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Rhizomania is a serious disease of sugar beet, caused by beet necrotic yellow vein virus (BNYVV). The disease can only be controlled by the use of resistant cultivars. The accession Holly contains a single dominant gene for resistance, called Rz. The identification of a locus for resistance that differs from Rz would provide possibilities to produce cultivars with multiple resistance to BNYVV. Inheritance of resistance to BNYVV was studied by screening progenies of crosses between resistant plants of the accessions Beta vulgaris subsp. maritima WB42 and B. vulgaris subsp. vulgaris Holly-1-4 or R104. Observed and expected segregation ratios were compared to elucidate whether the resistance genes in the three accessions are alleles or situated on different loci. STS markers, linked to the genes for resistance, were used to study the segregation in more detail. The results demonstrated that the genes for resistance to BNYVV inHolly-1-4 and WB42 are closely linked. The gene for resistance in R104 is at the same locus as in Holly-1-4, and also closely linked to the gene in WB42. As the Holly resistance gene has been named Rz, the name Rz2 is proposed to refer to the resistance gene in WB42. Consequently, the gene Rz should be referred to as Rz1.

A new classification for the cultivated and weed forms of beet is proposed. The paper includes a short description of concept and rules of the new system for the nomenclature of cultivated plants, as published in the 1995 International Code ofNamenclaturefor Cultivated Plants or Cultivated Plant Code. The classification has an open character, and is formed around the basic elements called culta (singular culton). Culta need not be grouped in a hierarchical way, as in botanical classification (basic elements taxa, singular taxon), but may be grouped according to cultivar. The rclevant culta for beet are the named cultivars and the newly formed cultivar groups.

Inheritance of resistance to beet necrotic yellow vein virus (BNYVV) was studied in segregating F2 and backcross families obtained from crosses between resistant plants of the sugar beet selection Holly-l-4 or the wild beet accession Beta vulgaris subsp . maritima WB42 and susceptible parents . Greenhouse tests were carried out, in which seedlings were grown in a mixture of sand and infested soil . Virus concentrations of BNYVV in the rootlets were estimated by ELISA . To discriminate resistant and susceptible plants, mixtures of normal distributions were fitted to loglo virus concentrations, estimated for segregating Fl, F2 and BC populations of both accessions . The hypothesis that Holly-1-4 contained one single dominant major gene was accepted . For WB42, results fitted with the hypotheses that resistance was based on either one (or more) dominant major gene(s) showing distorted segregation, or two complementary dominant genes, which are both required for resistance . Resistance from WB42 appeared to be more effective against BNYVV than resistance from Holly-l-4 .

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