© 2009 Plant Management Network.
Evaluation of Alternative Decay Control Products for Control of Postharvest Rhizopus Soft Rot of Sweetpotatoes
Brooke A. Edmunds, Colorado State University Extension, Brighton, CO 80601; and Gerald J. Holmes, Valent USA Corporation, Apex, NC 27502; (both previously of the Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695)
Edmunds, B. A., and Holmes, G. J. 2009. Evaluation of alternative decay control products for control of postharvest Rhizopus soft rot of sweetpotatoes. Online. Plant Health Progress doi:10.1094/PHP-2009-0206-01-RS.
Postharvest Rhizopus soft rot of sweetpotato, caused by Rhizopus stolonifer, is managed by minimizing injuries incurred during harvesting and packing, curing roots immediately after harvest, and applying fungicide during packing. The United States sweetpotato industry relies heavily on a single fungicide (dicloran) to control Rhizopus soft rot, however, many markets (export, infant food, and organic) no longer allow dicloran residues. Dicloran is currently the only fungicide labeled for control of Rhizopus soft rot. Thirty-three products were tested in nine individual experiments over a 5-year period to identify alternative control products. The reduced-risk fungicides boscalid plus pyraclostrobin (Pristine) and fludioxonil (Scholar) significantly reduced Rhizopus soft rot and performed similarly to dicloran. The biological products Bio-Save 10LP and 11LP suppressed Rhizopus soft rot although results were variable among tests. Generally recognized as safe (GRAS) treatments were ineffective in controlling soft rot by our methods.
Rhizopus soft rot, caused by the fungus Rhizopus stolonifer, is the most common postharvest disease of sweetpotatoes (Ipomoea batatas Lam.) (2). An estimated 2% of sweetpotatoes are lost to Rhizopus soft rot by the time they reach the retail market (3), but losses can be sporadic since only a few rotted roots can cause shipments to be rejected. R. stolonifer requires a wound for infection and to cause disease. Spores of R. stolonifer are ubiquitous in the environment, and in the presence of a suitable wound, they germinate to produce mycelia and pectolytic enzymes which cause a soft, watery rot. Whiskery, white mycelium that becomes covered with powdery, black sporangiospores is a characteristic sign of R. stolonifer (Figs. 1 and 2). An entire root can become completely rotted in 3 to 4 days after roots leave the packinghouse in good condition to arrive at their destination completely decayed.
Two key points where wounding can occur are during harvesting and packing. During harvest, roots are brought to the soil surface using specialized plows or chain diggers and hand-loaded into 20- or 40-bu capacity palletized bins. Wounds occur at the stem end where the root is removed from the plant in addition to wounds caused by digging equipment and the loading and transport of bins. Immediately following harvest, bins are transported to specialized storage rooms and roots are cured by exposure to high temperature (29°C) and relative humidity (85 to 90%). The curing process aids in healing harvest wounds and results in reduced losses during storage due to R. stolonifer and other diseases. After 5 to 7 days, the temperature is dropped to 13°C for long-term storage. In the United States, sweetpotato roots are commonly stored for up to 12 months after harvest, enabling producers to provide a year-round supply.
Roots are washed, graded, and shipped based on market demand. The packing process begins when pallet bins are brought out of storage and roots are poured into a large tank of water (dump tank) using either a forklift or mechanized system. The layout of individual packing lines varies, but all function to remove clinging field soil and sort the roots by grade (i.e., size, shape, and quality). A packing line has a series of overlapping components that remove soil through brushes and water sprays, apply fungicide, sort roots by size and defects, and load roots into fiberboard cartons. The cartons are usually stacked by hand onto pallets and loaded the same day into trailers for transport to market.
Sweetpotatoes can be wounded in three ways on packing lines: (i) by ‘skinning,’ when the skin or periderm is sloughed off the root; (ii) by broken root ends that are snapped off by packing line workers or the packing process; and (iii) by bruises when the root tissue is crushed during packing. Controlled experiments found that wounds caused by bruising result in increased susceptibility to infection by R. stolonifer as compared to the other types of wounds (9).
Management options for controlling Rhizopus soft rot of sweetpotato are limited. No cultivars are known to be completely resistant. Beauregard, the most commonly grown cultivar in the last 10 years, is considered moderately resistant to Rhizopus soft rot but susceptibility can vary greatly depending on preharvest conditions (4). This variability in susceptibility results in sporadic losses which can result in entire shipments being rejected at the point of sale.
To prevent losses from Rhizopus soft rot, the majority of sweetpotato packinghouses make prophylactic applications of the fungicide dicloran (Botran) as a spray or dip treatment on the packing line. Because the fungicide is applied so close to the time of consumption, the amount of residue left on the product is of great concern to regulatory agencies and consumers. As of January 2008, no residues of dicloran were allowed on exports to the European Union. Producers of infant food and organic foods also have a zero tolerance for dicloran residues. Because there are no other fungicides registered to control Rhizopus soft rot on sweetpotato, packers are forced to either lose these markets or risk losses incurred by shipping without fungicide protection.
The objective of this research was to identify alternative control products, including reduced-risk fungicides (newer fungicides considered to have a lower risk of human and environmental toxicity), biological products (formulations based on biological control organisms as the active ingredient), and GRAS compounds (products considered generally recognized as safe) for control of Rhizopus soft rot. The trial results for some individual years have previously been published (5,6,7,8).
Product Efficacy Trials
A total of nine individual trials were conducted at either the Central Crops Research Station in Clayton, NC, or the Horticulture Crops Research Station in Clinton, NC, over a period of 4 years. US no. 1 grade roots of cv. Hernandez were used in all trials as this cultivar is consistently susceptible to Rhizopus soft rot (1). All roots were grown commercially in North Carolina using standard cultural methods, cured after harvest, and stored at 13°C in a commercial storage facility until experiments were conducted. Roots were collected directly from bulk bins and transported to the research facility. One day prior to a trial, roots were gently hand washed with tap water and allowed to dry at room temperature.
The R. stolonifer isolate used in these trials was collected in 1992 from a sweetpotato root showing Rhizopus soft rot symptoms and signs and stored on silica crystals at 3°C (12). To produce inoculum, silica crystals were transferred to potato dextrose agar (Difco, Sparks, MD) and held at room temperature (22°C) to induce production of sporangiospores. After 6 days, approximately 10 ml of sterile water was added to each plate and the sporangiospores were dislodged by gently rubbing the surface with a bent glass rod. The resulting suspension was filtered through four layers of cheesecloth to remove mycelial fragments and agar pieces. The spore suspension was diluted with 0.01% octylphenol ethoxylate (Triton X-15, Dow Chemicals Co., Pennsauken, NJ) to a concentration of 106 sporangiospores/ml to reduce clumping. The suspension was kept refrigerated (1 to 2°C) overnight.
Roots were inoculated using a technique developed for sweetpotatoes (9). First, an impact bruise (8 mm diameter × 1 mm deep) was made to opposite sides of the mid-section of each root by the calibrated impact of a wood dowel (Fig. 3). Inoculum was introduced by brushing the R. stolonifer suspension over the wounded area with a 1-inch foam paintbrush (Fig. 4).
After allowing the roots to airdry (ca. 60 min), fungicide treatments were applied. All treatments, except copper ionization treatment, were suspended in 10 gal of tap water. For the copper ionization treatment, the target concentrations of copper ions in solution were produced on-site using commercial ionizing equipment (Superior Aqua Enterprises, Sarasota, FL) to treat 10 gal of tap water. All treatments were applied by submerging roots in the treatment suspension for 30 sec unless otherwise noted. Roots were gently agitated while submerged to ensure complete coverage. A total of 33 treatments (17 products) were tested against the standard dicloran (Botran 75W, Gowan Company, Yuma, AZ) treatment in nine trials (Table 1). Three control treatments in each trial included: (i) non-wounded, non-inoculated; (ii) wounded, non-inoculated; and (iii) wounded, inoculated.
Table 1. Products tested for efficacy against Rhizopus soft rot of sweetpotatoes, 2003-2008.
* Rate is expressed as amount of formulated product per 100 gal of water.
After treatment, roots were placed in commercial plastic storage crates (15 per crate, four replicates per treatment). The crates were arranged in a randomized complete block design and stored at 16°C. Roots were evaluated for Rhizopus soft rot incidence after 10 days in storage (Fig. 5). Infected roots were easily detected because 60 to 100% of the root mass was soft while non-infected roots were firm and showed no symptoms or signs of decay.
Disease incidence values for all trials and all treatments were combined and analyzed using ANOVA in SAS 8.0 (SAS Institute Inc., Cary, NC) to compare treatments across trials. Data were standardized by weighting decay incidence in treatments against decay in the wounded, inoculated control to account for variability in susceptibility. No significant trial or treatment by trial interaction was seen. The Student-Newman-Kuel’s test was used to separate means (P = 0.05).
Wound colonization counts were completed on the first six trials of Pseudomonas syringae (Bio-Save 10LP and 11LP, Jet Harvest Solutions, Longwood, FL) treatments. Non-inoculated roots were puncture wounded (1 mm diameter × 4 mm depth) and the appropriate Bio-Save treatment was applied as previously described. The roots were allowed to air dry and shipped on ice packs to the Jet Harvest Solutions testing lab (Longwood, FL) for determining colonization. Roots arrived within 24 h and P. syringae counts were completed by dilution plating onto nutrient yeast dextrose agar. Wound colonization counts were not done for the Metschnikowia fructicola (Shemer) treatments.
Effect of Decay Control Treatments on Rhizopus Soft Rot
The inoculation method utilized in these trials resulted in high levels of disease (mean = 95.8%, std. dev. = 7.3%) in the wounded, inoculated control across all experiments (Figs. 6, 7, and 8). No decay developed in the non-wounded, non-inoculated control and very little decay developed in the wounded, non-inoculated control (mean = 5.4%, std. dev. = 9.1%). Dicloran (Botran) performed well in all trials (mean = 10.4%, std. dev. = 9.9).
Fungicides. Boscalid plus pyraclostrobin (Pristine, 38WG, BASF Ag Products, Research Triangle Park, NC) was evaluated at four rates and in several different trials for a total of five treatments. These treatments resulted in low levels of decay that were not significantly different from dicloran (Fig. 6). No significant rate effect was seen, although 36.3 fl oz/100 gal resulted in the numeric highest level of control (mean = 10.9%, std. dev. = 4.2).
Fludioxonil (Scholar 50WP, Syngenta Crop Protection, Greensboro, NC) performed similar to dicloran in tests of two formulations, a wettable powder (50WP) and a soluble concentrate (250SC) No significant differences were found in efficacy of the two formulations or rates (Fig. 6). The treatment duration (30 vs. 60 second dip) had no significant effect on performance of fludioxonil (Scholar 50WP 16 oz (11.4% and 7.4%, respectively). Maxim 4FS (Syngenta Crop Protection), which is a seed treatment formulation for use only on sweetpotato at planting, was tested in early trials and significantly reduced Rhizopus decay (mean = 17.9%, std. dev. = 0.6%). Maxim 4FS is labeled for control of Rhizopus rot during transplant production but not postharvest use.
Fenhexamide (Elevate 50WDG, Arysta LifeScience North America, Cary, NC) was not effective in control of Rhizopus soft rot in this study. These results were not surprising as fenhexamide has also proven ineffective in the control of Rhizopus soft rot in other commodities such as strawberry and against R. stolonifer in vitro (13).
Sodium o-phenylphenol (SOPP, Freshgard 25, FMC Corporation, Philadelphia, PA) was moderately effective against Rhizopus soft rot. SOPP registrations were dropped in 2005 because it irritates skin and mucous membranes (i.e., eyes and throat) and was considered more hazardous to workers than dicloran.
Bio-fungicides. None of the biological treatments performed as well as dicloran, but some suppression of Rhizopus rot were indicated by three products (Fig. 7).
Bio-Save 10LP and 11LP are two different strains of non-pathogenic Pseudomonas syringae (strain ESC10 and ESC11, respectively). The Bio-Save 10LP is labeled for control of Mucor rot of apples, caused by Mucor piriformis which is closely related to R. stolonifer.
Averaged over all trials, treatments with P. syringae (except 22 oz of Bio-Save 11LP) were not effective in control of Rhizopus soft rot. A rate effect was seen in tests of Bio-Save 10LP but was not apparent in tests of 11LP (Fig. 7). There was no significant difference in efficacy at the high rate (70.5-oz rate) of each formulation, although the mean decay was lower in the LP10 compared to LP11 (41.2% vs. 54.5% decay). Based on an early trial showing high efficacy, Bio-Save 11LP was labeled for use on sweetpotatoes in 2005 (5,6). Subsequent testing has demonstrated high variability across trial dates. Wound colonization by P. syringae in the first six trials fell within the expected range for the treatment rate and was not significantly different among trial dates (data not shown), making it unlikely to be responsible for reduced efficacy.
The tank-mix of boscalid plus pyraclostrobin with BioSave LP11 improved control compared to either product alone. The cause of increased efficacy is unknown, although the different modes of action may have resulted in an additive effect.
Shemer (AgroGreen Minrav) is the trade name for a bio-fungicide developed in Israel; the active ingredient is a strain of Metschnikowia fructicola. Preharvest applications of M. fructicola have significantly reduced postharvest decay of strawberries which was caused by a mixed infection of R. stolonifer and Botrytis cinerea (10). None of the treatments with M. fructicola, with or without 0.01% potassium bicarbonate, reduced decay significantly.
Generally recognized as safe (GRAS) treatments. Rhizopus soft rot was not prevented by any GRAS product tested as decay in all of these treatments was similar to the inoculated control (Fig. 8). This was not surprising as most products tested are labeled as sanitizers, not fungicides. As in other commodities, many products are marketed to sweetpotato packers with claims of disease control efficacy but have limited supporting data. GRAS products were included in these trials to provide replicated trial data for judging efficacy.
Calcium chloride has been used as a successful postharvest treatment of peaches for control of brown rot caused by Monilinia laxa (15), but was ineffective against R. stolonifer in this trial. Potassium bicarbonate used alone did not significantly reduce decay. Potassium sorbate, a prepared-food preservative, also failed to reduce decay in these trials. Rhizopus soft rot of apricots was less for fruit treated by potassium sorbate, but not for fruit of nectarines or sweet cherries (11). Heating the treatment suspension and/or adjusting the pH may increase efficacy of potassium sorbate (14).
Tsunami 100 (Ecolab, St. Paul, MN), StorOx (Bio-safe Systems LLC, East Hartford, CT), and sodium hypochlorite are used to sanitize packinglines and commonly added to dump tanks on sweetpotato packinglines. While these products may reduce microbial counts in water, none were effective in controlling Rhizopus soft rot in these trials.
Copper ionization has been proposed as a sanitizing method for agricultural water and some companies are marketing the technique for disease control. Roots treated in copper ionized water for 30 or 120 sec developed as much Rhizopus soft rot as the wounded, inoculated control.
These trials have identified two reduced-risk fungicides, Scholar and Pristine, with the ability to effectively control Rhizopus soft rot of sweetpotatoes. The active ingredient in Scholar, fludioxonil, is known to be effective against Rhizopus soft rot of peaches and is labeled for postharvest use on that crop (16). Boscalid plus pyraclostrobin (Pristine) is effective at reducing Rhizopus soft rot of strawberries (13). Pristine is not labeled for use on sweetpotatoes; however, Scholar was registered in the United States in October 2008. Residue tolerance data required for treated roots sent to the European Union is expected in Spring 2010.
The biological control products were not as effective as the reduced-risk fungicides in these trials. Bio-Save 11LP showed suppression of Rhizopus soft rot and is currently labeled for postharvest applications on sweetpotatoes. Additional work is needed to optimize and stabilize efficacy of Bio-Save 11LP. These results also confirm that products labeled as sanitizers (i.e., bleach, acetic acid, copper ionized water, etc.) are ineffective against Rhizopus soft rot by the methods used in this study.
Trial products were only tested using a single application method: dipping in a treatment suspension for 30 or 120 sec. Many packing houses use overhead sprays over rotating brushes or a tank mix fungicide combined with food grade waxes to improve root appearance. Preliminary tests with dicloran show no significant difference between dip and overhead spray above rotating brushes (data not shown). Additional research is needed to understand the interaction of fungicides with waxes and the efficacy of other application methods. Furthermore, performance may be influenced by the method of inoculation. Disease incidence in the wounded, inoculated control is far greater than would be expected under normal commercial practices. However, dicloran, fludioxonil, and boscalid plus pyraclostrobin were highly effective under the conditions of our experiments.
This research was supported, in part, by the USDA-CSREES Risk Avoidance and Mitigation Program (Project No. NC09197), the North Carolina Sweetpotato Commission, and The IR-4 Project-Biopesticide Program. The authors thank Mike Adams for providing technical support.
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