Age-Dependent Expression of Resistance to Pseudoperonospora cubensis in Cucumber

Downy mildew, caused by the oomycete Pseudoperonospora cubensis, is a major disease of cucumber, but the developmental and experimental conditions under which resistance can be reliably recognized remain poorly defined. A series of experiments conducted during 2023-2026 examined susceptible cultivar SMR, resistant PI 197088 and PI 330628, their F1 hybrids, and F2 populations. In the 2026 follow-up, individually marked F2 plants were first inoculated at the 1-leaf stage under controlled laboratory conditions and later reinoculated in the field at 6-8 leaves with the same isolate, 273. Disease was then assessed at 7 and 15 days post inoculation (dpi) after this second inoculation, corresponding to approximately 10-12 and 18-20 leaves. One-leaf severity was unrelated to the 7-dpi field observation (r = 0.085 and 0.096 in PI 197088 × SMR and PI 330628 × SMR, respectively) and was only weakly associated with 15-dpi severity (r = 0.180 and 0.206). In contrast, the 7- and 15-dpi observations from the same field inoculation were strongly associated (r = 0.598 and 0.618). F1 plants were intermediate in quantitative disease severity, whereas the stringent full-resistance class (≤5% disease) was rare in the F2. Because developmental stage, inoculation history, and environment were not experimentally separable in 2026, the results are interpreted as evidence that resistance expression is strongly context dependent rather than as a test of plant age alone. For breeding, first-leaf screening should therefore not be used as a definitive predictor of the response expressed after later field challenge.