Cucumber Yield Improvement Through Breeding in the Southeast U.S.A.

Cucurbit Genetics Cooperative Report 12:9-10 (article 4) 1989

Todd C. Wehner
Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609

It is of interest to cucumber breeders in the U.S.A. to determine how much progress they have made in the improvement of yield. yield is a complex trait, and is affected by sex expression, disease resistance, and cultural practices, as well as other factors. All of those have been improved over the past decades by researchers in public and private institutes.

In a survey of cucumber breeders made in 1987, the primary objectives for trait improvement listed were yield, disease resistance, fruit quality, and earliness/sex expression (2). With emphasis on many traits, progress on any one will be slow because selection for many traits simultaneously reduces the selection intensity on each trait. In my recurrent selection program, 14 populations are subjected to a 5% selection intensity (20 families kept out of 400 tested) for yield, earliness, quality, and disease resistance (mostly anthracnose, downy mildew and gummy stem blight). Actual selection intensity is approximately 40% for yield and 50% each for earliness, quality and disease resistance (.05=.40 x .50 x .50 x .50). Thus, one would not expect much progress to be made where many traits are being selected simultaneously.

Nevertheless, progress has been made for yield, even after accounting for the contributions of improvement in cultural practices, sex expression and disease resistance. For example, the cultivars released form the public breeding programs run sequentially by Carroll Barnes, Richard Lower, and me in the Carolinas have lead to continuously improved yield even though many releases study was to estimate the improvement made for yield in pickling cucumbers grown in the southeast U.S.A.

Methods. Five cultivars which are most similar in type (gynoecious hybrid pickling cucumbers with resistance to the southern foliar disease) developed in the Carolinas over the last 2 decades were grown in trials in Clinton, NC under standard cultural practices (1). The trials were run in the spring when there was no foliar disease load, and in the summer when anthracnose, downy mildew and gummy stem blight were moderate to severe. The trials were run in 1981 through 1985 using 3 replications and 6 harvests. No summer trials were run in 1982 and 1983.

Irrigation was used to supplement rainfall. Weeds, diseases and insects were controlled as needed using labeled pesticides. Weight of all fruits produced, regardless of size (most being grade 2 and 3 with a diameter of 27 to 50 mm), were summed over harvests to get total yield. Yield was regressed on release date to determine the progress made per year. Release date is not completely accurate in determining when the material was developed. ‘Raleigh’, for example could have been released in 1985 if it had been given top priority in the program.

Results. It is interesting to note that the cultivars do not keep the same rank for any one trial as the overall mean (Table 1). In general, yield increased with each subsequent release, with an average of 0.4 t/ha each year of breeding. That yield progress was made even though other traits (such as fruit color, fruit shape, and length: diameter ratio) were being improved.

Since the 5 cultivars were tested under the same cultural practices, and had similar sex expression and disease resistance, progress in yield must have been due to direct improvement of the trait. Thus, I conclude that we have not hit a yield plateau in cucumber breeding, but have been working on so many traits that progress on each one of them is slow.

Table 1. Yield (t/ha) of disease resistant, gynoecious pickling cucumber hybrids grown in field trials (spring and summer) in Clinton, NC.^z

Release date	Cultivar	1981		1982	1983	1984		1985		Mean
		Spr	Sum	Spr	Spr	Spr	Sum	Spr	Sum
1969	Explorer	19	20	29	34	31	22	38	37	28.8
1973	Carolina	20	10	28	33	41	21	45	38	29.6
1975	Calypso	21	21	32	34	42	30	41	41	32.7
1979	Regal	24	18	40	38	41	27	48	41	34.6
1987	Raleigh	26	26	37	34	33	27	52	48	35.5

^z Data are means of 3 replications and 6 harvests.

Literature Cited

1. Hughes, G.R., C.W. Averre and K.A. Sorensen. 1983. Growing pickling cucumbers in North Carolina. N.C. Agric. Ext. Serv. AG-315.
2. Wehner, T.C. 1988. Survey of cucumber breeding methods in the U.S.A. Cucurbit Genet. Coop. Rpt. 11: 9-12.