Christopher A. Clark and Tara P. Smith

The LSU AgCenter sweet potato foundation seed program began operations at Chase, Louisiana, in Franklin Parish in 1949 with an emphasis on providing high-quality seed roots to the Louisiana sweet potato industry. At that time, seed roots were cut, and those found free of interior flesh color mutations were planted in a greenhouse to produce plants for field plantings. Individual plants demonstrating good yield and uniform root size were selected at harvest to use for the following year’s foundation seed production.

In 1999, the foundation seed program was converted to a virus-tested production system for seed roots. Research during the intervening period showed that the viruses that normally accumulate in sweet potato planting material in Louisiana could reduce yields of the Beauregard variety by 25 percent to 40 percent and could also negatively affect shape and skin color of the roots. Since the inception of virus-tested seed production, it has been an ongoing learning process to identify steps to produce planting material that is as free as possible of harmful viruses. This is important for a location where virus-infected sweet potatoes were grown for 50 years and where perennial morningglories, which are alternative hosts for the same viruses, have existed for many generations.

Since 1999, the only sweet potatoes grown on the Sweet Potato Research Station were either derived from true seed (sweet potato potyviruses are not thought to be transmitted through botanically “true” seed), from virus-tested tissue cultures or from seed roots produced at the station during the previous year. However, a study from 2007 to 2009 comparing yield and infection of different generations of seed indicated that foundation seed had from 20 percent to 80 percent reinfection, usually with a single virus. Subsequent generations of seed were nearly all infected with one and often multiple potyviruses.

Four viruses commonly infect sweet potatoes in Louisiana, and all are members of the same group of plant viruses (potyviruses) that are transmitted by aphids or in vegetative planting material. These are: Sweet potato feathery mottle virus (SPFMV) (Figure 1), Sweet potato virus C (SPVC), Sweet potato virus G (SPVG), and Sweet potato virus 2 (SPV2). These viruses are each spread by aphids in a nonpersistent manner, which means that an aphid can acquire the virus from an infected plant within seconds to minutes of feeding and can transmit the virus to a healthy plant immediately. Thus, these viruses can spread very rapidly when infected plants are present. Previous research showed that infection of Beauregard sweet potatoes with any one of these viruses causes minimal effect on the plant, but as the plant becomes infected with two, three or all four viruses, yield declines. Subsequent research conducted on other commercial varieties, including Bayou Belle, Orleans and Evangeline, indicates that they, too, exhibit similar yield declines when infected with multiple potyviruses.

Challenges remain for providing healthy sweet potato plants at the farm level because of the rapid rates of reinfection with the complex of potyviruses that contribute to cultivar decline. Beginning in 2010, efforts were put in place to address reinfection of foundation seed at the station. Several strategies have been employed, including producing seed used for research at an isolated off-site location, aggressive herbicidal control of morningglories focusing on perennials, and in 2014 and 2015, removing plants with virus symptioms in selected seed-production fields. Since 2012, virus-tested plants grown in the greenhouses at the station have been used to plant small fields at the Northeast Research Station in St. Joseph, where sweet potatoes are not routinely produced. Roots from these fields were harvested and planted for use in research plots at the Sweet Potato Research Station the following year to produce plants for research. These seed roots were tested in 2012, 2013 and 2014 by grafting them to the Brazilian morningglory, Ipomoea setosa, to test for the presence of sweet potato viruses. No virus reinfection was detected in seed produced for research at the Northeast Research Station during those seasons.

Shoots from sample lots of seed roots produced at the Sweet Potato Research Station were also grafted onto indicator plants during each winter to determine incidences of reinfection. In 2012-14, this was done by grafting on I. setosa. In the meantime, a test that amplifies and detects the nucleic acid of viruses was found not only to be far less time consuming but also slightly more accurate in detecting the potyviruses in storage roots. This approach replaced grafting in 2015. Ten of 18 seed lots from different fields and cultivars produced in 2013 had seed roots in which potyviruses were detected by indexing on the indicator host Ipomoea setosa, and incidence ranged from 3 percent to 42 percent of roots (Table 1).

During 2014, 24 plants with virus symptoms were removed from seed fields from early June to late August. These plants were near an infected perennial morningglory growing along a highway right of way. Three of 18 seed lots produced in 2014 included infected roots with incidence of 1 percent to 2 percent in each of those lots (Table 1). No symptomatic plants were found in seed fields in 2015, and only one seed lot included a single infected root (Table 1). Efforts are ongoing to identify best management practices for producing virus-tested foundation seed at the station.

In 2015, a National Clean Plant Network for Sweetpotatoes (NCPN-SP) was formed as part of a larger group of networks for other specialty crops, including, grapes, fruit trees, citrus, hops, berries and roses. Changes in foundation seed operations and membership in NCPN-SP have positively affected outcomes in the AgCenter sweet potato foundation seed program. Specifically, through efforts associated with the National Clean Plant Network, production of virus-tested sweet potato vine cuttings of AgCenter-developed cultivars is increasing. This allows the station to increase the flow of clean material to the Louisiana industry and to cooperators in other states that lack the infrastructure to support production of virus-tested plants. AgCenter researchers are also enhancing virus-testing capabilities by implementing a nationally standardized protocol for target viruses adopted at the 2015 NCPN-SP virus workshop and expanding the capacity to use existing assays to more thoroughly test plants in the foundation seed system.

The National Clean Plant Network is an outlet to share techniques for tissue culture and virus indexing and also to share clean germplasm. In addition, it is providing funds for critical needs to improve infrastructure and foundation seed production systems in Louisiana and other clean plant centers across the United States. In 2015 and 2016, supplemental lighting was installed in the Sweet Potato Research Station tissue culture receiving greenhouse, which will allow for a more rapid increase of plants (Figure 2). Through extension and outreach, the network provides an additional framework to provide information on the value of clean plant systems to AgCenter stakeholders, the sweet potato industry and end users.

Christopher A. Clark is a professor in the Department of Plant Pathology and Crop Physiology, and Tara P. Smith is director of the Northeast Region and research coordinator at the Sweet Potato Research Station.

Figure 1. Sweet potato plants showing symptoms characteristic of potyvirus infection. Photo by Christopher A. Clark

Figure 2. Research is enhanced through the new and improved greenhouse at the Sweet Potato Research Station. Photo by Tara P. Smith

Table 1. Results of indexing for potyviruses of sample lots of seed roots produced at the Sweet Potato Research Station.