Partial Resistance to Erwinia carotovora subsp. carotovora and Plant Vigour among F1 Hybrids of Zantedeschia Cultivars

The potential of breeding Zantedeschia cultivars for resistance to soft rot caused by Erwinia carotovora subsp. carotovora (syn. Pectobacterium carotovorum subsp. carotovorum) was determined. Hybrids of six cultivars (‘Back Magic’, Galaxy’, Pink Persuasion’, Sensation’, ‘Treasure’ and ‘Florex Gold’) were screened using a non-destructive resistance test (leaf disk test) and scored different levels of susceptibility. Some hybrid families were less susceptible than the very susceptible reference ‘Florex Gold’, while all hybrid families were more susceptible than the partial resistant reference Z. rehmannii. The half-sib family of ‘Pink Persuasion’ was more resistant, but less vigorous than the half-sib family of ‘Florex Gold’. We concluded that resistance and vigour may be negatively correlated in the deployed cultivars and that resistance should be bred from other sources than the used cultivars. INTRODUCTION Bacterial soft rot caused by Erwinia carotovora subsp. carotovora (syn. Pectobacterium carotovorum subsp. carotovorum) occurs worldwide in many crops. This soil-borne, facultative anaerobic pathogen causes maceration and rotting of parenchymatous tissue of all plant organs, eventually resulting in plant death (Pérombelon and Kelman, 1980). The pathogen is difficult to restrain because of many reasons, such as absence of effective bactericides (Blom and Brown, 1999), its genetic variability (Avrova et al., 2002; Gardan et al., 2003), its wide host range, its broad array of virulence factors and because infection can be latent (Pérombelon, 2002). It is an important disease in Zantedeschia spp. and the major disease in cultivars of the section Aestivae (Kuehny, 2000; Wright and Burge, 2000). Plants turn yellow when disease has initiated, produce a foul smell and can become completely macerated resulting in plant death within a few days (Wright, 1998). Bacteria are spread mechanically during cultivation and symptoms can become visible during all stages of plant growth and development. Conditions are favourable for soft rot when plants are under stress during low soil aeration, high temperature or high relative humidity (Funnell and MacKay, 1999; Wright and Burge, 2000). Cultural measures such as drainage, mulching, soil ventilation and time of tuber lifting can reduce disease development (Funnell, 1993; Wright and Burge, 2000; Wright et al., 2002), but better control of the disease could be achieved by the use of resistant cultivars. Most commercial cultivars have been developed from interspecific hybrids within the section Aestivae, mainly of Z. albomaculata, Z. elliotiana, Z. rehmannii and Z. pentlandii (Funnell, 1993). The level of genetic control of the resistance should be determined in order to value the potential of this resistant germplasm for breeding. The aim of the present study was to determine whether resistance is under genetic control and if it is possible to breed for resistance to soft rot using cultivars as parents. To this end, hybrids were developed of six cultivars of the section Aestivae. Resistance levels of offspring were determined using a non-destructive leaf disk test. Proc 21 IS on Breeding Ornamentals, Part II Eds: G. Forkmann & S. Michaelis Acta Hort 651, ISHS 2004 64 MATERIALS AND METHODS Plant Material Hybrids were retrieved after hybridising six cultivars in all directions (‘Black Magic’, ‘Galaxy’, ‘Pink Persuasion’, ‘Sensation’, ‘Treasure’ and ‘Florex Gold’). Plants were cultivated as described by (Snijder et al., 2003) with the only modification that tubers of the parents were treated with gibberellic acid (100 ppm) before planting, as is common practice in commercial production. Tubers of the two-year-old seedlings (Table 1) were not treated with gibberellic acid before planting, because gibberellic acid can decrease the number of leaves (Brooking and Cohen, 2002), while leaves were needed for the resistance test. Growth Characteristics Growth characteristics (plant length, number of leaves and chlorophyll content) were observed at two months after planting. The tuber weight was determined during planting. Chlorophyll content was determined of the youngest leaf by a SPAD-502 instrument (Minolta Camera Ltd.). This instrument estimates leaf chlorophyll content in situ, by transmitting light through a leaf at wavelengths of 650 and 940 nm. The signal of 650 nm is absorbed by chlorophyll, while the signal of 940 nm is not. The ratio of transmittance of both signals through the leaf is a measure for the amount of chlorophyll, resulting in a unitless SPAD (Soil Plant Analysis Development)-value. Three measurements were made on the abaxial side of the upper half of the oldest leaf, two months after planting and averaged to reduce variability (Adamsen et al., 1999). Experimental Procedure Hybrids were analysed for resistance to bacterial isolate E. carotovora subsp. carotovora PD 1784 (obtained from the Dutch Plant Protection Service) in their second growing season using a leaf disk test, with which individual plants can be tested without being destructed (Snijder et al., 2003). Two to four leaves of each seedling were evaluated in one experiment, which was split into 15 plots. The observations were done at three or four days after inoculation, depending on the progress of the infection on the reference genotypes ‘Florex Gold’ (susceptible) and Z. rehmannii (partial resistant), which were represented in all plots with two leaves. Resistance Test and Statistical Analyses The resistance test (Fig. 1) and its statistical analysis were done as described by (Snijder et al., 2003). Results from the different replicates and plots were related to ‘Florex Gold’, resulting in an index for macerated leaf disk area (M), where ‘Florex Gold’ was set at 100 as suceptible reference. Growth characteristics were analysed by generalised linear models (GLMs). The chlorophyll content (measured in SPAD units) and plant length were estimated assuming a normal distribution, tuber weight by assuming a lognormal distribution and the number of leaves assuming a Poisson distribution. All analyses were done with the statistical software package Genstat 6, release 6.1 (GenStat, 2002). RESULTS AND DISCUSSION After crossing the six cultivars in all directions, marked differences appeared in fruit set and in seed viability. ‘Florex Gold’ and ‘Treasure’ were good mothers, because they delivered many viable seeds that produced healthy plants (Table 1.). ‘Galaxy’ and ‘Pink Persuasion’, however, were bad mothers, since they did not produce viable offspring and were therefore not incorporated in Table 1. Some crossings produced chimerical or pale plants that appeared unhealthy and were therefore omitted from the experiment. This hybrid variegation may be caused by combined plastome-genome incompatibility and biparental inheritance of plastids as was observed by Yao and Cohen (2000) in hybrids of Z. aethiopica and Z. odorata and by