Cucurbit Genetics Cooperative Report 16:1-2 (article 1) 1993
Paul C. St. Amand and Todd C. Wehner
Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609
It is often desirable to know if resistance data from one test is positively and highly correlated with other tests. A high positive correlation indicates that testing once in any year is a good measure of resistance and a low correlation indicates that multi-year tests are necessary to determine the resistance of a cultigen (breeding lines, cultivars, and plant introduction accessions). The objective of this study was to calculate correlations among years for foliar ratings of gummy stem blight in field-grown cucumbers.
Field tests were conducted in 1981, 1982, 1983, and 1986 to determine the resistance of diverse cucumbers to gummy stem blight. Each test was conducted at the Horticultural Crops Research Station at Clinton, N.C. with 3 (1982) or 6 (1983, 1986) replications, except for the 1981 test, which was conducted at the Horticultural Crops Research Station at Castle Hayne, N.C. without replication. Field plots were rated for foliar lesions using a 0 to 9 scale (0 = no foliar symptoms, 9 = plant dead). The rating system was modeled after the categories developed by Thompson and Jenkins (2). Plots were inoculated with equal numbers of spores from 12 isolates of Didymella bryoniae. Plants were sprayed at the vine-tip-over stage (4 to 6 true leaves) to run-off using a Solo back-pack sprayer at 15 to 20 psi. Overhead irrigation was used (25 to 38 mm/week) to spread the inoculum and encourage uniform disease development. Every 3rd row (4th row for 1986) was planted with susceptible ‘Wisconsin SMR 18’ to enhance the uniformity of disease spread. Plots were 6 m long (1981) with 40 plants each, or 3 m long (1982, 1983, 1986) with 30 plants each and were planted on raised, shaped beds 1.5 m apart (center to center) separated at each end by 1.5 m alleys. Standard cultural practices were used for crop production (1). A randomized complete block design was used for all tests. One rating was given for each plot 7, 14 and 21 days after inoculation, except in 1981 which was rated only 7 days after inoculation. Data were analyzed using PROC REG of SAS (SAS Institute, Cary, N.C.).
Disease ratings were greatest in 1982 and the mean rating was very similar for years in 1981, 1983, and 1986 (Table 1). The range of disease ratings was greatest in 1981 and was smallest in 1983 and 1986, Pearson product-moment correlations and Spearman rank correlations between years were moderate to high (Table 2); however, the highest correlations are based on small numbers of cultigens and the correlations tended to decrease as cultigen number increased. Rankings of cultigens were usually less correlated than were actual ratings, indicating that changes in rank often occurred. Wyszogrodzka et al. (3) calculated the Pearson correlation coefficient between gummy stem blight field tests conducted in Florida and Wisconsin. That correlation (r=0.42) was moderate and similar to those reported here for tests involving larger numbers of cultigens.
Table 1. The mean, standard deviation, median, minimum and maximum of foliar disease ratings of cucumber for gummy stem blight for four field tests conducted in different years. Also shown is the number of cultigens rated in each test.z
Year |
Number of cultigens |
Mean |
Standard deviation |
Median |
Minimum |
Maximum |
| 1981 | 31 | 5.6 | 3.1 | 8.0 | 1.5 | 9.0 |
| 1982 | 46 | 6.3 | 2.0 | 6.6 | 1.7 | 9.0 |
| 1983 | 45 | 5.1 | 0.8 | 5.1 | 2.9 | 8.2 |
| 1986 | 36 | 5.5 | 0.9 | 5.5 | 3.6 | 7.2 |
z Foliar symptoms were rated from no symptoms (0) to complete necrosis (9).
Table 2. Correlations between years for foliar disease ratings of cucumber cultigens inoculated with gummy stem blight. Pearson product-moment correlation coefficients are above the diagonal and Spearman rank correlation coefficients are below the diagonal.z
Year |
||||
Year |
1981 |
1982 |
1983 |
1986 |
| 1981 | – | 0.57 (31) *** | 0.68 (10) * | 0.93 ( 6) ** |
| 1982 | 0.56 (31) *** | – | 0.73 (14) *** | 0.73 ( 8) * |
| 1983 | 0.82 (10) ** | 0.84 (14) *** | – | 0.66 (32) *** |
| 1986 | 0.70 ( 6) NS | 0.64 ( 8) NS | 0.48 (32) ** | – |
z The symbols NS, *, **, and *** indicate not significant, or significant at the 0.05, 0.01, or 0.001 levels, respectively. Numbers in parenthesis indicate the number of cultigens shared between tests.
Literature Cited
- Hughes, G.R., C.W. Averre, and K.A. Sorensen. 1983. Growing pickling cucumbers in North Carolina. N.C. Agric. Ext. Serv. AG-315.
- Thompson, D.C. and S.F. Jenkins. 1985. Pictorial assessment key to determine fungicide concentrations that control anthracnose development on cucumber cultivars with varying resistance levels. Plant Dis. 69: 833-836.
- Wyszogrodzka, A.J., P.H. Williams, and C.E. Peterson. 1986. Search for resistance to gummy stem blight (Didymella bryoniae) in cucumber (Cucumis sativus L.) Euphytica 35: 603-613.