Rapid apple decline (RAD), a phenomenon that causes sudden decline and death of young apple trees in high-density orchards, is a pressing issue. This problem has been associated with abiotic (flood, drought, freeze) and biotic stressors (fungi, bacteria, and insects). Ambrosia beetles (AB) (Xylosandrus spp.) have been linked to stressed trees suffering from RAD. However, the direct association is still not clear. We evaluated the effects and interactions of rootstock (‘B. 9’, ‘M. 7’, and ‘G. 41’) and water stress (flood, drought, control) on physiological responses and AB colonization. Specifically, a greenhouse experiment was performed in spring 2023 on 14-month-old potted ‘MAIA1’ trees. Trees were subjected to water stress for 20 d. Water stress treatments consisted of a saturated pot-in-pot system for flood, water with-held for the duration of the experiment for drought, and irrigation based on soil moisture, maintaining the pots between 0.7 and 1 water fraction volume for control trees. The experiment had a randomized complete block design, was replicated 10 times, and had a factorial treatment structure. Measurements were made every 2 d for soil moisture, fluorometry, and spectrometry data, every 4 d for leaf gas exchange and midday water potential, at 7 and 14 d for ethanol volatiles, and at the termination of the experiment for AB colonization. Under control conditions, ‘G. 41’ trees had higher transpiration, stomatal conductance, and assimilation rates than ‘B. 9’ and ‘M.7’ trees. However, after 4 d of stress, ‘G. 41’ showed a more rapid decline in leaf gas exchange parameters relative to other rootstocks. Drought-stress trees from the three rootstocks declined 4 d faster than flood trees during the first 12 d. However, this decline occurs more abruptly in ‘G. 41’ and ‘M. 7’ trees. From 16 to 20 d, drought and flooded trees have a similar response. Under control conditions, all three rootstocks have similar quantum efficiency values. Conversely, under water stress, ‘G. 41’ and ‘M. 7’ declined faster than ‘B. 9’ trees, with a more severe response to drought for ‘G. 41’ and ‘M. 7’ trees. Chlorophyll index (Ci) showed lower values for flood and drought trees after 10 d for ‘G. 41’ and ‘M. 7’ trees. For ‘B. 9’, this was only true in drought trees. Flooded trees had AB tunnels three times longer than control, and drought trees were the only ones where AB larvae were observed, which was linked to higher ethanol levels.
