1Research Institute of Crop Production, Division of Phytomedicine, Drnovska 507, 161 06 Praha 6, Ruzyne, Czech Republic

Background and objectives
After a reliable method for detection of plum pox virus (PPV) in flowers and leaves of peach trees was worked out [1], research on resistance of peach cultivars to PPV started in the Czech Republic in 1994. Trees of 34 peach and two nectarine cultivars were evaluated visually and serologically for resistance to PPV [2]. Peach cultivars were grouped into four categories: medium resistant, tolerant, medium susceptible and highly susceptible. Correlation of susceptibility of peach cultivars to PPV and gustatory properties of fruits harvested from infected trees was determined.

Materials and methods
Trees of peach and nectarine cultivars were grown in a 15-year-old orchard and naturally infected with PPV. The presence of PPV in all tested trees was proved by ELISA. The intensity of symptoms in leaves and fruits was evaluated visually. The relative concentration of PPV was established in flowers by determination of the lowest dilution with positive reaction using ELISA. Fruits from individual cultivars were harvested and gustatory properties determinated in five fruits of every peach or nectarine cultivar evaluated.

Results and conclusions
Peach cultivars Candor, Envoy, Favorita Morettini 3, Flamencrest, Harcrest, Harmony, Maycrest, Spring Lady, Triestina and Velvet were rated as medium resistant to PPV. Most of cultivars were good to taste, sweet and aromatic. Medium sweet and less striking were F. Morettini 3, Harcrest, Harmony and Triestina. Cultivars Canadian Harmony, Harken, Rosired 1, Springrest and Suncrest were found to be tolerant to PPV. All tolerant cultivars were good-tasting, sweet and aromatic. Cultivars Springbrite, Sunhigh, Maria Aurelia (nectarine), Ellerbe, NJC 102, Madison, Adriatica, Harson and Veteran were characterized as medium susceptible to PPV. Fruits of Springbrite, Sunhigh and NJC 102 were good-tasting, sweet and aromatic; the fruits of other cultivars were rather standard or substandard in taste. Cultivars Ambergold, Fortuna, Spotlight, Sunhaven, Record aus Alfter, Harbinger, Harbrite, Redhaven, Somervee, Catherina, NJC 106 and Maria Emilia (nectarine) were characterized as highly susceptible to PPV. The taste of fruits of most of these cultivars was substandard, less sweet and insipid. Only the fruits of cultivar Record aus Alfter were very good, sweet and winey tasting.

The best gustatory properties of fruits were found to be in positive correlation with resistance or tolerance of peach cultivars to PPV. Fruits from infected trees of most of medium resistant and tolerant cultivars to PPV were good to taste, sweet and aromatic, while those of highly susceptible cultivars used to be best of substandard quality.

1. Polak J, 1995. Zeitschrift fur Pflanzenkrankheiten und Pflanzenschutz 102,16-22.
2. Polak J, 1998. Acta Horticulturae, in press.