Rhizopus soft rot

Introduction:

Disease management

Effective management strategies for Rhizopus soft rot on sweetpotato include resistant varieties, proper curing after harvest, and decay control product applications on packinglines. Resistant varieties. The sweetpotato industry readily accepts new cultivars, which leads to a quick shift in the most widely grown cultivar. Beauregard, released in 1987, is currently the dominant cultivar grown in the U.S. (Rolston et al., 1987). In the last ten years, seven new cultivars have been released in the U.S.: Bienville (La Bonte et al., 2003), Carolina, Covington (Yencho, personal communication), Ruby (Collins et al., 1999), Evangeline (La Bonte et al., 2008), Ruddy (Bohac et al., 2002), and White Regal (Bohac et al., 2001). Yield and quality characteristics, rather than disease resistance, are the primary focus of modern breeding efforts. Rhizopus soft rot resistance screening is used to describe most newly released cultivars, but is not a primary selection factor used by breeders. The method of testing Rhizopus resistance is usually not specified and/or may vary between research groups, making direct comparisons difficult using only the information from the cultivar release publication. Replicated resistance screening of modern cultivars and breeding lines showed that white-fleshed cultivars tend to be more susceptible than orange-fleshed cultivars (Clark and Hoy, 1994). Notable exceptions included cv. Hernandez and Jewel, which are both orange fleshed and moderately susceptible to R. stolonifer. Holmes and Stange (2002) confirmed that cv. Hernandez is more susceptible than cv. Beauregard when injured. Beauregard is considered to be moderately resistant to R. stolonifer although sporadic, heavy losses during shipping are known to occur. No cultivar has been found that is completely resistant to Rhizopus soft rot. Curing. Curing immediately after harvest generally eliminates losses to R. stolonifer by healing wounds occurring during harvest. The current recommended curing process is to expose the roots to high temperature (29°C) and high relative humidity (90%) for five to seven days (Kushman, 1975). The time length is important because increased sprouting (a negative quality in stored roots) can result if roots are held at high temperatures for extended periods (Hall, 1992). Curing induces suberization of wounds followed by new periderm formation (this process was called wound ‘cork’ or ‘phellum’ in early research), effectively healing the wounds (Weimer and Harter, 1921). The new periderm consists of three to seven layers of flattened cells, with the first layer formed after four days in the curing environment (Walter and Schadel, 1982; Walter and Schadel, 1983; Strider and McCombs, 1958). McClure (1959) and Tereshkovich and Newsom (1964a) suggested that re-curing prior to shipping would adequately heal wounds occurring during the packing process, resulting in reduced loss to decay. This may be sufficient for healing skinning injuries, but wound periderm formation has been found to be irregular in bruise type wounds (Daines, 1943; Strider and McCombs, 1958). Commercial packinghouses have not embraced re-curing as a management strategy due to the time and resources required to raise the root temperature to 29 C, which is required for re-curing to be effective (McClure, 1959; Tereshkovich and Newsom, 1964a). McClure (1959) suggested a hot water dip to quickly boost root temperature but this may result in additional problems such as infection by other pathogens or increased sprouting during transport. Decay control products. R. stolonifer is most commonly managed by packingline applications of dicloran (also known as DCNA or Botran®). Dicloran, a chlorinated nitro-aniline, is a broad spectrum fungicide registered for postharvest use on sweetpotatoes and in-field use for several fruits, vegetables, and ornamentals. Use of dicloran on sweetpotatoes to control Rhizopus soft rot was first reported in 1964 (Martin). Dicloran is considered by the Fungicide Resistance Action Committee (FRAC) to be at low to medium risk for resistance development. Forced resistance has occurred in culture when R. stolonifer was grown on dicloran-amended media (Webster et al, 1968); however, no additional reports have been published on this phenomenon. Another fungicide, SOPP (sodium o-phenylphenol), was developed in the mid-1950’s and used for Rhizopus soft rot control of sweetpotatoes for many years. SOPP is considered more difficult to work with than dicloran, as SOPP is corrosive to the skin and mucous membranes (i.e., eyes and throat). There also have been anecdotal reports of negative effects on root quality including root skin color changes. SOPP registrations were dropped in 2005. As of 2005, no commercial fungicides or decay control products, other than dicloran, were labeled for control of Rhizopus soft rot on sweetpotatoes. New chemistries have been tested for postharvest use on fruits to control Rhizopus soft rot. Commercial formulations of boscalid alone and boscalid + pyraclostrobin were effective in the control of Rhizopus on strawberry (Sallato et al., 2007). A boscalid+pyraclostrobin treatment also significantly reduced postharvest decay of Rhizopus fruit rot of nectarine with two preharvest applications within eight days of harvest (Holb et al., 2005). Boscalid+pyraclostrobin is the active ingredient in Pristine® which is registered for preharvest treatements to control Rhizopus soft rot on stone fruits. Fludioxonil is another new chemistry which shows promise for controlling Rhizopus soft rot of sweetpotato. R. stolonifer is considered sensitive to fludioxonil in vitro with an ED50 ranging from 0.01 to 0.09 mg/L (Olaya et al., 2007). Postharvest applications of fludioxonil effectively controlled Rhizopus soft rot of peaches, plums, and nectarines (Förster et al., 2007; Northover and Zhou, 2002; Yoder et al., 2001). Prior to 2002, there were no published studies on the use of fludioxonil on sweetpotatoes. Fludioxonil is the active ingredient in Scholar® which is labeled for postharvest applications to control Rhizopus soft rot of stone fruits. There has been a growing interest in the use of biological control organisms for control of postharvest diseases of fruits and vegetables. As of 2002, no commercial formulations were registered for use against R. stolonifer (McSpadden Gardener and Fravel, 2002). In recent years, studies have identified biological control organisms effective against Rhizopus soft rot on fruit. Application of a Trichoderma harzianum Rifai emulsion reduced the Rhizopus lesion size in apple, pear, peach, and strawberry (Batta, 2007). A yeast antagonist, Cryptococcus laurentii, effectively controlled postharvest decay of strawberry by Rhizopus stolonifer when applied alone or following a hot water dip (Zhang et al., 2007). Preharvest applications of the yeast Metschnikowia fructicola reduced postharvest decay on strawberries caused by a mixed infection of R. stolonifer and Botrytis cinerea (Karabulut et al., 2004). Pichia membranefaciens, another yeast antagonist, completely inhibited Rhizopus soft rot of peach when applied at a high concentration (Qing and Shiping, 2000). Several bacterial antagonists have been identified as effective against R. stolonifer. Strains of Pantoea agglomerans reduced Rhizopus soft rot on plum (Frances et al. 2006), apple, pear (Nunes et al., 2001), peach, apricot and nectarine (Bonaterra et al., 2003). Another bacterium, Enterobacter cloacae, reduced Rhizopus decay on peach (Wilson et al., 1987). Pseudomonas syringae strain ESC-10 (available commercially as Bio-Save 10LP) significantly reduced Rhizopus decay in peaches (Northover and Zhou, 2002). No organism has been identified with effectiveness in controlling R. stolonifer infection of sweetpotato. Surface and water disinfectants/sanitizers have also been considered for use in controlling postharvest R. stolonifer infections by reducing the spore load in packingline wash water. Chlorine is the most commonly used water sanitizer in the produce industry. Chlorine added to hydrocooling and dump tank water was ineffective in controlling the development of Rhizopus soft rot on strawberries and tomatoes (Ferriera et al., 1996; Vigneault et al., 2000; Bartz et al., 2001). An in vitro study showed that volatile chlorine was able to reduce Rhizopus spore germination and mycelium growth (Avis et al., 2006). Applications of volatile chlorine also significantly reduced postharvest Rhizopus development on table grapes and strawberries (Lisker et al., 1996; Avis et al., 2006). These studies are in the preliminary stages and issues addressing human health and metal oxidation of equipment (common concerns with chlorine use) have not been investigated. A more recently developed sanitizer is peroxyacetic acid (PAA), a strong oxidizer, available in commercial formulations such as Oxidate®, Tsunami 100®, and StorOx®. Narciso et al. (2007) found what they thought was a residual effect of preharvest PAA sprays on strawberries, which significantly reduced postharvest decay attributed to combined Botrytis and Rhizopus infection. In reality, the preharvest applications functioned to reduce the spore numbers on the fruit rather than having a residual effect. Copper ionization is a water treatment using charged copper ions to reduce microbial counts of human pathogens in swimming pools and drinking water. There is interest in using ionization in postharvest systems as an alternative to chlorine. There is only anecdotal evidence to suggest that postharvest quality is improved by using copper ionized treated water on sweetpotato packinglines. No systematic studies on the control of plant pathogens have been completed. Calcium chloride has been used alone or in combination with biological control agents to enhance the effectiveness of disease control. Postharvest dip treatments of calcium chloride significantly reduced the development of brown rot of peach (caused by the fungus Monilinia laxa) (Thomidis et al., 2007). Qing and Shiping (2000) found a significant increase in the efficacy of a yeast antagonist against R. stolonifer on nectarines when applied in combination with a 2% (wt/v) solution of calcium chloride. The proposed mode of action of the calcium treatment is to reduce the activity of cellulolytic enzymes produced by pathogens (Wisniewski et al., 1995). Other decay control techniques are in development; plant and fungi-derived volatile compounds and ultraviolet light treatments have shown promise for controlling postharvest diseases of fruits and vegetables caused by R. stolonifer. Effectively functioning as fumigants, volatiles have an advantage over liquid decay control products as the commodity remains dry (especially important for moisture-sensitive fruit such as strawberries and cut flowers) and can be used during transit. Limited research has identified plant-derived volatiles which reduce in vitro Rhizopus mycelial growth, including acetaldehyde, benzaldehyde, and cinnamaldehdye (Avissar and Pesis, 1991; Utama et al, 2002). In 2001, the fungus Muscador albus was found to emit powerful volatiles capable of reducing the growth of other fungi (Strobel et al., 2001). Controlled studies found these volatiles were able to reduce R. stolonifer growth in vitro (Mercier and Jimenez, 2004). None of these volatile treatments are ready for commercial implementation. Exposing tomatoes and sweetpotatoes to low dose ultraviolet light (UV-C, 254 nm) for 5-7 minutes resulted in a significant reduction in Rhizopus soft rot development as a result of an induced resistance response in preliminary trials (Stevens et al., 1997; Stevens et al., 2004). The effect of UV-C treatments (also referred to as radiation hormesis) on quality characteristics of sweetpotatoes is unknown, although the no external damage was seen in these trials. This research holds promise as a non-chemical management method which could be incorporated into existing packinglines; however, commercial scale research has not been completed. In summary, alternative fungicides, biological controls and other treatments are capable of reducing decay caused by R. stolonifer. None have been tested on sweetpotatoes. There is a pressing need to identify and label effective alternative decay products and treatments as dicloran use is prohibited in many markets, including the European export market and buyers for infant food companies.