Cucurbit Genetics Cooperative Report 4:17-19 (article 8) 1981
A. P. M. den Nijs and A. C. van der Giessen
Institute for Horticultural Plant Breeding (IVT), P. O. B. 16, Wageningen, The Netherlands
Five different isolates of Didymella bryoniae (Auersw.) Rehm, causal fungus of fruit and stem rot, were separately used to inoculate three cucumber genotypes with different levels of resistance, to detect possible differences in pathogenicity. Three tests were carried out, the first in 1978 and the last two in 1980. The isolates had been collected from 1974 to 1977 by Van Steekelenburg (Research Institute for Plant Protection) and were subcultured manifold on malt extract agar (1). The plant material consisted of two IVT breeding lines with different levels of resistance against fruit and stem rot and were derived from cv. Rheinische Vorgebirge (2). Susceptible check was cv. Levo. The experimental design was a split block with eight replications of two plants per treatment randomly arranged in two plastic tents. The plants were inoculated in the potted plant stage, as described earlier (1). We inoculated the plants with each isolate separately, while shielding the remainder of the plants and taking care to keep the treatments apart.
Average visual disease ratings according to the scale in (1) of all 16 plants per treatment are in Table 1. These are the results of one test only, inoculated October 10, 1980 and evaluated October 20, 1980. The data from the other two tests are similar. The ratings in the water-control treatment may reflect damage from water-logging due to the extremely high relative humidity under the tents. ANOVA of all the data revealed that the apparent resistance level of the three genotypes differed significantly over all isolates (F2/14 = 23.97**). The isolates were also significantly different (F3/35 = 29.02**). Separate analyses per host genotype established that on line A, isolate M77-3 was more pathogenic than all others with M74-3 as second most pathogenic, both significantly differing from the water-control. On line B both isolates were equally pathogenic and both differed significantly from the others. On ‘Levo’ their pathogenicity was similar, and all isolates induced significantly more symptoms than the water-control. Yet there was no significant interaction between isolates and host genotypes (F10/70 = 1.94), so from this test we cannot conclude that physiological races of D. bryoniae exists, although differences in overall pathogenicity were clear. The results of the other two experiments pointed in the same direction. There was a noteworthy large difference in the coefficient of variation of the two lines (67%) in comparison with that of cv. Levo (26%). The lines appear to be segregating. There was almost complete concordance of the ranking of all five isolates on susceptible cv. Levo in all three tests. Apparently no relative change in pathogenicity has occurred in these isolates for over two years, despite frequent subculturing under blacklight and the fortuitous selection for in vitro sporulation (1).
The value of the resistance found in the potted plant stage depends on how well it holds out in plants during the cropping period. Initial results of correlation studies with related breeding lines were not reassuring (3). We have investigated whether symptomless plants in our tests were indeed free of the fungus following 4-5 weeks of recovery after the test. Therefore, we attempted to reisolate the fungus from pieces (length 0.5-1 cm) of disinfected internodes cultured according to (1). We also applied the same technique to plants with mild, moderate and severe symptoms in the potted plant stage, but at the time of dissection, all plants were without symptoms. The plants had been inoculated with very pathogenic isolate M77-3 or with M74-4. Fungal growth was assessed following five days of incubation of the stem pieces.
From almost all of 67 dissected plants the fungus was reisolated from the hypocotyl. From less that half of the plants we also obtained fungal colonies from higher internodes, often from the first one or from the top of the plant (9th to 15th internode). Only one to three internodes per plant yielded positive reisolation results. A summary is in Table 2. The most resistant line contained most plants from which the fungus was reisolated. From plants that were severely diseased in the potted plant test we obtained less often fungal growth than from moderately or slightly attacked plants. From one-third of the plants that did not show symptoms in the potted plant test, we were able to reisolate Didymella, so these plants apparently carried it without visible damage. The results of both isolates differed slightly.
These preliminary data caution against overreliance on the potted plant test. Other reisolation attempts out of inoculated plants of up to four months of age indicate that the fungus may be present in the plants for a long time unnoticed, and possibly break out in disease when the environment and condition of the plants are right.
Table 1. Average disease rating of five isolates of Didymella bryoniae on two lines and cv. Levo in a potted plant test.
Isolate |
Line A |
Line B |
Levo |
Mean |
| M74-2 | 0.61z | 0.60 | 1.76 | 0.99 |
| M74-3 | 1.60 | 1.30 | 2.30 | 1.73 |
| M74-4 | 1.08 | 0.43 | 2.05 | 1.19 |
| M75-3 | 1.20 | 0.59 | 1.51 | 1.10 |
| M77-3 | 2.29 | 1.28 | 2.33 | 1.97 |
| Mean | 1.36 | 0.84 | 1.99 | 1.40 |
| Water | 0.16 | 0.06 | 0.39 | 0.20 |
z Average of 8 plots of 2 plants.
Table 2. Number of plants from which Didymella bryoniae was reisolated, as a fraction of the number of plants tested (plants were inoculated in a potted plant test 6 weeks before dissection).z
Line |
Isolate |
0 |
1 |
2 |
3 |
Mean |
| A | M77-3 | – | 3/3 | – | 1/8 | 4/11 |
| M74-4 | 2/6 | – | 1/1 | 0/2 | 3/9 | |
| B | M77-3 | 1/2 | 3/5 | 8/8 | – | 12/15 |
| M74-4 | 2/8 | 0/1 | 0/1 | – | 2/10 | |
| Levo | M77-3 | – | 0/1 | 3/5 | 0/5 | 3/11 |
| M74-4 | – | 1/2 | 1/5 | 1/4 | 3/11 | |
| Mean | 5/16 | 7/12 | 13/20 | 2/9 | 27/67 |
z Only internodes above hypocotyl considered.
Literature Cited
- Giessen, A. C. van der and A. P. M. den Nijs. 1981. In vitro growth and sporulation of Didymella bryoniae and a glasshouse method for screening for resistance. Cucurbit Genetics Coop. Rpt. 4:6-8.
- Meer, Q. P. van der, J. L. van Bennekom and A. C. van der Giessen. 1978. Gummy stem blight resistance of cucumbers (Cucumis sativus L.). Euphytica 27:861-864.
- Steekelenburg, N. A. M. van. 1981. Comparison of inoculation methods with Didymella bryoniae on Cucumis sativus L. Euphytica (accepted).