Verticillium Wilt Resistant Germplasm-Release of Clone LRC18-21 and Derivatives

American Journal of Potato Research, Jul/Aug 2004 by Lynch, D R, Chen, Q, Kawchuk, L M, Driedger, D

ABSTRACT

Verticillium wilt is an important disease affecting potato tuber yield and quality. In North America the major commercial cultivars are susceptible, and management strategies for control rely mainly on soil fumigation and crop rotation. We describe a Solarium chacoense clone (LRC18-21) with single gene (V^sub c^) resistance to Verticillium wilt as well as germplasm derived from the original clone. LRC18-21 (diploid S. chacoense), LRC418-21 (tetraploid S. chacoense) and LRC373-5 (diploid S. tuberosum/chacoense hybrid) and LRC4373-5 (tetraploid S. tuberosum/chacoense hybrid) have been released to Potato Introduction Station (NRSP-6, Sturgeon Bay, WI) for distribution to interested parties. Transfer of the V^sub c^ gene to commercial cultivars could provide effective and economical control of Verticillium wilt.

Accepted for publication 23 April 2004.

ADDITIONAL KEY WORDS: colchidne, 2n pollen, tuberosum-wild species hybrids, unilateral sexual polypoidization

INTRODUCTION

Verticillium wilt, caused by Verticillium albo-atrum Reinke and Berthier or Verticllium dahliae Kleb., is a common vascular disease causing severe yield and quality losses in many crops. In a recent survey Verticillium wilt was ranked as the most important disease of both seed and commercial crops and the second greatest constraint on tuber yield in North America (Powelson and Rowe 1993). Symptom expression and the magnitude of stem invasion by the pathogen are poorly correlated (Corsini et al. 1988, Mohan et al. 1990; Lynch et al. 1997) and it is important to distinguish between tolerance (no visual symptoms, often associated with late vine maturity, but significant stem invasion) and resistance (no visual symptoms with low levels of stem invasion). Tetraploid sources of multi-genie resistance in North America include A66107-51, A68113-4 (Corsini et al. 1990) and B0169-56 (Goth et al. 1994). Lynch et al. (1997) identified a single gene (V^sub c^) in a clone (LRC18-21) of the diploid wild species Solatium chacoense that confers high levels of resistance to Verticillium wilt. The original diploid clone, a tetraploid clone (ploidy manipulated using colchicine), and hybrids between haploids of S. tuberosum and the wild species clone have been released to the Potato Introduction Station (NRSP-6, Sturgeon Bay, WI) for distribution to interested parties. We provide a brief description of the characteristics of the original germplasm and the derivatives.

DESCRIPTION

The identification and confirmation of the single gene resistance (dominant) in LRC18-21 is described by Lynch et al. 1997. Crosses between LRC18-21 (male) and haploids of S. tuberosum cv Atlantic (female) were made in 1998 and segregating populations, grown out in the field in Vauxhall (Alberta, Canada) in 1999, were selected for tuber production (four haploid-wild species hybrids with 15 to 30 genotypes per family). The vines of genotypes that produced tubers were screened for resistance (24 plants per clone) to Verticillium albo-atrum as described by Lynch et al (1997) (Table 1). Genotypes that showed no foliar symptoms and low stem colonization (

Unilateral polyploidization using 2n gametes is an efficient way to produce tetraploid populations from diploid S. tuberosum haploid-wild species hybrids (Peloquin et al. 1991). Therefore we screened the hybrids identified in Table 1 for pollen viability (acetocarmine stained) and 2n pollen as described by Chen et al. (2003). Results presented in Table 2 indicate that while 2n pollen production does occur in most of our resistant hybrids, except for LRC373-1 (11.3%), the phenomenon occurs only at low levels. Attempts to cross the resistant diploid hybrids with tetraploid cultivars and breeding clones met with limited success (very low seed set and no tetraploid hybrids generated). Much higher levels of 2n pollen (up to 28%) occurred in some of the susceptible hybrids (data not shown), which suggest that by increasing the size of the population, higher 2n pollen-producing resistant progeny could be identified.

Since the resistance is controlled by the single V^sub c^ gene, an attractive option to facilitate use of the germplasm is to produce tetraploid clones by chromosome doubling. Tetraploid derivatives of the original diploid clone, as well as selected haploid-wild species hybrids, were produced in 2002 using colchicine and a tissue culture technique developed by Chen et al. (unpublished). Ploidy was confirmed by root-tip chromosome counts (Chen et al. 2003). Transmission of the resistance to the tetraploid germplasm was confirmed as outlined by Lynch et al. 1997. The tetraploid derivatives have been successfully used as both male and female parents in crosses with tetraploid cultivars and breeding clones.

The glycoalkaloid content of S. chacoense is less predictable than that of S. tuberosum, and while some genotypes only contain chaconine and solanine, others may contain numerous less common glycoalkaloids (Osman et al. 1976; Osman 1980; Sinden et al. 1986; Sanford et al. 1996). The original diploid clone (LRC18-21) plus a number of the haploid-wild species hybrids in our study were selected to determine the glycoalkaloid composition and concentration and therefore assess potential hazards in the utilization of the germplasm. The glycoalkaloids were identified by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) as outlined by Driedger (2002) and quantified using a colorometric method (Coxon et al. 1979). The composition and concentration of glycolakaloids in the germplasm is included in Table 3.