Symptoms of cold damage vary with the intensity and duration of the cold event. Chilling injury can occur on tropical species at temperatures above freezing, with symptoms of discoloration, reddish or black spotting or blotches, or necrosis of the leaflets (Figs. 1 and 2). The youngest, partially expanded leaf is typically less affected than older foliage, and the petioles and rachis will usually be the last tissues to become necrotic (Fig. 3). Premature fruit drop is also an early symptom of chilling injury. As the temperature experienced decreases to freezing or below, the primary symptom is foliar necrosis, with the same within-plant distributional pattern followed as with chilling injury. Severe freezes cause necrosis of the spear leaf base and secondary saprophytic bacteria and fungi colonizing this dead tissue can cause a basal spear leaf rot (Fig. 4). In this situation, the spear leaf can easily be pulled out, and the mushy base will emit a foul odor. If these secondary microbes reach the meristematic tissue, the primary meristem itself can be killed, resulting in the death of the palm.
Prolonged cold, but not necessarily freezing, temperatures can cause a wilt and eventually death of any green tissue remaining after the coldest weather, particularly in very tropical species such as Cocos nucifera (Fig. 5).
Cold injury can easily be confused with potassium deficiency since both affect the older leaves on a palm. However, leaflet necrosis is uniform within leaflets and among leaflets from the base to the tip of the rachis with cold injury. Potassium deficiency results in necrosis being concentrated at the tips of leaflets and those symptoms will be most severe at the leaf tips and virtually non-existent towards the leaf base. Cold injury will typically affect the lower half or more of the canopy, whereas severe leaf necrosis will only be present on about one whorl of the oldest leaves with potassium deficiency. The reddish or black spots or blotches associated with chilling injury in some species can occur on leaves of any age, whereas the translucent yellow-orange or necrotic spotting caused by potassium deficiency will only be seen on the oldest leaves in the crown.
Cold damage can have physical (e.g., ice formation or desiccation) or physiological causes, depending on the species and the severity and duration of the cold weather. Gradual decreases in temperatures can result in palms becoming cold acclimated and therefore more tolerant of cold temperatures, whereas a rapid decrease to the same temperature can have much more severe effects on palms.
Cold damage can occur in any species, but the temperature at which damage will occur is species-dependent and affected by the degree of cold acclimation achieved prior to exposure to cold temperatures. Recent research has shown that potassium-deficient palms are much more severely affected by cold temperatures than those that are potassium-sufficient.