Symptoms of potassium (K) deficiency vary among species but always appear first on the oldest leaves. Older leaflets of some palms such as Dictyosperma album are mottled with yellowish spots that are translucent when viewed from below (Fig. 1). In other palms such as Dypsis cabadae, Howea spp. and Roystonea spp., symptoms appear on older leaves as marginal or tip necrosis with little or no yellowish spotting present. The leaflets in Roystonea, Dypsis, and other pinnate-leaved species showing marginal or tip necrosis often appear withered and frizzled (Fig. 2).
In fan palms such as Livistona chinensis, Corypha spp., Washingtonia spp., and Bismarckia nobilis, necrosis is not marginal but is confined largely to tips of the leaflets (Fig. 3). In Phoenix roebelenii, the distal parts of the oldest leaves are typically orange with leaflet tips becoming necrotic. The rachis of the leaves usually remains green, however, and the orange and green are not sharply delimited as with magnesium deficiency (Fig. 4). This pattern of discoloration holds for most palm species that show discoloration as a symptom. In P. canariensis, there may be little discoloration, but leaflets show fine (1-2 mm) necrotic spotting and extensive tip necrosis. These necrotic leaflet tips in most Phoenix spp. are brittle and often break off, leaving the margins of affected leaves irregular.
In Caryota spp. and Arenga spp., chlorotic mottling is minimal or non-existent, but early symptoms appear as irregular necrotic spotting within the leaflets and/or leaflet tip necrosis (Fig. 5). In most other palms, including Cocos nucifera, Elaeis guineensis, Dypsis lutescens, Chamaerops humilis, Hyophorbe verschafeltii, and others, early symptoms appear as translucent yellow or orange spotting on the leaflets and may be accompanied by necrotic spotting as well (Fig. 6). As the deficiency progresses, marginal and tip necrosis will also be present. The most severely affected leaves or leaflets will be completely necrotic and frizzled except for the base of the leaf, leaflets, and the rachis (Fig. 7). Symptoms decrease in severity from tip to base of a single leaf and from old to new leaves within the canopy (Figs. 7 and 8). In severe cases, the entire canopy will consist of a reduced number of leaves, all of which will be chlorotic, frizzled, and stunted. The trunk will begin to taper (pencil-pointing) and death of the palm often follows (Figs. 9 and 10).
Manganese deficiency, leaf spot diseases, royal palm bug damage, soil salt injury, nutrient toxicities, water stress
Manganese (Mn) and potassium deficiencies can be extremely similar from a distance, but close examination should reveal characteristic spotting and marginal necrosis in K deficiency or necrotic streaking for Mn deficiency. Potassium deficiency symptoms are also more severe toward the leaf tip and are less so at the leaf base. The reverse is true for Mn deficiency.
Leaf spot diseases are typically distributed throughout the canopy and not concentrated on the oldest leaves, but there are exceptions to this generalization.
Old damage from royal palm bugs appears similar to that of K deficiency on royal palms, but the leaflet frizzling caused by royal palm bugs is typically observed in mid-canopy and extends the length of the leaf. K deficiency will be concentrated towards the tips of the oldest leaves in this species.
Soil salt injury, water stress, and various chemical toxicities also appear as tip necrosis concentrated towards the tips of the oldest leaves. Leaf nutrient or soil analyses may be helpful for distinguishing these similar disorders.
Potassium deficiency is caused by insufficient K in the soil but can be induced or accentuated by high N:K ratios in the soil. Potassium is readily leached from sand or limestone soils which have very low cation exchange capacities.
Potassium deficiency is very common on palms grown in highly leached sandy soils. It is less common in container substrates. Potassium deficiency is perhaps the most widespread of all nutrient deficiencies, occurring in most palm-growing regions of the world. It is quite severe in southern Florida and much of the Caribbean region. Although most species of palms are susceptible to some degree, genera such as Veitchia, Ptychosperma, Adonidia, and Archontophoenix are notably resistant to K deficiency. Potassium deficiency is the leading cause of mortality in Roystonea growing in southern Florida landscapes.
Visual symptoms alone may be sufficient for diagnosis of this disorder, although leaf nutrient analysis may be helpful in distinguishing late stage K deficiency from Mn deficiency. These two deficiencies can be extremely similar from a distance, but close examination should reveal characteristic spotting and marginal necrosis in K deficiency or necrotic streaking for Mn deficiency. Potassium deficiency symptoms are also more severe toward the leaf tip and are less so at the leaf base. The reverse is true for Mn deficiency.
In Elaeis guineensis, K deficiency has been associated with increased incidence of diseases such as Fusarium wilt, Cercospora leaf spot, and Ganoderma butt rot.