Cucurbit Genetics Cooperative Report 8:74-75 (Article 28) 1985
Breeding for Resistance to Cucumber Mosaic Virus in Courgette and
Pink, D. A. C. and D. G. A. Walkey
National Vegetable Research Station, Wellesbourne, Warwick, CV35 9EF, UK
Cucumber mosaic virus (CMV) is the only economically important virus infecting courgette and vegetable marrow (C. pepo) plants in the UK. There are no resistant cultivars available but a resistance breeding programme was started at the NVRS in 1980. We decided not to use a wild species as a source of resistance because this would require a lengthy backcross procedure and since other groups of workers were already working along these lines (6, 7, 8) there seemed little point in duplication. Also unlike those workers who are breeding for resistance to Watermelon mosaic virus (WMV) 1 and 2 in addition to CMV, we are only concerned with CMV resistance. We therefore decided to screen for resistance within C. pepo. Resistance has been reported previously (2, 3, 10) but apparently this was not incorporated into any recently released commercial cultivars.
Plants for screening were grown in an aphid-proof glasshouse and mechanically inoculated at the cotyledon stage. They were scored for systemic infection, on a O (no symptoms) to 5 (severe mosaic and stunting, necrosis or early death) scale, 10 to 14 days later. Symptomless plants were rechallenged by inoculating all leaves and those still scored as O were tested for virus content by back-inoculation to plants of Chenophodium quinoa. Plants were considered resistant if CMV could not be detected by back-inoculation from uninoculated leaves. Plants that developed local lesions on their inoculated cotyledons (indicating a successful inoculation) but no systemic infection were also considered resistant.
Screening of 64 accessions of C. pepo
from Europe, UK, USA and Mexico revealed resistant plants in several open-pollinated cultivars. The highest frequency was in the cv. Cinderella (9), a bush-type Halloween pumpkin bred by Dr. A. M. Rhodes, University of Illinois, Urbana, from the cross Uconn x Connecticut Field (CF) with 3 or 4 backcrosses to CF. CF is susceptible to CMV (9) so that the resistance is presumably derived from Uconn, however, we have not tested this cultivar. In addition to symptomless plants there were plants with symptom scores 1 to 4 indicating the resistance is quantitative, however, at present we have no information on the genetics of the resistance.
The resistance of Cinderella was increased by selection of resistant plants (9) and appears to be fixed at the S4 generation. The resistance is effective against eight strains of CMV known to differ in their pathogenicity and virulence (4) and will hopefully prove to be durable in the field. However, our first attempt at evaluating the resistance in the field failed because establishment of the virus in the spreader plants was poor and there were too few aphid vectors present in the plot. The resistance is influenced by the environment and is more effective at higher temperatures and higher light intensities (5), which is fortunate since the resistance will therefore be enhanced in the field.
It is unfortunate for us that resistance was found in a pumpkin since a backcross programme is now necessary (at least we are starting with something that is edible!). We are at the first backcross generation using several courgette cultivars as recurrent parents. Some of these possess the silvery- leaf trait and in view of the possible role of this in giving protection against aphid vectors (1) we intend to combine this trait with the resistance. Although we are not actively breeding for resistance to powdery mildew we are selecting against increased susceptibility to this disease.
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Ph.D. thesis, Cornell University.