Helminthosporium-complex leaf spot

A number of leaf spot fungi cause variously shaped lesions on the leaf surface of many palm species. High rainfall or frequent overhead irrigation are often instrumental in their spread. If only a single leaf is affected, removal and disposal of that leaf is a simple and effective control. Some leaf spot fungi move in as secondary problems on palm leaves that are deficient in nutrients or have received some sort of damage. Leaf spot diseases caused by various Bipolaris and Exserohilum fungi (often called Helminthosporium-complex leaf spots) affect a broad range of palms with characteristically round, dark brown lesions that eventually merge and form large blighted areas.

The disease is easily spread by overhead irrigation. Chlorothalonil and mancozeb fungicides have provided good controls. Cercospora leaf spot is frequently a problem on Rhapis palms, and Cylindrocladium on kentia (Howea forsteriana). Anthracnose caused by a Colletotrichum fungus can affect a large number of palms, particularly where overhead irrigation is used. Stigmina (Exosporium fungus) leaf spot can be a particular problem on date palm (Phoenix) species. Graphiola leaf spot or `false smut’ can become a significant problem on landscape palms during periods of high rainfall.

The disease becomes conspicuous when the fungus responsible produces its grayish-black fruiting bodies which rupture through both leaf surfaces. Copper based fungicides and maneb or mancozeb have halted infections if applied in the early stages. Pestalotiopsis leaf spot affects a number of species. It seems to be a particular problem on date palm (Phoenix) species on which lesions often first appear on the rachis tissue.

Graphiola leaf spot or `false smut'

Causes elongated diamond-shaped lesions on the leaf surface. Both copper and a wide range of fungicides have been used to combat this fungal disease. In California and Arizona, diamond scale (Sphaerodothus neowashingtoniae) attacks California fan palm (Washingtonia filifera), causing premature death of the leaves.
This fungus produces black, diamond-shaped sporulating structures on the leaf blade and stem. Removing infected leaves is the best control; the effectiveness of fungicides is not reported.

Sooty mold

This superficial fungal disease, caused by Capnodium spp., is more a nuisance than a life-or-death problem on palms. When present, it is always associated with infestations on sucking insects such as palm aphid, scales, or mealybugs. These insects excrete `honeydew’, a waste product high in sugars that the sooty mold fungus feeds upon. The fungus appears on the leaf surface (and sometimes the trunk) as a conspicuous black, sooty deposit. Heavy infestations will interfere with the food-manufacturing efficiency of the leaf. The best control is to keep the palm free of honeydew-producing insects.

Phytophthora bud rot of coconut (Cocos nucifera)

One of the more common diseases encountered in palms in wet tropical climates. It is primarily a warm season disease. This soil borne fungus causes collapse or brown-out of the younger foliage and emerging leaf. Discoloration of the internal tissue of the stem is evident, often accompanied by a foul smell. Phytophthora can also cause a leaf spot. Good control of bud rot is accomplished by drenching the soil with metalaxyl (Subdue), or applying a foliar spray of fosetyl aluminum (Alliette) at label rate. Foliar applied fosetyl aluminum will translocate to the roots of the palm, whereas metalaxyl will reduce populations of this soil borne fungus in the root zone. Overwatering and planting too deeply aggravate incidences of Phytophthora.
Root rot and discoloration of vascular tissue caused by Phytophthora

Pink rot or Gliocladium blight

This fungal disease is a serious problem on Chamaedorea species and areca palms in Florida, and queen, date and Washingtonia palms in California. The causal agent is not active at temperatures above 85? F, thus it is primarily a winter disease in Florida, while remaining active most of the year in coastal California. Oozing lesions occur on the stems, and leaves turn brown and droop. The fungus produces salmon-pink, powdery fruiting bodies. The disease is easily spread if affected leaves are pulled off the plant prematurely, thus leaving an entrance for new disease inoculum. Mancozeb fungicides provide good control.

Is increasing in frequency on palms in Florida, but is not yet terribly common. This soil borne fungus generally enters the palm through wounds, and causes the disintegration of the trunk or bud. It can also infect leaves of young palms. A cross-section through the trunk will reveal blackened fruiting bodies. Affected palms will blow over easily.

Decline of coconut (Cocos nucifera) caused by Ganoderma

Has become a serious and incurable disease of older landscape palms (usually 15 or more years old). The disease progresses from the older leaves upward, which turn brown and droop from the trunk. Wounds on the lower portions of the trunk or roots favor entry of the fungus. The fruiting body of the fungus is a conspicuous bracket or `conch'. The disease spreads rapidly from plant to plant, and the fungus can persist in the soil for many years. Affected palms must be completely removed and destroyed and the soil fumigated. All visible conchs should be collected and destroyed. If ganoderma has been diagnosed in a landscape site, it may be best to replant with a broad-leafed tree, as no palm can yet be declared reliably resistant.

`Conch' of Ganoderma fungus on trunk of affected palm

Canary Island date palms (Phoenix canariensis) with Fusarium wilt

Fusarium wilt is a problem in California and Florida. The disease frequently causes an uneven decline in the canopy of an infected palm, with leaflets on only one side of a single leaf dying first. The water and food conducting tissue within the leaves is usually discolored. Canary Island date palm is killed by the disease; Phoenix dactylifera is affected non-fatally. In Florida, Washingtonia robusta is also susceptible. Pruning tools are known to transfer the fungus from tree to tree, and should be sterilized before using again on a different tree.

Causes a wet blight of the emerging spear leaf which can spread downward to the irreplaceable bud. Affected spear leaves often will pull easily from the bud. A foul odor frequently accompanies the damage. It often follows hard on the heels of recent cold damage to a palm. Bud drenches with copper-based fungicides have been a common treatment for bacterial bud rots, but their effectiveness has never been absolutely proven.

Necrotic inflorescence, an early sign of lethal yellowing

Lethal yellowing is an incurable disease of many palm species caused by a mycoplasma-like organism (a form of life sometimes described as intermediate between a virus and a bacterium) vectored by a leaf hopper bug (Myndus crudus). The disease organism is now resident in at least Palm Beach, Broward, Dade, Monroe, Lee and Collier counties of Florida, southern Texas, Mexico and parts of Africa. The disease often begins with the blackening of young inflorescences on infected palms. On coconuts, developing fruits will suddenly drop off the stems. One by one, mature leaves may begin to yellow on the palm, until all leaves in the canopy wilt and die. On other species (and some varieties of coconut as well) the yellowing may not be conspicuous; instead, leaves collapse and the palm quickly dies. The only practical control is to avoid planting highly LY susceptible palms. The decline caused by the disease can be temporarily suspended (though not cured) with a program of injections of tetracycline antibiotics, but only on palms with a developed trunk. Injections can be maintained until a resistance replacement palm achieves acceptable size, after which the infected palm is allowed to die.
Coconut palm (Cocos nucifera) with lethal yellowing
Christmas palm (Adonidia merrillii) with lethal yellowing

Some palm species characteristically develop vertical fissures on their trunks. When these appear on palms that normally do not express them, it is usually an indication of water problems. Both too much or too little soil moisture can result in small cracks on the trunk, as can overly deep planting. Large scale trunk splitting is often associated with an over-abundance of water. Trunk cracking can also occur as a consequence of cold damage.

Trunk constriction

At the point along their length where active growth is taking place, palm stems complete their caliper growth (that is, increase in diameter) before elongating. The optimal caliper that a palm species will achieve is partially determined by the intrinsic character of the species and partially by the quality of the growing conditions at that point in time. If nutrition or water supply is limiting, or if some other type of environmental stress occurs (a freeze, for example), the palm stem may fail to achieve the same increase in diameter as occurred in past years. As conditions improve, the stem will once again reach optimum caliper. The result over the long term will be a constriction in the trunk at the point where the stem was actively growing when the stresses occurred. In older palms, it is sometimes possible to `read' the past history of growing conditions by the patterns of constrictions that appear along the length of the trunk.

This syndrome is often related to that of trunk constriction. `Pencil-pointing’ refers to a sudden, unnatural narrowing of the stem towards the crown of the palm. It is often associated with acute nutrient deficiencies, but can also be caused by continuous overtrimming of the canopy. If conditions improve, the palm will return to its normal caliper growth, and a trunk constriction will develop at the point where pencil-pointing was observed.

Lightning strike on coconut palm

A direct lightning hit on a palm is usually fatal. Sudden collapse of the crown, trunk splitting and/or bleeding, and dark streaks on the trunk are all possible symptoms of lightning damage.
Trunk bleeding caused by lighning strike

Tall palms that have reached close proximity to high voltage powerlines have been observed with yellowed or necrotic leaves despite regular fertilization and no evidence of pests or disease, suggesting that the electromagnetic fields around these lines can injure them.

Many herbicides can cause damage to palms. Telltale signs of herbicide injury include distorted and under-sized new growth and patches of dead tissue on the leaves. Damage from some pre-emergent herbicides may take months to become apparent. Consequently, special care should be taken when using weed-killing chemicals around landscape palms, avoiding any contact of the chemical with new roots, or any green tissue on the palm. Only herbicides labeled for use around palms should be applied.

Certain palm species (fishtail palms, Caryota spp., for example) flower and fruit once and then die. On clustering species with this habit, new stems are produced that continue the growth of the palm, while solitary palms will have to be replaced.

Leaf burn on the windward side of palms planted near the shore is often indicative of salt injury. It usually follows a period of high winds. A sudden intrusion of salt water into the root zone can cause an overall decline and death of the plant. The best way to deal with this problem is to plant in exposed coastal locations only those palms with high salt tolerance.