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Cucurbit Genetics Cooperative Report 23 1-3 (article1) 2000

A New Source of Resistance to Meloidogyne incognita (Kofoid & White) Chitwood Identified in Cucumis

Jin-Feng Chen1 and Stephen Lewis2

1Departmentof Horticulture, Nanjing Agricultural University, Nanjing 210095, China; 2 Department of Plant Pathology and Physiology, Clemson University, Clemson, SC 29634

Introduction: Cucumber (Cucumis sativus L.) is considered very susceptible to major pathogens active in temperate growing regions (Fassuliotis, 1979). In this regard, susceptibility to nematodes can be a serious limiting factor in commercial production of cucumber. Several cucumber-wide screenings have been conducted to identify sources of resistance to nematodes (Meloidogyne ssp.). These tests evaluated hundreds of cucumber cultigens (cultivars, breeding lines, nd plant introduction accessions), but no source of resistance to M. incognita was identified (Winstead and Sasser, 1956; Fassuliotis and Rau, 1963; Walters et al., 1993).

Some related Cucumis species, such as C. metuliferus E. Meyer ex Naudin, C. anguria, L., C. ficifolius A. Rich., and C. longipes Hook., have been found to possess high resistance to various root-knot nematode species include inM. incognita (Fassuliotis, 1967; Norton and Granberry, 1980). But fortunately all attempts to hybridize these wild relatives with commercial cucumber have failed. (Whitaker, 1930; Natra, 1953; Smith and Venkat Ram, 1954; Deakin et al., 1971; Fassuliotis and Nelson, 1988).

A successful cross has been made and confirmed between cucumber and C. hystrix Chakr. (2n = 24) (Chen et al., 1997) This interspecific hybridization is the first repeatable cross between a cultivated cucumis species and a wild relative. Since there is cross-compatibility between C. sativus and C. hystrix, the economically important characters if C. hystrix are of great interest to cucurbit scientists and breeders. In this paper, we introduce the resistance to root-knot nematode (M. incognita) in C. hystrix and the transmission of resistance from C. hystrix to its interspecific F1 hybrid with cucumber.

Materials and methods:Plant materials. C. hysteria Xishuangbanna No. ! and No. 2 cucumber (Cucumis sativus L. var. xishuangbannesis Qi et Yuan) used in this study were from the original collection of J.F. Chen (Chen et al., 1994) the Northern Chinese cucumber cultivar 'Beijing jietou' (V05A464) was obtained from Dr. C.Z. Qi, of the Vegetable Research Institute Chinese Agricultural Academy of Sciences, Beijing. The Southern Chinese cucumber 'Erzhaozi' was obtained from Mr. Z.B. Gong, of the Chengdu Seed Company, Chengdu, Sichuan Province.

Matings. Reciprocal interspecific hybridization between C. hystrix and cucumber, and subsequent embryo rescue were performed as described previously (Chen et al., 1997). The diploid sterile F1 progeny (2n = 19) went through chromosome doubling as previously described (Chen et al., 1998). The BCF1 was made by crossing the chromosome-doubled F1 (with genome HHCC, where H represents the genome of C. hystrix, C the C. sativus) to the original diploid cucumber stock parent.

Inoculation. Nematode and inoculation Meloidogyne incognita race 3 was cultured on greenhouse-grown tomato, Lycopersicon esculentum Mill. cv. Rutgers. Nematode inoculum was obtained by collecting eggs with 0.5% NaOCl as described by Hussey and Barker (1973).

Experimental design, plant culture and data collection. Seeds were germinated in vermiculite in a greenhouse, and seedlings at the two-leaf stage were transplanted into 4-inch pots filled with pure sand. Plantlets from in vitro culture were also transplanted into the same media at the same time. Plants were fertilized weekly with a commercial nutrient formulation (N: P: K = 20: 20: 20), and kept in a greenhouse at 28 ˚ C. Four days after planting, two holes 2-3 cm in depth and 00.6 cm in diam. were made with a bamboo stock around the plant roots. One ml of inoculum containing 2,500 eggs was pipetted into each hole (5,000 eggs for each plant). There were five replications of each cultigen to determine the ability of the nematode reproduce. Plants were placed on a table in a completely randomized design. Seven weeks after inoculation, the root systems were carefully washed free of sand, and evaluated for number of galls. The number of galls for each root system was counted, and a gall index was calculated using a 0 - 5 scale, with 0 = no galls, 1 = 1 to 2, 2 = 3 to 10, 3 = 11 to 30, 4 = 31 to 100, and 5>100 galls. Normalized data were subjected to analysis of variance using SAS (SAS Inst., Cary, NC), and the means of gall indices were tested using Tukey's Studentized Range (HSD) Test.

Results and Discussion: To identify the resistance in cucumis hystrix Chakr. and evaluate the transmission of resistance to the progenies of its interspecific hybrid with cucumber, a screening was conducted in a greenhouse using C. hystrix, four cucumber cultigens, and three reciprocal interspecific hybrids at two ploidy levels, and one BCF1 progeny.

After 45 days, there were, on average, only about three galls that could be seen in each C. hystrix root system,m. In contrast, over 100 galls could be counted in each cucumber root system tested.

C. hystrix had a high level of resistance to M. incognita with mean gall index of 1.8, while cucumbers were confirmed as being highly susceptible with a mean gall index of 4.8-5.0. The resistance was partially transferred to the interspecific hybrid. The mean gall index changed from 4.8 to 3.4, which is about mid-point between the resistant parent C. hystrix (1.8) and the susceptible parent cucumbers (4.8 - 5.0). This transmission was also observed when the chromosome=doubled F1 and BCF1, the statistical analysis indicated no significance between them.

There is only limited variability in mean gall index among plans of hybrids, or the parents. No significant difference in mean gall index was observed between the reciprocal F1 plants, which indicates that the expression of resistance in the progeny is not influenced by the maternal parent. Meanwhile, the mean gall index of the F1 plant was intermediate between both parents, indicating that neither resistance nor susceptibility is dominant.

In summary, the results revealed a high level of resistance (~ three galls in each root system) in C. hystrix, while cucumber was highly susceptible. The resistance was partially transmitted to the F1 when the reciprocal interspecific hybrid was made. This resistance was further transmitted to the BCF1 progeny when the F1was backcrossed to the cucumber.

The benefit-to-cost ratio for the development of resistant crop cultivars in the United States was estimated at $300 for every $1 spent (Bottrel, 1979). Plant resistance was identified as the highest research priority in management of plant-parasitic nematodes (Bird, 1980). This success of interspecific hybridization between cucumber and its wild relatives (Chen et al., 1997) is of great importance to cucumber genetics and breeding. Introgression of root-knot nematode resistance from C. hystrix to cucumber must be advanced by further backcrossing. Strategies need to be developed to facilitate the gene transmission,such as more knowledge of cytogenetics and in combination with marker-assisted selection.

Literature Cited

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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
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