Cucurbit Genetics Cooperative Report 16:77-78 (article 28) 1993
Xingping Zhang and Bill Rhodes
Horticulture Department, Clemson University, Clemson, SC 29634-0375
Martyn and Netzer (1) reported Citrullus sp. PI 296341-FR to be 100% resistant to races 0 and 1 of Fusarium oxysporum f. sp. niveum but segregating for resistance to race 2. We report here inheritance studies with this PI.
Materials and Methods. Seeds of parents, PI 296341-FR, New Hampshire Midget (NHM) and F1, F2 and BC1 generations were surface sterilized and germinated. Eight-day seedlings with fully expanded cotyledons were used for inoculation. Roots of the seedlings were rinsed with water and then dipped in a spore suspension for 10 min. Inoculation concentration of each race of fusarium was 10 spores/ml. Inoculated plants were single planted in styrofoam trays and moved into the greenhouse. Plants were watered daily and fertilized weekly. The temperature in the greenhouse was 26 ± 1-2°C. Wilting began 5 to 7 days after inoculation. Data collected before transplanting (18 days post inoculation) and after planting (50 days after inoculation) are shown in Table 1. A set of crosses were made in the summer using resistant PI 296341 and F1 individuals. Resistant F1 plants were selfed or backcrossed in each case to produce F2 and backcross progeny.
Results. The data support Martyn and Netzer’s observation that the resistance genes in the resistant parent PI 296341- FR were not fixed. Inheritance to each race in this PI will be discussed separately.
Race 0 Resistance: F1 progeny from reciprocal crosses with NHM are segregating 1:1 resistant to susceptible, indicating heterozygosity of one or more dominant genes for resistance in PI 296341. Segregation in the F2 population from a selfed resistant F1 plant fits a 9:7 ratio, suggesting an interaction between nonallelic genes. Segregation of the backcross population with NHM also indicates modifier(s) which overcome the dominant gene for resistance.
Race 1 Resistance: Dominance for resistance to race 1 in PI 296341 is indicated. This result is in agreement with Netzer and Weintall (2). However, the presence of a susceptible class in the F1 indicates other modifier gene(s). The susceptible class from the testcross to the recessive parent is much higher than expected, also indicating segregation of modifier gene(s). At 18 days, the F2 progeny segregate 3:1 R:S as expected, but the susceptible class has increased by 50 days.
Race 2 Resistance: Both resistant and susceptible individuals exist in the parent PI 296341. The cross of a resistant individual with the susceptible NUM parent results in predominant fly, but not exclusively, susceptible individuals. Thus, it is clear that resistance to race 2 is governed by at least one recessive pair of genes. If a 13:3 model is hypothesized for the F2 generation, i.e., a dominant gene from NHM is epistatic over a recessive gene for resistance in PI 296341, then the data fit the model perfectly. If the backcross to the susceptible parent is assumed to be AaBb x aaBB, then all the backcross progeny will carry the B gene that is epistatic over a. If the few progeny scored as resistant in the F1and backcross populations were actually susceptible, the hypothesis that one or more recessive genes is interacting with a dominant gene is consistent with the data.
Our data suggest epistasis between resistance genes in PI 296341 and the susceptible cultivar New Hampshire Midget.
Table 1. Resistance/susceptible distribution in different families inoculated with races 0, 1 and 2 of Fusarium oxysporum f. sp. niveum.
Resistant:Susceptible |
|||||
Races of the Pathogen |
Families |
18 days |
50 days |
Hypothesis |
p |
| Race 0 | PI 296341 | 15:0 | 15:0 | 1:0 | 1.00 |
| NHM | 0:15 | 0:15 | 0:1 | 1.00 | |
| 296341xNHM | 9:11 | 9:11 | 1:1 | 0.70-0.50 | |
| NHMx296341 | 13:7 | 10:10 | 1:1 | 0.99-0.95 | |
| (296341xNHM)NHM | 9:27 | 4:32 | 0:1 | 0 | |
| (296341xNHM)F2 | 70:24 | 52:42 | 9:7 | 0.90-0.75 | |
| Race 1 | PI 296341 | 15:0 | 15:0 | 1:0 | 1.00 |
| NHM | 0:15 | 0:15 | 0:1 | 1.00 | |
| 296341xNHM | 16:4 | 15:5 | 3:1 | 0.99-0.95 | |
| NHMx296341 | 17:3 | 16:4 | 3:1 | 0.75-0.50 | |
| (296341xNHM)NHM | 12:23 | 6:29 | 1:3 | 0.25-0.10 | |
| (296341x29634l)F2 | 67:28 | 61:34 | 2:1 | 0.75-0.50 | |
| Race 2 | PI 296341 | 10:5 | 8:7 | 1:1 | 0.75-0.50 |
| NHM | 2:13 | 0:15 | 0:1 | 1.00 | |
| 296341xNHM | 4:14 | 1:17 | 0:1 | 0 | |
| NHMx296341 | 3:17 | 2:18 | 0:1 | 0 | |
| (NHMx296341)NHM | 1:39 | 1:39 | 0:1 | 0 | |
| (NHMx296341)F2 | 20:72 | 18:77 | 3:13 | >0.90 | |
Literature Cited
- Martyn, R.D. 1991. Resistance to races 0, 1, and 2 of Fusarium wilt of watermelon in Citrullus sp. PI 296341-FR. HortScience 26(4):429-432.
- Netzer, D. and C. Weintall. 1980. Inheritance of resistance in watermelon to race 1 of Fusarium oxysporum L. sp. niveum. Plant Dis. 64:853-854.