figure 2) for inoculations as described by Edmunds and Holmes, 2009. Spores were dislodged from the surface of the culture plate by adding 0.01% Triton-X solution and scraping the surface using a flame-sterilized scalpel. The resulting spore suspension was poured through four layers of sterile cheesecloth in a funnel. The final spore concentration was adjusted to 106 sporangiospores/ml based on hemacytometer counts. A total volume of 751 ml was prepared to inoculate 2100 sweetpotato roots. The sweetpotato roots were wounded with a potato peeler or rubber band impelled wooden dowel (also known as the wackadero) (Figure 3), to inflict the scrape wound and bruise wound, respectively (Figure 4). The roots were then inoculated by brushing the spore suspension over the wounded area using a foam paintbrush (Figure 5). 

The sweetpotato roots were left to air dry before applying the treatments. While the spore suspension dried out, the mixtures of dicloran (Figure 6) and Bio-Save (Figure 7) were prepared, by mixing 18 ml in 15 L water and dissolving 67 g in 15 L water, respectively. When the spore suspension had dried out, roots were either placed directly in storage (non-treated controls), submerged for 30s in the dicloran (dicloran dip), submerged for 30s in Bio-Save (Bio-Save dip), placed immediately in the curing room for 24 hours (re-curing), submerged for 30s in Bio-Save and re-cured for 24 hours (Bio-Save dip and re-cure), sprayed with Bio-Save using a backpack sprayer (Bio-Save spray) or sprayed with Bio-Save then re-cured for 24 hours (Bio-Save spray and re-cure). For the spray treatments, roots were placed in baskets and a 2-gal capacity backpack sprayer was used (Figure 8), spraying over all the roots and turning the roots to ensure coverage of the entire root (Figure 9). All roots except for those that received the re-curing treatment (Figure 10) were placed under storage conditions (60oF) immediately following treatment application. It was however discovered that the curing conditions may not have been ideal because the heating was off resulting in lower temperatures than recommended, thus roots were cured an additional day with the right conditions. Consequently, all roots that were re-cured were moved to storage conditions after 48 hours in the curing room. All the roots in the storage room (Figure 11) were then evaluated for RSR incidence 14 days after inoculation, by counting how many roots out of the total inoculated had soft rot (Figure 12). 

  • 150-day inoculations

These were performed between 14th February – 26th February 2021. This time, roots were washed with tap water (Figure 13) one week prior to inoculations and treatment application, then air dried and kept on baskets under storage conditions until inoculation time. Due to limited root availability, only the bruise wound was used (Figure 14). Additionally, while 10 roots were used per cultivar for each replication within a given treatment for cultivars Beauregard, Covington and Bellevue, Bayou Belle had up to 7 roots in some cases and Orleans had up to 8 roots.  The root size was also more variable since there were not many roots to choose from to obtain uniform roots. 7-day-old cultures of R. stolonifer were used to prepare a spore suspension for inoculations as described above. The final spore concentration was adjusted to 106 sporangiospores/ml based on hematocytometer counts. A total volume of 704.5 ml was prepared to inoculate approximately 1050 sweetpotato roots. Wounding, inoculation, and application of treatments was performed as described above, except that only the rubber band impelled wooden dowel was used to wound the roots (bruise wound). Additionally, deeper containers were purchased for dip treatments (Figure 15). All roots were incubated under storage conditions (60o F) and evaluated for RSR incidence at 14 days after inoculation, by counting how many roots out of the total had soft rot.

  • Experimental design and statistical analysis

The roots were arranged in baskets in a completely randomized design with 3 replications. The total number of sweetpotato roots with RSR was recorded at 14 days, for both sampling times. The root counts were converted to percent, by dividing the roots with RSR by the total roots inoculated. The data were analyzed for each sampling time as full models for main effects treatment and cultivar using generalized linear mixed models (PROC GLIMMIX) due to the binary nature of the outcome (rot or no rot). While only the treatment main effect was significant at 130 days, both main effects (treatment and cultivar) were highly significant at 150 days after harvest. The mean percent rot was determined for each treatment and for each cultivar using PROC UNIVARIATE, and these were used to generate bar graphs to show differences among treatments. Mean separation among treatments was based on Fisher’s least significant differences (LSD) among the odds of RSR, at a critical level of P < 0.05. All analysis was performed in SAS 9.4 (SAS Institute, Cary, NC).

Literature cited

Edmunds, B.A. and Holmes, G.J., 2009. Evaluation of alternative decay control products for control of postharvest Rhizopus soft rot of sweetpotatoes. Plant Health Progress, 10, p.26.

Holmes, G.J. and Stange, R.R., 2002. Influence of wound type and storage duration on susceptibility of sweetpotatoes to Rhizopus soft rot. Plant disease, 86, pp.345-348.

Sweany, R.R., Picha, D.H. and Clark, C.A., 2020. Hot‐water baths, biologicals and re‐curing effects on Rhizopus soft rot during sweetpotato packing. Plant Pathology, 69, pp.284-293.

 

 

Research results and discussion:

Year 1 (2019/2020)

In order to show the effectiveness of each treatment among all cultivars, bar graphs were generated that demonstrated the response of all five cultivars, to a treatment at both 120 and 145 days after harvest. The results will be presented in the same order as the objectives and will end with a look at dicloran and the non-treated control.

  • Bio-Save 10LP ( syringae strain ESC-10) applied as a dip

At both 120 and 145 days after harvest, Bio-Save applied as a dip was quite effective for managing RSR. At 120 days, the highest percent rot (41.7%) was observed in the cultivar Bellevue, with no significant differences among the other cultivars (Figure 11). At 145 days, the overall incidence was much lower (<4.3%), with three of five cultivars having no rot hence no significant differences among all the cultivars. Applied as a dip, it was, therefore, more effective for all cultivars except for Bellevue at 120 days, and for all cultivars at 145 days. 

  • Bio-Save 10LP ( syringae strain ESC-10) applied as a spray

A much higher incidence was observed at 120 days after harvest (Figure 12), for all cultivars indicating the spray was not effective. This is most likely due to insufficient coverage of the sweetpotato roots with the Bio-Save during the spraying process. At 145 days, minor adjustments were made regarding the number of times roots were sprayed over, and so the RSR incidence was significantly lower (<3.3%) among all the cultivars. At both times, all cultivars responded the same, with no significant differences between them. Applied as a spray, the Bio-Save was more effective for all cultivars at 145 days, at which time it provided up to 100% control.

  • Re-curing for 24 hours

Re-curing sweetpotato roots had variable effects on the RSR incidence among the sweetpotato cultivars. At 120 days after harvest, it seemed most effective for Bayou Belle and Covington that had 16.7% and 20% rot, respectively. For Bellevue however, it only provided up to 37% control (Figure 13). Bellevue responded similarly at 145 days, with the greatest proportion of rot (75.3%), and at 145 days Beauregard had the least proportion (11%). The variability observed suggests that it may be worth evaluating different re-curing periods to determine the optimal. Since some cultivars had low incidence, suggests that it does have the potential for application.

  • Bio-Save dip combined with re-curing 24 hours

When roots were dipped in Bio-Save then re-cured, the RSR ranged from 10%-38% among all cultivars, with Bellevue having the highest and Covington the least at 120 days (Figure 14). This strategy seemed more effective at 145 days, when 3/5 of the cultivars had no rot, but Bellevue still had 21% of its roots rotting. 

  • Bio-Save dip spray combined with re-curing 24 hours

This was more effective at 145 days, than at 120 days probably due to improved coverage at 145 days. Since re-curing was also not as effective, coupled with the inadequate spray, all cultivars had between 20% – 50.3% rot, with the highest in Bellevue and the least in Covington (Figure 15). At 145 days, the improvement was probably related to the adjustments in the spray process hence more a function of the Bio-Save than the re-curing. All cultivars responded the same at 145 days, with not more than 4.3% rot.

  • Dicloran (the standard practice)

As expected, this provided at least 87% and 96% control at 120 days and 145 days, respectively (Figure 16).  The same level of efficacy was provided regardless of cultivar, hence no significant differences among cultivars at each sampling time. At 145 days, the overall RSR incidence was lower than that at 120 days, with 4/5 cultivars having no rot.

  • Non-treated control

The non-treated roots generally had among the highest RSR incidence, ranging from 16.7%-67% at 120 days, and 13.3%-50% at 145 days (Figure 17). At 120 days, Beauregard, Bellevue and Covington had the highest proportion of rot, while Orleans had the least. At 145 days, Covington and Beauregard had the most rot (50% and 44%, respectively), while Bayou Belle had the least proportion of rot.

The results also showed that cultivars respond differently even to the same treatment (Table 2), thus what may be required in the future are cultivar-specific recommendations for managing RSR as no one strategy may work the same for all sweetpotato cultivars. The total rot for each cultivar shows that Bellevue had the greatest proportion of rot (42.5%) at 120 days, and even though an overall reduction in RSR incidence for all cultivars, it had the greatest proportion of rot at 145 days with 18.6% rot (Table 2). Regarding the treatments, the greatest proportion of rot at 120 days was observed in the roots not treated, sprayed with Bio-Save, or Re-cured. At 145 days, roots re-cured, and those not treated had the greatest proportion of rot. At both sampling times, dicloran was the most effective treatment. There seemed to be a general decline in the RSR incidence at 145 days compared to 120 days, demonstrated by the lower proportion of rot for all cultivars and treatments (apart from re-cured and non-treated roots). It will be worth exploring in future studies how cultivars respond to given treatments, with regard to RSR incidence, changes at various times in storage and what factors may be responsible.

Table 2. Mean RSR Incidence for each cultivar and treatment at 120 and 145 days. The total rot per treatment or cultivar is pooled across cultivars or treatments, respectively.

Treatment/Cultivar

Bayou Belle

Beauregard

Bellevue

Covington

Orleans

Total rot/treatment

 

————————-120 days (%) —————————–

Bio-Save dip

3.3

11.0

41.7

3.3

3.3

12.5

Bio-Save spray

46.7

52

37.7

63.3

50.0

49.9

Re-cure 24 h

16.7

44.7

63.0

20.0

36.7

36.2

Bio-Save dip + Re-cure 24h

26.7

14.7

38.0

10.0

16.7

21.2

Bio-Save spray + Re-cure 24h

23.3

48.0

50.3

20.0

26.7

33.7

Dicloran

6.7

3.7

8.3

13.3

13.3

  9.1

Non-treated

43.3

67.0

58.3

46.7

16.7

46.4

Total rot/cultivar

23.8

34.4

42.5

25.2

23.3

 

 

————————-145 days (%) —————————–

Bio-Save dip

3.3

0

4.3

0

0

1.5

Bio-Save spray

0

0

0

0

3.3

0.7

Re-cure 24 h

30.0

11.0

75.3

40.0

43.3

39.9

Bio-Save dip + Re-cure 24h

0

0

21.0

0

6.7

5.5

Bio-Save spray + Re-cure 24h

0

0

4.3

0

3.3

1.5

Dicloran

0

3.7

0

0

0

0.7

Non-treated

13.3

44.0

25.0

50.0

33.3

33.1

Total rot/cultivar

6.7

8.4

18.6

12.9

12.9

 

 Overall, the biological product Bio-Save was comparable to the standard dicloran, especially at 145 days after harvest. Both methods of application seemed to provide similar levels of control, during the second sampling. When used in combination with re-curing, the Bio-Save was effective too. Because re-curing roots still resulted in high RSR incidence, the effects were most likely due to the Bio-Save. It cannot however be ruled out that re-curing could play a role, especially since Sweany et al (2020), observed it was effective. Further studies will evaluate the effects of different re-curing periods on RSR incidence, as this would provide a cost-effective strategy for managing RSR. Dicloran as expected was exceptional both at 120 and 145 days, which explains why it has been relied on for decades. The biological product Bio-Save was the most promising of all the alternatives evaluated, regardless of application methods, both on its own and when combined with re-curing for 24 hours. Two previous studies (Edmunds and Holmes, 2009; Sweany et al., 2020) that used the same product, did observe suppression but with variability. Their ease of application, ability to use in combination with other strategies, and with already existing equipment make biological products desirable (Stockwell and Stack, 2007).

Literature cited

Edmunds, B.A. and Holmes, G.J., 2009. Evaluation of alternative decay control products for control of postharvest Rhizopus soft rot of sweetpotatoes. Plant Health Progress, 10, p.26.

Stockwell, V.O. and Stack, J.P., 2007. Using Pseudomonas spp. for integrated biological control. Phytopathology97, pp.244-249.

Sweany, R.R., Picha, D.H. and Clark, C.A., 2020. Hot‐water baths, biologicals and re‐curing effects on Rhizopus soft rot during sweetpotato packing. Plant Pathology, 69, pp.284-293.

 

Year 2 (2020/2021)

With the exception of one Bayou Belle root, there was no rot on all roots wounded with the potato peeler (scrape wound) and so the data will only show the incidence of RSR associated with roots wounded with the rubber-band impelled wooden dowel (bruise wound). To show the effectiveness of each treatment at each sampling time, bar graphs were generated that demonstrated the effects of a particular treatment on all five cultivars at both 130 and 150 days after harvest. The results will be presented in the same order as the objectives and will end with a look at dicloran and the non-treated control.

  • Bio-Save 10LP ( syringae strain ESC-10) applied as a dip

The Bio-Save applied as a dip was not effective at 130 days based on the high incidence of RSR observed for all cultivars (>73%). At the second sampling, however, there was a drastic improvement with 85% control for Bayou Belle and up to 100 % control on cultivar Bellevue (Figure 16). At 150 days, the Bio-Save effectively controlled the RSR in all cultivars, with no significant differences in the RSR incidence among the cultivars.

  • Bio-Save 10LP ( syringae strain ESC-10) applied as a spray

When applied as a spray, the Bio-Save was also not effective at 130 days, even though there were slight variations among cultivars (Figure 17). At 150 days, the Bio-Save resulted in up to 100% control in the cultivars Bellevue and Covington, which were not significantly different from Bayou Belle and Orleans that had less than 10% RSR. The only exception at 150 days was cultivar Beauregard which had 50% RSR, the highest among all cultivars, but less than that observed at 130 days.

  • Re-curing for 24 hours

The effects of re-curing were variable among the sweetpotato cultivars and at each sampling time. (Figure 18). Overall, there was a significant decline in RSR incidence for all cultivars between the two sampling times. At 130 days after harvest, re-curing resulted in the least RSR incidence for Covington which had 52 % RSR. At 150 days, in addition to the overall reduction for all cultivars, the re-curing was more effective for most cultivars with the lowest RSR incidence observed in Bayou Belle, Bellevue, and Covington which all had <20% RSR. Even though Beauregard had a reduction in RSR incidence from the first to the second sampling, it had the highest incidence (57%) at the second sampling. The low RSR incidence observed especially at the second sampling suggests that re-curing could have potential, but different cultivars may require different re-curing periods based on the variability in responses.

  • Bio-Save dip combined with re-curing 24 hours

When the roots were dipped in Bio-Save and then re-cured, the RSR ranged from 57%-90% among all cultivars at 130 days, and 37-83% at 150 days. At 130 days, all cultivars except for Covington had >89% of the roots rotting (Figure 19). Even though there was a reduction in RSR for all cultivars at 150 days, RSR incidence was still quite high (>60%) for the cultivars Bayou Belle, Beauregard, Bellevue, and Orleans. The re-curing seemed to have caused a greater incidence of RSR compared to when the Bio-Save was used on its own, so the combination of treatments was not effective.

  • Bio-Save spray combined with re-curing 24 hours

Like the Bio-Save applied as dip, RSR incidence at 130 days was quite high with all cultivars except Covington having greater than 87% (Figure 20). Except for Bayou Belle which had a slight increase in RSR incidence at 150 days, there was a small reduction in RSR incidence for all other cultivars. At 150 days, the RSR incidence ranged from 50% in Covington to 95% in Bayou Belle, thus the Bio-Save spray combined with re-curing was also not effective in the second year. In fact, the RSR incidence was even greater when treatments were combined compared to when the Bio-Save applied as a spray was used alone. The Bio-Save was applied the same way at both sampling times, just by different individuals, so the slight reduction is probably age-related as has been shown by Holmes and Stange (2002).

  • Dicloran (the industry standard)

As expected, the fungicide dicloran provided at least 97% and 93% control at 130 days and 150 days, respectively (Figure 21).  The same level of efficacy was provided regardless of cultivar, with some cultivars like Bellevue having no RSR, hence no significant differences among cultivars at each sampling time.

  • Non-treated control

The non-treated roots had among the highest RSR incidence at 130 days, ranging from 49%-80% (Figure 22). Covington had the lowest RSR incidence, with Bayou Belle having the highest and the other three cultivars in between. At 150 days, there was a significant reduction for all cultivars with Bellevue only having 3% of its roots rotting indicating the influence of storage time on RSR.

When the results are combined across cultivars to show the treatment efficacy at each sampling time (Figure 23), the fungicide dicloran offered the greatest control at 130 days (99%), with no significant differences among the other treatments which ranged from 74-90%. The different sources of roots used in the second year as well as the extra day in the curing room at 130 days are two possible explanations for the differences observed from the first year’s results and the generally higher incidence at 130 days.  At 150 days, the RSR incidence was lower across all treatments, and treatment differences were noticeable with the lowest number of roots rotting on roots treated with the fungicide dicloran (4%). The Bio-Save applied as dip and spray provided 93 and 88% control, respectively. When combined with re-curing, the efficacy of the dip and spray dropped to 38% and 23%, respectively indicating that combination of the treatments was not effective option. Interestingly, the non-treated roots had lower RSR incidence than the biological and re-cure treatments both alone and combined in both years (67% in year 1 and 22% in year 2) suggesting that some treatments may have increased the susceptibility of the roots to RSR. Re-curing, when pooled across cultivars, was not bad indicating it has some potential, thus there might be a need to investigate the ideal times and conditions for the different cultivars. 

The results showed the variations in response that can exist amongst cultivars even to the same treatment. Regardless, the biological product Bio-Save regardless of application method and provided good control for the most part, especially at the second sampling time where it was second to dicloran. The fungicide dicloran performed as expected, with the best control at both sampling times for all cultivars. This time around, however, the Bio-Save was only effective on its own and not when combined with re-curing, which seemed to increase the RSR incidence and negatively impact the efficacy of Bio-Save observed on its own.

Literature cited

Holmes, G.J. and Stange, R.R., 2002. Influence of wound type and storage duration on susceptibility of sweetpotatoes to Rhizopus soft rot. Plant disease, 86, pp.345-348.

Participation Summary

Educational & Outreach Activities

1 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

50 Ag professionals participated
Education/outreach description:

Year 1 (2019/2020)

While no outreach has been conducted yet, it is planned to have either a workshop with sweetpotato growers, packers and agricultural professionals in Louisiana or to incorporate our outreach in the sweetpotato research station field day, that attracts a diverse audience yearly. Black Gold farms, a prominent sweetpotato packer have in the past hosted the sweetpotato field day. In the event a workshop was more feasible, Black Gold would still be considered as a potential host. During these outreach events results in the form of a talk will be shared based on the 1st year results in year 1 and of both years results in year 2. Hopefully an interactive discussion session will also be held with growers and packers, following the presentation of results. Educational handouts, research briefs will be shared with all in attendance. Oral presentations will also be made at the National Sweetpotato Collaborators meeting in January 2021 and possibly other meetings/conferences thereafter. All the results will be published in the Horticulture journal for the scientific community and results will be disseminated to extension and research workers in various states.

Year 2 (2020/2021)

So far, an oral presentation entitled “Evaluating Options for the Control of Rhizopus Soft Rot on Sweetpotato” was given at the 98th American Phytopathological Society Southern Division 2020 virtual meeting. Since this was a scientific meeting, the audience was primarily comprised of graduate students, faculty, and industry representatives. This year, the results obtained from both years and other similar research will be shared as a video during the virtual sweetpotato field day in August. The virtual sweetpotato field day targets various sweetpotato stakeholders including growers and packers who would benefit greatly from the research. Since the project was extended, plans are to have a workshop with sweetpotato growers, packers, and agriculture professionals later in the Fall. Black Gold farms are a possible host for the workshop if it would take place in person. During the workshop, results from both years would be shared, and hopefully, this would be an interactive session to learn and get feedback from the attendees. The last planned outreach would be to give an oral presentation at the National Sweetpotato Collaborators group meeting if it takes place since it was not held this year. The results will eventually be published in the Horticulture journal for the scientific community and extension publications shared with various extension personnel in various states.

Project Outcomes

Project outcomes:

Year 1 (2019/2020)

The project is still in progress, but it is anticipated that this project will provide very useful knowledge that will contribute to future sustainability by resulting in the reduction of fungicide use to control RSR, hence meeting the needs of growing markets such as the European Union, the baby food market and organic market. Additionally, growers would have more options available to manage Rhizopus soft rot, which would be less harmful to the environment or organisms. Socially, it could contribute to a better quality of life, due to the increased availability of good quality roots for consumers.

Year 2 (2020/2021)

Based on the results obtained from both years, the biological product Bio-Save provided good control, though this was dependent on how long the roots had been stored, the treatment combination, and the cultivars suggesting the need for cultivar and time-specific control options. Overall, applied as a dip, it was more effective and, in some cases, comparable to the dicloran thus could potentially be used by growers/packers to manage RSR. The re-curing treatment both alone and in combination with Bio-Save was not effective for all cultivars in both years, thus cannot be recommended at this point. Having another alternative comparable to dicloran would result in reduced fungicide use, improved environmental stewardship and better quality of produce for the various markets.

Knowledge Gained:

Year 1 (2019/2020)

So far, it has been a learning experience getting acquainted with other products and/or strategies for managing RSR, some that I had not worked with before. 

Year 2 (2020/2021)

It has both been a learning experience as well an eye-opener to some research aspects that could be looked at in the future. During the project, one thing that came out was the fact that many other variables that could be overlooked influence the control efficacy of the various treatments. A holistic approach would therefore be valuable if feasible.

Recommendations:

Year 1 (2019/2020)

In as much as it is too early to give recommendations, there are a few things that could be improved on based on the first-year experience. Firstly, it may be worth in the future to apply treatments at actual packing facilities as it is not always easy to imitate the real situations. It would also be great to involve growers with packing facilities as a way of accounting for the root variability due to location. Seeing the uncertainty with root yields and availability in storage due to floods and end rots, and taking into account the root variability, it may be worth planting all cultivars in various locations, especially in locations not as affected by floods to increase the number of roots available for screening. Lastly, it will be great to perform a cost-benefits analysis with growers incorporated into the study, should any of the strategies being tested in this study be found to be comparable to the standard dicloran.

Year 2 (2020/2021)

Based on the experiences during the entire project, the first recommendation would be to develop RSR management recommendations specific to the time in storage and cultivar because different responses were observed. Secondly, it would be good to test the products used in this study at an actual packing facility before making recommendations to growers to get an idea of what it would be like in real life. Additionally, it would be good to perform a cost-benefits analysis with growers incorporated into the study, using Bio-Save which showed the most potential in comparison to the standard used fungicides. Based on the variability observed related to root source it would be good to use roots from various locations representing various growing conditions so that recommendations would be specific to certain areas.  Collaborating with sweetpotato growers would be a good strategy to have increased root numbers for evaluations since this was one limitation in both years.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.