Sweet Potato Disease Research Conducted at Burden Research Plantation Christopher A. Clark and Mary W. Hoy Department of Plant Pathology & Crop Physiology

Major Areas of Research Disease resistance –Lines from the LSU AgCenter breeding program are screened every year for resistance to major diseases. Sweetpotato breeding is a team effort involving people in the LSU AgCenter Depts. of Horticulture, Entomology, Plant Pathology & Crop Physiology, and the Sweet Potato Research Station at Chase. –Sweet potato germplasm from the U.S. collection is being evaluated to find a source of resistance to viruses. Cultivar decline –To determine the role of pathogens in the decline in yield and quality of cultivars (varieties) and how to reduce this decline.

Disease Resistance Fusarium wilt and root-knot nematode reactions are assessed in greenhouse trials on the Baton Rouge campus. Resistance to storage rots is evaluated using storage roots produced at Burden. These are inoculated with four different pathogens after harvest. Resistance to Streptomyces soil rot is evaluated in a plot in which the soil is infested and maintained at pH>6 to favor disease.

Disease Reactions of Important Sweet Potato Cultivars Developed at the LSU AgCenter Cultivar Year Released Strep. Soil Rot Fus. Wilt Root knot Bact. Soft rot Fus. Root rot Rhiz- opus Centennial1960SI-RSR Jasper1974RRI-RI Travis1980I-RI S Beauregard1987I-RRSSRR Hernandez1992I-R I II Bienville2002RRRVRR S = susceptible, I = intermediate, R = resistant, and V = variable

Advanced Lines for 2002 Soil Rot Data from 2001 Plot at Burden LineUS #1 YieldTotal Yield % Roots with soil rot Rating Beauregard I-R Jewel S L I-R L I L R L I-R L I-R L I-R L R L I-R L I-R

Fusarium Wilt and Root-knot Nematode Evaluations - Campus Greenhouse LineWilt IndexWilt RatingMi eggs/plant Root knot rating Beauregard4R584,000S Jewel54,800R* Porto Rico68S568,000S L99-350R2,200R L00-57R720HR L00-812R800HR L S4,120R L00-644R0HR L00-752R464,000S L R8,440R L00-992R24,000I L R Wilt Index: 0 = all plants healthy, 100 = all plants dead. * Jewel was considered as resistant early in the search for resistance to root knot in sweetpotato, but since then, greater levels of resistance have been found.

Storage Rot Evaluations – 2000 Burden Plantation LineBSRFRRJBRRSR Beauregard18S1003R Jewel1R1203R Excel8R112567S Bienville13I950R L I8031I L S1606R BSR = Bacterial soft rot, left column = mean lesion size in mm FRR = Fusarium root rot, mean lesion size in mm JBR = Java black rot, mean % roots totally decayed RSR = Rhizopus soft rot, left column = mean % roots totally decayed Ratings (S,I,R) are only given where sufficient disease developed to differentiate the reactions of the known susceptible and resistant standards. Some years, there is not sufficient disease development to allow this rating, as in the case of FRR and JBR in 2000.

Virus Resistance Objective – to develop approaches for screening sweetpotato germplasm for resistance to several viruses that infect sweetpotato and to use these methods to find sources of resistance to specific viruses that occur in Louisiana, such as Sweet potato feathery mottle virus (SPFMV) and the associated complex of Potyviruses, and Sweet potato leaf curl virus (SPLCV).

Virus Resistance Procedure for 2002 Plants used: –virus-tested plants of ~30 traditional U.S. cultivars –seedlings from 4 crosses between parents reported to have virus resistance and parents considered horticulturally acceptable by U.S. standards Methods: –A single 3-plant plot was planted of each line to be tested. On July 10, each plant was cut back and then one plant was grafted with a sweetpotato scion infected with Sweet potato leaf curl virus, another plant was grafted with a source of russet crack, and the third plant was left as a control. Evaluations: –Severity of symptoms on each plant, whether the control becomes naturally infected, and yield and appearance of storage roots.

Example of a Graft used for Virus Inoculation

Cultivar Decline It has been observed on many crops, especially those such as sweetpotato that are propagated vegetatively, that yields of cultivars (varieties) may decline over the years after they are first released. The graph on the next page illustrates this decline for the three most popular sweetpotato cultivars in the U.S. There may also be a decline in certain quality attributes such as color of skin or shape of the storage roots. Foundation seed programs, such as the one at the Sweet Potato Research Station in Chase, LA, have for many years worked to eliminate mutations, one cause of cultivar decline. Recently, they have begun producing ‘virus-tested’ seed –Virus-tested (VT) seed is generated from tissue culture plants that have been tested for known viruses and found to be apparently free of these viruses. Since it is not possible to prove the absence of all viruses, it is called virus-tested not virus-free.

Cultivar Decline in USA MT/Ha

Cultivar Decline Research Objectives –To determine the role of individual viruses and combinations of viruses in causing decline. –To determine the role of the bacterial soft rot pathogen, Erwinia chrysanthemi, in causing decline and also in causing ‘souring’ when soil becomes flooded. –To compare the rate of decline in different cultivars from different countries.

Role of Viruses in Decline In collaboration with Dr. Rodrigo Valverde, we are trying to identify the viruses that occur in sweetpotato, develop methods to detect them in plants, and to survey for their frequency in the field. Viruses found so far include: –SPFMV in almost 100% of samples. –Two other potyviruses, which are similar to SPFMV and appear to be common. They are temporarily designated LSU-1 and LSU-5. –SPLCV in the ornamental sweetpotatoes with black foliage (Blackie, Ace of Spades, and Black Beauty) and in some breeding lines. –“C-6”, an uncharacterized virus found in most of the same black ornamental sweetpotatoes as SPLCV (Blackie, Ace of Spades, and Black Beauty).

Once viruses are isolated, they are inoculated into Beauregard sweetpotatoes that are ‘virus-tested’ to determine, in field plots, the effects the viruses have alone and in combination on yield and quality. –To permit us to compare plants infected with known viruses or combinations of viruses, the planting material is produced in a greenhouse during the winter under conditions controlled to prevent unwanted infection. –Plots of infected and noninfected sweetpotato are separated in the field by rows of soybeans and the whole field is treated with a systemic insecticide to prevent aphids or whiteflies from transmitting viruses between plants in different plots.

Example of Field Plot

Virus Effects on Yield of Beauregard Results of two replicated tests in 2000, VT = virus-tested control.

Effects of Viruses - Results In numerous tests over many years, SPFMV by itself has had little measurable effect on yield and quality. In six tests over 3 years, SPLCV has consistently caused yield reductions similar to those in the preceding graph, and also has caused grooving and skin darkening on Beauregard roots. Only one year of reliable data has been collected for LSU-1, but it appears that combinations of potyviruses such as SPFMV and LSU-1, may cause yield reductions and lighter root skin color even though individually the same viruses have little effect. There is not enough data to determine the effects of C-6. Tests are continuing on the potyvirus group and C-6 viruses.

Role of Bacterial Root and Stem Rot Caused by Erwinia chrysanthemi

Introduction Erwinia chrysanthemi is a bacterium that causes soft rot diseases on many plants. These diseases typically are favored by conditions such as flooding in which the plant is deprived of oxygen. Beauregard is more susceptible than other sweetpotatoes to bacterial soft rot. The problem is made more difficult by the fact that the bacterium can be latent in roots or vines that are used for planting until conditions favor disease development. When flooded, especially in Aug-Sept while soil is hot, sweetpotatoes develop a syndrome known as souring. The symptoms can range from mere discoloration of the skin to collapse of areas of the cortex of the storage root to complete soft rotting of the root which is accompanied by a sour smell. The role of pathogens in this phenomenon has not been completely determined. We are investigating the role of Erwinia chrysanthemi alone and in combination with a soil-borne yeast-like fungus, Geotrichum, in inducing this syndrome.

Objectives To develop methods to detect latent populations of the bacteria. To determine what effects E. chrysanthemi has on Beauregard production and how environment affects disease development. To determine how the bacteria spread in sweetpotato production system. To use information from studies above to develop management systems.

Methods Two types of tests have been conducted since 2000: –Effect of Erwinia chrysanthemi - Plants were inoculated at the time of transplanting with either high or low concentrations of E. chrysanthemi and the crop was allowed to develop under prevailing weather conditions. –Role in Souring - Healthy plants were transplanted and grown to near maturity under prevailing conditions. One week before harvest, the crowns of plants were flooded with inoculum of E. chrysanthemi and/or Geotrichum and then the plots were flooded for one week. Tests were harvested in August while soil temperatures were high and in October when they were lower in both 2000 and 2001.

Effect of Erwinia chrysanthemi on Yield Drought conditions 3% stem rot 100% stand Vines of inoculated plants appeared stunted and chlorotic.

Effect of Erwinia chrysanthemi on Yield – 2001 early Flooding rain from Tropical Storm Allison after transplanting. Stem rot killed many plants within 2 weeks of rain % stand

Effect of Erwinia chrysanthemi on Yield – 2001 late Planted after TS Allison. No stem rot. 100% stands. No apparent difference in vine growth.

Total Yield of Sound Sweetpotatoes after inoculating with Erwinia chrysanthemi and/or Geotrichum and flooding - August, 2000

Results –Effect of Erwinia chrysanthemi – Weather conditions were dramatically different for each of the three tests conducted and results were also dramatically different. The heat/drought stress of 2000 appeared to result in stunting of infected vines, while the flooding stress associated with TS Allison in early June, 2001 resulted in high mortality of plants due to stem rot. The second planting in 2001 was not exposed to either of these stresses and there were no apparent effects of inoculation with the bacterium. –Role in Souring – In Aug., 2000 when soil temperatures exceeded 100 F, significantly more roots of Beauregard broke down when inoculated with both pathogens than in any of the other treatments. Some souring occurred in all treatments in all tests and no other significant differences were observed.

International Cultivar Decline Study Sweetpotato researchers from several countries collaborated in planning a study to compare selected cultivars for decline in different locations. –Cultivars included: Beauregard (LA-USA), Jonathan (Peru), NC-262 (Malaysia), Tanzania (East Africa), and Xushu-18 (China). We have added to our plots Bienville (LSU) and Picadito (So. FL and Cuba). Each participant began with virus-tested plants of each cultivar from the same source. –These are planted in the field and monitored for changes in yield and %virus infection progressing from VT through generations (G1, G2…) in subsequent years in the field. It is hoped that at a later time it also will be possible to determine mutation occurrence.

Total Yields of International Cultivar Decline Study – Burden, 2001

Virus Reinfection of International Decline Cultivars

Preliminary Observations It would take many tests over many years to accurately differentiate yield decline, but it appears that some cultivars such as Beauregard decline somewhat faster than others such as Xushu-18. When grown in a farmer’s field, most of the cultivars became reinfected with at least one virus during the first year. However, some such as Tanzania and Picadito had lower rates of reinfection. These will be included in studies on resistance to viruses.