Managing Rhizopus Soft Rot on Sweet Potatoes

Linda Benedict, Ferrin, Donald M., Smith, Tara, Picha, David H., Clark, Christopher A., Villordon, Arthur O., Labonte, Don R.  |  6/10/2009 2:10:36 AM

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Sweet potato “seed,” which are actually sweet potatoes, are saved for planting the next year. See illustration on pages 18-19. (Photo by Mike Cannon)

Christopher A. Clark, Donald M. Ferrin, Don LaBonte, David H. Picha, Tara P. Smith and Arthur Villordon

Sweet potatoes are susceptible to Rhizopus soft rot, a disease caused by a common fungus that can destroy sweet potatoes after they are removed from storage and washed but before they arrive at the market. For more than 50 years, this disease has been managed effectively by applying a fungicide, usually dichloronitroaniline (Botran), to sweet potatoes on packing lines after they have been washed. One area of research has been to find an alternative means of controlling this disease so that sweet potatoes could be packed safely without fungicide.

Many tests conducted in Louisiana and North Carolina have evaluated the efficacy of new chemicals categorized as either Generally Recognized as Safe (GRAS) or reduced-risk fungicides or biological control agents for control of Rhizopus soft rot. In general, the GRAS products did not provide satisfactory control while the biological-control agents gave some control but less control than the reduced-risk fungicides, which provided control equivalent to Botran. One reduced-risk fungicide, fludioxonil (Scholar), received U.S. Food and Drug Administration approval in November 2008 for use on sweet potatoes. However, in September 2008 the European Union and other countries set for this chemical maximum residue levels equivalent to the limit of detection – essentially making it impossible to use this product on sweet potatoes destined for those markets.

For disease to develop, all components of the disease triangle must be present: a susceptible host (in this case, sweet potato storage roots), a pathogen (Rhizopus stolonifer) and a favorable environment for disease to occur. To manage Rhizopus soft rot without relying on fungicides will require an integrated program that addresses each leg of this disease triangle. In the 1990s LSU AgCenter researchers began screening sweet potato varieties for resistance to Rhizopus soft rot. Those studies showed that the variety Beauregard is more resistant than older varieties. Resistance is relative, however, and Rhizopus soft rot can occur on Beauregard even though it is more resistant than the varieties Jewel or Hernandez (Figure 1).

In a survey of commercial packing lines, on many occasions Beauregard could have been packed without using a fungicide, but on some occasions Rhizopus soft rot exceeded the two percent tolerance threshold set for this disease. For several years, an effort has been made to develop breeding lines with even greater levels of resistance than Beauregard, and now several lines produce reasonable yields and are more resistant. This will allow evaluation of whether resistance can be relied on as a principal means of disease management.

Resistance is the easiest and most economical tool for controlling diseases. But in the case of Rhizopus soft rot on sweet potato, the expression of resistance also can be affected by several factors. The U.S. Department of Agriculture Risk Avoidance and Mitigation Program (RAMP) funded a large study from 2003-2008 involving scientists from the LSU AgCenter, Mississippi State University, North Carolina State University and Auburn University. The study focused on understanding the cultural and environmental factors that affect soil insects and postharvest diseases on sweet potatoes in an effort to improve their management with reduced reliance on pesticide use. Beauregard sweet potatoes were collected from more than 100 plots in Louisiana farmers’ fields over three years, and their susceptibility to Rhizopus was examined in relation to weather data, cultural practices and field conditions. Figure 2 shows the extreme range of variation in susceptibility of Beauregard from different fields in 2006.

Although the factors that contribute to this variation are complex, correlations were observed with soil moisture, soil phosphorus (P) and potassium (K), and time in storage. Some of the factors such as rainfall that induce this extreme range of susceptibility cannot be controlled, but monitoring them can possibly identify lots of sweet potatoes that will require treatment at packing to control Rhizopus and identify others that will not require treatment. This could potentially reduce fungicide applications. Other practices, such as P and K fertilization, are subjects of ongoing research to determine if they can be manipulated in the field to increase resistance to Rhizopus.

Rhizopus soft rot requires a wound for infection, and the type and severity of wounding determine whether disease will develop. Therefore, part of the RAMP project involved working with sweet potato packers to identify the causes of wounds on packing lines and ways to reduce them. The amount of the pathogen present also determines whether infection will occur, and sanitation of the packing area is important to reduce this inoculum. As an output of the RAMP project, an extension bulletin, Postharvest Handling of Sweet Potatoes, published jointly by the cooperating states outlines proper storage and handling of sweet potatoes to reduce wounds and improve sanitation. The publication is available through local extension sources and online at www.lsuacenter.com and by typing the title in the search box.

These improvements alone, however, have not completely eliminated the Rhizopus soft rot problem. To supplement these methods, preliminary tests were conducted to investigate whether dips in hot water for several minutes during packing can reduce disease incidence. Initial results were promising (Figure 3), and further research is in progress aimed at evaluating whether hot-water sprays can be used on packing lines.

Prophylactic applications of Botran fungicide provide growers a simple, inexpensive and highly effective means to protect their sweet potato crop from Rhizopus soft rot as it makes its way to market. The development of an integrated pest management approach requires a detailed and thorough understanding of all complex disease triangle factors that contribute to disease development. Several factors – improving the genetic resistance of sweet potatoes, modifying the environment to enhance that resistance, using proper sanitation to reduce populations of the pathogen on the packing line and using hot water to reduce the pathogen on the sweet potatoes – are all necessary and important components of an integrated management system. This, researchers hope, will allow the sweet potato industry to manage Rhizopus soft rot effectively while also reducing prophylactic fungicide use.

Christopher A. Clark, Professor, and Donald M. Ferrin, Assistant Professor, Department of Plant Pathology and Crop Physiology, LSU AgCenter, Baton Rouge, La.; Don LaBonte, Professor, and David Picha, Professor, School of Plant, Environmental and Soil Sciences, LSU AgCenter, Baton Rouge, La.,; Tara P. Smith, Assistant Professor, and Arthur Villordon, Associate Professor, LSU AgCenter Sweet Potato Research Station, Chase, La.

(This article was published in the spring 2009 issue of Louisiana Agriculture.)

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