Insect, disease, and weed management

0945. Relationships between preharvest conditions and increased susceptibility of sweetpotatoes to Rhizopus and bacterial soft rots in Louisiana and North Carolina. (back)

B.A. EDMUNDS¹*, C.A. Clark², G.J. Holmes¹, E.D. Gray^2
1. NCSU, Dept of Plant Path.;². LSU AgCenter, Dept of Plant Path. & Crop Physiology

In 2004-06, the interaction of preharvest factors and susceptibility to Rhizopus (Rhizopus stolonifer) and bacterial (Erwinia chrysanthemi) soft rots was studied in 73 Louisiana and 124 North Carolina sweetpotato fields (cv. Beauregard). Preharvest parameters (including pesticide use, soil nutrients, and weather) were recorded for each field and roots were harvested. After 100 days storage, one-half of the roots were uniformly wounded and inoculated with a R. stolonifer suspension. The remaining roots were stab-inoculated with a pipet containing a suspension of E. chrysanthemi cells. Preharvest parameters were related to disease incidence after 7 days using Pearson correlations (p<0.05). There was a wide range in susceptibility to both diseases found in roots from different fields in all three years. A negative correlation was found between soil phosphorus index and R. stolonifer in all 3 years in LA. This relationship was not found in North Carolina and may be due to the differences in the range of the soil phosphorus index (Louisiana: mean=51, range=6-104; North Carolina: mean=141, range 35-279). In both states, seasonal soil moisture levels correlated negatively to E. chrysanthemi and positively to R. stolonifer susceptibility. The results suggest there are complex interactions of field variables that have a profound influence on susceptibility to both diseases and that the two pathogens respond differently to these variables.

1045. Sweet Potato Leaf Curl Virus: Efficiency of Whitefly Transmission (back)

Alvin M. Simmons, Kai-Shu Ling*, Howard F. Harrison, D. Michael Jackson U.S. Department of Agriculture, Agricultural Research Service, U.S. Vegetable Laboratory, Charleston, SC 29414.

Sweetpotato whitefly, Bemisia tabaci (Gennadius) is a widespread pest. It is highly adaptive and feeds on taxonomically diverse species of plants. This whitefly damages plants directly by feeding on leaves, but the problem is compounded because B. tabaci is known to vector numerous plant viruses, including Begomoviruses (Geminiviridae). Sweet Potato Leaf Curl Virus (SPLCV) is transmitted by this whitefly. However, information on the efficiency of its transmission by B. tabaci is lacking. Laboratory experiments were conducted on viral transmission, acquisition, and retention of the SPLCV on Ipomoea setosa or sweetpotato seedlings by B. tabaci. Symptom observations on indicator plants (I. setosa) and Real-time PCR techniques were used. Data on transmission, acquisition, and retention will be presented. The findings will provide knowledge on the epidemiology of SPLCV in the sweetpotato field.

1100. Efficient Regeneration and Selection of Virus-free Sweetpotato Plants from Sweet Potato Leaf Curl Virus Infected Materials and Their Effects on Yields in Field Trials. (back)

Kai-Shu Ling^1*, D. Mike Jackson¹, Howard Harrison1, Zvezdana Pesic-VanEsbroeck², Mary Hoy³ and Christopher A. Clark³.
¹U.S. Department of Agriculture, Agricultural Research Service, U.S. Vegetable Laboratory, Charleston, SC 29414.
²Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695.
³Department of Plant Pathology and Crop Physiology, Louisiana State University, Baton Rouge, LA 70803.

Sweet potato leaf curl virus (SPLCV) is an emerging virus disease in sweetpotato (Ipomoea batata) in the U.S. The incidence of SPLCV infection on sweetpotato increased dramatically in recent years due to the explosion of whitefly, Bemisia tabaci (Gennadius) populations. Among several sweetpotato viruses in the U.S., SPLCV is considered to be the most detrimental to production. In this study, we applied a meristem shoot-tip culture technique to generate virus free plants from 30 SPLCV infected heirloom and commercial cultivars and USVL breeding lines. Numerous plantlets were regenerated from specimens of the 30 genotypes from the USPI collection. Individual plants were considered virus-free if disease-like symptoms were not observed on grafted indicator plants (Ipomoea setosa) and Real-time PCR assays were negative. SPLCV-free and SPLCV-infected plants of five sweetpotato cultivars and one breeding line were included in two field trials at Charleston, SC in 2007 to assess the impact of the virus. Within cultivars, virus-cleaned plants usually outperformed virus-infected plants.

1200. Differential Clomazone, Herbicide Tolerance among Sweetpotato Genotypes. (back)

Howard F. Harrison*, Jr. and D. Michael Jackson, U.S. Vegetable Laboratory, ARS-USDA, Charleston, SC.

Clomazone (Command 3ME) is a broad spectrum preemergence herbicide that is registered for use in sweetpotato [Ipomoea batatas L. (Lam.)]. It controls several important annual weeds that are not controlled by the other sweetpotato herbicides. Following clomazone application for weed control in the plant beds of the U.S. Vegetable Laboratory breeding program, we observed genotypic differences in response to the herbicide. The most susceptible clones exhibited extensive foliar bleaching; whereas, bleaching was not observed on the most tolerant genotypes. All clones were rated for bleaching, and 12 with differential response to the herbicide were selected for further evaluation. Most of the highly susceptible clones that we identified originated from the USVL program. In a greenhouse concentration-response experiment, the most tolerant clones (Beauregard and SC 1149-19) were injured less by 16 mg clomazone/kg potting soil than the most susceptible clones were by 1 mg/kg. This indicates that there are substantial differences in clomazone tolerance among sweetpotato clones. Similar differences in foliar bleaching were observed in a field study; however, clomazone injury did not appear to greatly reduce yields of even the most susceptible clones. Susceptibility to clomazone is undesirable, and clones exhibiting the trait should be removed from sweetpotato breeding programs.

P3. Jackson, D. M.* and H. F. Harrison, Jr. 2008. A conservation tillage system for sweetpotato: Effects on pests and beneficial insects. (back)

In conventional tillage systems, sweetpotatoes are grown on beds formed from bare soil, which can lead to erosion before the expanding root system stabilizes the soil. Conventional tillage systems also favor the establishment of annual weeds until the sweetpotato canopy is fully established. Conservation tillage systems for sweetpotato may help alleviate these problems. Therefore, we grew three genotypes of sweetpotato (Beauregard, Ruddy, and SC1149-19) in either conventionally tilled plots or in a killed-cover crop tillage system at the U. S. Vegetable Laboratory, Charleston, SC, in 2002 2004. The four tillage treatments were: (1) conventional tillage, hand-weeded (CT-HW), (2) killed-cover crop, hand-weeded (KCC-HW), (3) conventional tillage, weedy (CT-WE), and (4) killed-cover crop, weedy (KCC-WE). In the fall, one-half of the pre-formed beds were planted to a winter cover crop of an oat and crimson clover mixture, while the other one-half of the beds were left fallow then re-bedded before sweetpotato slips were planted. One-half of each 4-row plot (100 plants per plot) was hand-weeded while the other one-half was not. The center-two rows of sweetpotatoes from each plot were harvested, weighed, and rated for insect damage. The insect resistance of Ruddy held up well under the killed-cover crop conditions, and this cultivar had significantly higher percent of clean roots and lower infestations by soil insect pests than the two susceptible genotypes. In general, injury to sweetpotato roots by soil insect pests was significantly lower in the KCC plots than in the CT plots. Pitfall traps and fire ant sampling indicated that more insect predators were present in the killed cover crop plots.

P4. Sweet Potato Leaf Curl Virus: Virus Reservoir in Species of Wild Morning Glory

Kai-Shu Ling*, Howard Harrison, Alvin Simmons, and D. Mike Jackson.
U.S. Department of Agriculture, Agricultural Research Service, U.S. Vegetable Laboratory, Charleston, SC 29414.

Recent increases in populations of the Sweetpotato leaf curl virus (SPLCV) vector, the sweetpotato whitefly, Bemisia tabaci (Gennadius), led to a dramatic increase in the disease in sweetpotato (Ipomoea batatas). Knowledge of crop or weed species that occur in sweetpotato growing areas and can serve as sources of the virus is critical for devising effective disease management strategies. In this experiment, over 120 plant species consisting of diverse wild and cultivated plant species were tested as alternate hosts. SPLCV infection was determined by symptom expression on the tested plants as well as with Real-time PCR assays. The collection included 49 Ipomoea species, and 41 of these proved to be the alternate hosts for SPLCV. SPLCV was detected in several wild morningglories collected in Charleston County during 2007. This suggests that cultivated or indigenous Ipomoea species may serve as a source for SPLCV infestation of clean sweetpotatoes.