An epidemiological analysis of the dilemma of plant age and late blight ( Phytophthora infestans ) susceptibility in potatoes

While the age-related susceptibility of potatoes to late blight is well-known, the universality of this relationship is unclear. This study aimed at resolving the dilemma of the age-related susceptibility in late blight epidemics by analysing the disease progress curves of three potato cultivars (Folva, Novano and Nofy). The following epidemiological parameters were analysed: incubation period (IP), standardised apparent infection rate (SAIR), inflection point (InflPoint), time to reach 1% severity (T1) and relative area under the disease progress curve (rAUDPC). Cultivar had a pronounced effect on IP. However, neither plant age nor age × cultivar had a significant effect on IP. Overall, cultivar was the most important determinant of susceptibility to late blight. Differences between plant ages for late blight susceptibility were more evident in 2019 compared to 2020. Susceptibility to late blight increased with plant age according to rAUDPC, InflPoint and T1. Based on SAIR, younger (Folva) and middle-aged (Novano) plants were the most susceptible in 2019, whereas no differences were apparent between plant ages in 2020. Based on principal component analysis, the first component (Comp.1) accounted for ~79% (2019) and ~ 81% (2020) compared to ~21% (2019) and ~ 19% (2020) of the second component (Comp.2). Comp. 1 was strongly associated with rAUDPC, InflPoint and T1, thus, the conclusions based on these parameters, i.e. susceptibility increases with plant age, were more likely than with SAIR, which on the other hand was strongly associated with Comp.2. In conclusion, our study showed that the susceptibility of potatoes to late blight increases with plant age.