Cucurbit Genetics Cooperative Report 14:125-128 (article 43) 1991
T.A. Zitter, D.A.C. Pink, and D.G.A. Walkey
Department of Plant Pathology, Cornell University, Ithica, NY 14853, Institute of Horticultural Research, Wellesbourne, Warwick, CV35 9EF, U.K.
Cucumber mosaic virus (CMV) is the only virus of any economic importance infecting zucchini and vegetable marrow crops in the United Kingdom (4). Although some resistance was reported in Cucurbita pepo L. ‘Delicata’ (2), this resistance does notappear to have been incorporated into any present day cultivars. Screening at the seedling stage of 64 accessions of C. pepo from Europe, UK, US and South America to two UK strains of CMV revealed resistance in several open pollinated cultivars (6). The highest frequemncy of resistant plants was found in the pumpkin ‘Cinderella’, a bush-type Halloween pumpkin bred from the cross UConn x Connecticut Field (Dr. A.M. Rhodes, per. comm.). The resistance was expressed as prevention of systemic infection. Inoculated cotyledons developed chlorotic local lesions identical to those found on susceptible plants but the true leaves remained symptomless. the resistance was shown to be under complex genetic control (3) and was influenced by environment, being more effective at higher temperatures and light intensities (5). The resistance was also equally effective against six strains of CMV including one strain from the US (4). We report here reaction of ‘Cinderella’ plants to two strains of CMV from NY known to differ in their virulence (1), and describe the effect of varying pre-and post-inoculation temperatures on symptom expression.
Methods. The test plants used were ‘Cinderella’, ‘Cinderella S.’, a line selected from Cinderella, ‘Cobham Bush Green’ (CBG), a vegetable marrow previously shown to have moderate resistance, and the highly susceptible yellow zucchini ‘Goldrush’ (6). The seeds were pre-germinated on moist cellulose wadding and transplanted into 12 cm pots filled with Fisons Levingtons compost and placed in troughs containing modified Hewitt’s nutrient solution in controlled environment cabinets. The temperature in the cabinets prior to and after inoculation with virus varied between experiments. Single plant randomization was used throughout.
The M strain of CMV was isolated from marrow in Essex, England and has previously been shown to cause severe symptoms in susceptible C. pepo lines (6). The FNY and SNY strains originated from muskmelon and differ greatly in aphid transmissibility and replication in muskmelon and summer squash (1). All three strains were maintained in Goldrush. Crude virus inoculum was used in experiments 1 and 2, while partially purified inoculum prepared by low and high speed centrifugation ws used in experiments 3 and 4. Seedlings were mechanically inoculated at the cotyledon stage. Plants were scored for systematic infection of the true leaves 12 days after inoculation using a 0 (symptomless) to 5 (severe mosaic) scale (6). Any plant without systemic sysmptoms or no chlorotic local lesions (CLL) on cotyledons were discarded from the data as being escapes. Selected plants were assessed for their virus content using ELISA or by indexing to Chenopodium quinoa.
Results. In experiments 1 and 2 the three strains were tested for their severity on the cultivars selected. In experiment 1 there was no significant difference in virulence between the CMV strains when tested on Cinderella, CBG or Goldrush; however, there were significant differences (p = 0.001) betwen cultivars. The resistance of Cinderella, was equally effective against all strains, and the ranking of cultivars for mean symptom scores was Cinderella (0.7) , CBG (3.4) , Goldrush (4.3). In experiment 2 comparisons were made between Cinderella, Cinderella S3, and Goldrush. Cinderella S3 was slightly more resistant than Cinderella but analysis of variance showed the difference was not significant. The resistance of both liens was effective against all three strains. The strains caused similar levels of symptoms on Cinderella and Cinderella S3 , but CMV-FNY and -SNY caused significantly (p , 0.001) less symptoms on Goldrush than did CMV-M.
In experiments 3 and 4, the effect of pre-inoculation temperature and inoculum concentration and the effect of pre-and post-inoculation temperature were studied using Cinderella and Goldrush grown in three cabinets set at 15, 20 and 25 ˚ C until inoculation. After inoculation all three cabinets were set at 20 ˚ C. Fifty plants each of Cinderella and Goldrush were grown in each cabinet, 45 of these plants were inoculated and five plants were uninoculated. The plants were inoculated with a semi-purified virus preparation of undiluted 10-1 and 10-2 dilutions. Each dilution was inoculated to 15 plants of each cultivar in each cabinet. Diluting the inoculum or latering the pre-inoculation temperature had very little effect upon the response of Goldrush to infection by CMV (Table 1). The only significant effect was a reduction in the mean symptom score of plants grown at 25 ˚ C prior to inoculation with the 10-2 dilution. There was a significant percentage weight loss at any temperature for all treatments involving Goldrush. In contrast all Cinderella plants inoculated with the 10-2 dilution suffered no significant weight loss. Plants in these three treatments also had the lowest mean symptom scores with many plants scoring 0. There was, however, a significant (p , 0.001) interaction between pre-inoculation temperature and inoculum. Plants inoculated with the 0 and 10-1 dilutions had the highest mean symptom scores when grown at 25 ˚ C prior to inoculation, in contrast this temperature gave the lowest mean symptom score for plants inoculated with 10-2 dilution.
In experiment 4 plants were again grown at the three temperatures prior to inoculation. Each cabinet contained 48 plants of each cultivar. Thirty-six plants were inoculated with the semi-purified inoculum and 12 plants were uninoculated. After inoculation plants were moved between cabinets to give all possible combinations of pre- and post-inoculation temperature combinations. All Goldrush plants displayed severe symptoms at all pre- and post- inoculation temperature regimes (Table 2). In contrast the growing conditions for the Cinderella plants significantly affected symptom severity. Plants raised at 25 ˚ C prior to inoculation had much less severe symptoms than those raised at 15 or 20 ˚ C regardless of the post-inoculation conditions. Likewise, a post-inoculation temperature of 25 ˚ C resulted in reduced symptom severity compared with 15 or 20 ˚ C. Pre- and post-inoculation temperatures of 154 and 20 ˚ C had little effect on symptom severity. Mean % weight loss of Cinderella plants was generally correlated with symptom severity (Table 2) except that plants grown continuously at 25 ˚ C showed less % weight loss than plants grown under the other temperature regimes despite having a mean symptom score of 4.5.
Table 1. Effect of pre-inoculation temperature and inoculum concentration on mean symptom score (left) and percent weight loss (right) of Goldrush and Cinderella.
Cultivar |
|||||||
Goldrush |
Cinderella |
||||||
Dilution |
0 |
10-1 |
10-2 |
0 |
10-1 |
10-2 |
|
Pre-inoculation temperature |
|||||||
| 15 ˚ C | 5.0 71.6a | 4.9 63.8a | 4.8 48.2b | 1.7 46.5c | 0.8 23.6c | 0.7 0.0 0 | |
| 20 ˚ C | 5.0 51.a | 4.9 53.1b | 4.6 36.0b | 1.7 20.5c | 1.5 19.4c | 0.5 1.9 | |
| 25 ˚ C | 5.0 43.5b | 4.9 55.488 | 3.7 45.388 | 2.2 23.78 | 2.6 22.9b | 0.1 8.7 | |
Significance of weight loss: ap 0.001, bp 0.01, cp 0.05
Table 2. Effect of pre-inoculation temperature on mean symptom score (left) and weight loss (right) of Goldrush and Cinderella.
| Cultivar | |||||||
| Goldrush | Cinderella | ||||||
| Post-inoculation | 15 | 20 | 25 | 15 | 20 | 25 | |
| Pre-inoculation temperature | |||||||
| 15 ˚ C | 5.0 58.4a | 4.7 52.5a | 4.5 47.5a | 3.6 30.6b | 3.4 35.9b | 2.0 28.5b | |
| 20 ˚ C | 5.0 36.8b | 5.0 69.2a | 4.6 51.9a | 3.3 39.2b | 4.2 45.4a | 2.6 31.0b | |
| 25 ˚ C | 5.0 43.6b | 5.0 54.1b | 4.5 25.4b | 1.6 11.7 | 1.8 18.9b | 1.8 0.0 | |
Significance of weight loss: ap 0.01, bp 0.05
In general, the results using ELISA and local lesion assays to estimate virus titer were in good agreement with the subjective symptom scores. Statistical correlations between ELISA results and symptom scores could not be made because it was necessary to dilute samples from plants with scores of 2 and 3 by 10-2 and with scores of 4 and 5 by 10-3. However, by taking a spectrophotometer reading of twice that given by the healthy control as a threshold for positive virus detection, CMV was only just detectable in plants with a symptom score of 1 and increasingly higher readings were obtained with plants with scores of 2 up to 5 (A405 0.24-1.16). virus was also easily detectable in inoculated cotyledons showing CLL on cinderella plants with scores of 0, but not in the uninoculated true leaves of these plants. virus could not always be detected in inoculated cotyledons which did not show CLL.
Literature Cited:
- Banik, M.T. and T.A. Zitter. 1990. Determination of cucumber mosaic virus titer in muskmelon by enzyme-linked immunosorbent assay and correlation with aphid transmission. Plant Dis. 74:857-859.
- Enzie, W.D. 1940. The Geneva Delicata squash. Farm Res. 6:12.
- Pink, D.A.C. 1987. Genetic control of resistance to cucumber mosaic virus in Cucurbita pepo. Ann. Appl. Biol. 111:425-432.
- Pink, D.A.C. and D.G.A. Walkey. 1984. Resistance in marrow (Cucurbita pepo L.) to different strains of cucumber mosaic virus. J. Agric. Sci., Camb. 103:519-521.
- Pink, D.A.C. and D.G.A. Walkey. 1985. Effect of temperature and light intensity on resistance in marry (Cucurbita pepo) to cucumber mosaic virus. J. Agric. Sci., Camb. 104:325-329.
- Walkey, D.G.A. and D.A.C. Pink. 1984. Resistance in vegetable marrow and other Cucurbita spp. to two British strains of cucumber mosaic virus. J. Agric. Sci., Camb., 102:197-205.