Project Number: 6607-21000-010-07
Project Type: Specific Cooperative Agreement

Start Date: Aug 22, 2007
End Date: Feb 28, 2009

Objective:
The objective of this research is to identify orange, moist-fleshed sweetpotato genotypes that are resistant to sweetpotato leaf curl virus (SPLCV). A secondary benefit will be to develop systems that might be used for developing resistance to other viruses in sweetpotato.

Approach:
Evaluations will be conducted in two stages. An initial screening will be conducted by micrografting tissue culture plantlets of virus-tested accessions from the germplasm repository with nodes from plantlets infected with isolate SWFT-1 of SPLCV-US. After nine weeks, leaves will be removed, frozen leaf tissue will be ground to a fine powder in liquid nitrogen using a mortar and pestle and total DNA will be extracted using GenElute¿ Plant Genomic DNA Miniprep Kit (Sigma-Aldrich, St. Louis, MO), according to the manufacturers¿ directions. The titer of SPLCV will be determined by real-time PCR using the method of Kokkinos and Clark (2006). To limit the number of accessions to a manageable number, we will concentrate on approximately 50 high-yielding genotypes that might be of greatest value in efforts to incorporate resistance into horticulturally acceptable cultivars. A group of standard genotypes will be included: susceptible = Bienville and 1-2 others identified in the first phase, intermediate = Beauregard, and resistant = Xushu-18 and Picadito. Genotypes identified as suppressing replication of SPLCV in phase 1 (i.e. having low titer estimated by real-time PCR) will be regenerated and grown in the greenhouse to produce storage roots large enough for core-grafting and subsequent use as `seed¿ roots. Preliminary studies found that the most severe symptoms occurred in sprouts in plant beds and thus it is important to evaluate field performance in beds as well as in the production field. As many roots as possible up to a maximum of 10 will be core-grafted with cores from storage roots of Beauregard infected with isolate SWFT-1 of SPLCV. Control and core-grafted roots will be stored for 2-3 months and bedded in field beds in March of year 2. Control and core-grafted roots will be bedded separately and once the black plastic mulch is removed, the beds will be covered with AgroFabric 19® to exclude insect vectors of viruses. Vine cuttings from these beds will be used to transplant replicated field plots to compare yield of SPLCV-infected with control plants of each genotype. A single composite leaf sample will be collected at 35 days after transplanting from the SPLCV-infected plants of each genotype, DNA extracted, and the titer of SPLCV estimated by real-time PCR to verify that the titers in the field correspond to those previously estimated in tissue culture.