Boron Deficiency

Symptoms

Boron deficiency causes a wide array of symptoms, not only among species of palms, but also within a single species. Symptoms always occur on newly emerging leaves, but remain visible on these leaves as they mature and are replaced by younger leaves.

One of the earliest symptoms of B deficiency on Dypsis lutescens and Syagrus romanzoffiana is transverse translucent streaking or puckering (Fig. 1) on the leaflets. In many species including Cocos nucifera, Elaeis guineensis, and S. romanzoffiana, mild B deficiency may be manifested as sharply bent leaflet tips, commonly called "hookleaf" (Fig. 2). These sharp leaflet hooks are quite rigid and cannot be straightened out without tearing the leaflets. In some species, these "hooks" drop off. Another symptom associated with chronic B deficiency in S. romanzoffiana is the production of weak, narrow leaflets towards the tips of newly emerging leaves. These leaflets often drop off, leaving the rachis tip devoid of leaflets (Fig. 3).

Boron deficiency can be very transient in nature, often affecting a developing leaf primordium for a very short period of time (e.g., 1 to 2 days). This temporary shortage of B can cause necrosis of the primordial spear leaf for a distance of about 1 to 2 cm. When such leaves eventually expand, this "point" necrosis affects the tips of all leaflets intersected by that necrotic point, the net result being the appearance of a blunt, triangular truncation of the leaf tip. This pattern can be repeated as many as 3 times during the development of a single leaf of Cocos nucifera (about 5 weeks) (Fig. 4).

One of the most common symptoms of B deficiency is the failure of newly emerging spear leaves to open normally. They may be tightly fused throughout their entire length, or the fusion can be restricted to basal or distal parts of the spear leaf. In a chronic state, multiple unopened spear leaves may be visible at the apex of the canopy (Fig. 5).

Perhaps the most unusual symptom of chronic B deficiency is the tendency for the entire crown to bend in one direction (Figs. 6 and 7). This is one form of epinasty that can also cause twisting of petioles and leaves or sharp bends in the petiole, resulting in a new leaf growing downward along the trunk (Fig. 8). These epinastic symptoms are believed to be caused by a B deficiency-induced decrease in indoleacetic acid oxidase activity and therefore excessive auxin concentrations within the leaves. Boron deficiency can even cause branching in species that do not normally branch (Fig. 9).

Boron deficiency in its acute form produces yet other symptoms. Often leaves emerge greatly reduced in size and crumpled in a corrugated fashion (accordion-leaf) (Figs. 10 and 11). Palms may grow out of these symptoms or the deficiency can kill the meristem (Fig. 12). Boron-deficient palms often abort their fruits prematurely and inflorescences may have extensive necrosis near their tips (Figs. 13 and 14). 

May Be Confused With

Manganese deficiency in Cocos nucifera produces symptoms similar to those of B deficiency, but no other common deficiency produces symptoms that could be confused with those of B deficiency. Lethal yellowing also causes inflorescence necrosis and premature fruit drop, however, the calyx end of fallen coconuts from LY-infected Cocos nucifera will be blackened, whereas coconuts from B-deficient trees will not have this blackened end.

Cause

Boron is readily leached through most soils, with a single heavy rain event temporarily leaching most available B out of the root zone. When this leaching stops, B released from decomposing organic matter will again provide adequate B for normal palm growth in most cases. Thus, mild, transient symptoms of B deficiency are believed to be caused by leaching. On the other hand, in drier climates B binds tightly to soils as they dry, and soil drying is believed to be the cause of more chronic B deficiencies in palms.

Occurrence

Boron deficiency is very widespread on palms growing in wet climates throughout the world, but can also occur in desert situations. It was almost unknown in Florida until a series of high rainfall summers occurred in the mid 2000s, when it became a widespread and serious problem on palms within the state. Boron deficiency has been observed in container-grown palms, but it is rather rare in this environment.

Additional Comments

Because B deficiency can be highly transient in nature and because deficiencies affect only leaf primordia developing within the bud area, leaf analysis is not particularly useful. Leaf analysis informs you of the B status of the palm 4 or 5 months ago when the sampled leaf itself was in the critical developmental stage within the bud. The B status of the palm may have changed considerably one way or another during 4 or 5 months since the affected leaf became old enough to sample. Thus, leaf analysis, or even leaf symptoms, unless the deficiency is chronic (regularly occurring), cannot tell you about the current B status of a palm.

December 1, 2010