Wj. Dekogel et al., SEASONAL-VARIATION IN RESISTANCE OF CHRYSANTHEMUM CULTIVARS TO FRANKLINIELLA-OCCIDENTALIS (THYSANOPTERA, THRIPIDAE), Euphytica, 97(3), 1997, pp. 283-288
Seasonal variation in the level of host plant resistance can have impo
rtant consequences for the repeatability of tests to measure host plan
t resistance to insect pests. In the present study, the levels of resi
stance to Frankliniella occidentalis of a susceptible and a partially
resistant cultivar of chrysanthemum, Dendranthema grandiflora grandifl
ora, were determined throughout the year. Thrips damage, reproduction
and adult female survival were determined on excised leaves in Petri d
ishes under uniform conditions. Strong seasonal fluctuations were obse
rved in these three characteristics. On leaves from plants grown in wi
nter, damage, reproduction, and survival were higher than on leaves fr
om plants grown in summer. Clear differences in resistance were observ
ed between the susceptible and the partially resistant cultivar on lea
ves from plants grown in winter, while differences disappeared in summ
er. Damage on both cultivars and survival on the susceptible cultivar
were negatively correlated with mean daily solar radiation during plan
t growth, suggesting that the level of resistance depends on light int
ensity during plant growth. This was confirmed in an experiment carrie
d out in summertime with shaded and unshaded plants. Leaves from contr
ol plants, grown under high light intensity had a higher level of resi
stance than leaves from shaded plants grown under reduced light intens
ity. There were clear differences in resistance between the cultivars
under shaded conditions (low light intensity), but not under unshaded
control conditions (high light intensity).