Pests and Diseases
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Insect pests and a number of diseases can attack and damage palms and in extreme cases can kill them. If the growing point of a palm is invaded by insects or infected by disease the strength of the palm can be greatly reduced, and if it is destroyed, the palm will die. Cycads are less affected by pests and diseases, their tough leaves are most vulnerable only during the stage that they are emerging and before they have hardened up.
In the cool climates, attack on palms and cycads by pests and diseases is not as much a problem as it is in tropical climates where several generations of insects may breed within one year and where there are a large number of diseases that thrive in the warmth and humidity. This is especially the case with fungal diseases. The cool climates have the advantage in that during the cooler months many species of insects go into decline and in some climates are killed by cold. The marked separation of the seasons arrests the spread of some diseases and gives one the time to keep control.
Some pests and diseases are common to most countries, while each country has some that are common mainly to it Therefore mention of all of the types is not possible in this section.
Insects and mites can attack palms and damage the new growth causing the leaves to look unsightly. Older palms are not attacked to the same extent as younger ones, or if they are, the damage is less noticeable.
Insects generally fit into one of two catagories, those that cause damage by sucking sap from the leaves and those that cause damage by chewing at the leaves.
These insects are oval in shape, about 2 mm in size, are grey, pale pink or pale yellow in colour and have a white wax covering their bodies. They multiply rapidly into large colonies that can often look like patches of white fungus and feed by sucking sap from the leaves and stems. They thrive in warm moist places and can live in the soil or on the plant. If they affect the bud of a palm, new leaves may emerge distorted They also have the tendency to colonise on the underside or in the crown of cycads, again sucking sap. Sometimes they may be associated with ants who feed on honeydew that they produce. Mealy bug infestations can be controlled by spraying with dimethoate.
Mites And Spider Mites:
These sucking insects are usually invisible to the eye and breed rapidly in warm dry conditions. They can tend to go unnoticed until signs of their damage are obvious. They colonize on the underside of palm leaves, withdrawing the sap causing the leaves to go brown and die. The European mite and the two-spotted mite are known to invade not only the leaves but also the bud and crownshaft of some palms, causing their growth to be arrested. Control is achieved by spraying with dicofol or other specific miticides often twice to break the insects breeding cycle.
There are many species of these pests that suck sap from the leaves of palms and cycads. They can vary in shape and colour and commonly encountered types are oval in shape with armoured shells and sometimes waxy to touch, usually visible and do not seem to move about. While many of the types are specific to certain countries, some species, white palm scale (Phenacaspis eugeniae), palm scale (Parlatoria proteus), fern scale (Pinnaspis aspidistrae), soft brown scale (Coccus hesperidum) and pink wax scale (Ceroplastes rubens) are known to attack palms and cycads. Their damage is greatest on palms and can cause yellowing of the leaves with the infestation of sooty mould in shaded areas. Scale can be controlled by spraying with maldison or dimethoate.
A number of the species of thrips feed on dead leaves or decaying matter while some will attack palms causing considerable damage. They are small in size and colonize on the underside of palm leaves chewing by rasping and sucking the sap causing the leaves to develop silver-white patches. The adults fly from plant to plant, lay eggs and the young thrips feed on the plant then often move into leaf litter at the base of the plant, develop into adults then take flight to lay eggs again. They are particularly prevalent after warm dry weather. Control by spraying with dimethoate or maldison.
These insects resemble tiny white moths and some species attack palms infesting the underside of the leaves and sucking sap from the plant Small infestations can be controlled by hosing and if this is not effective it should be sprayed with dimethoate or demeton-S-methyl.
Caterpillars Of Butterflies And Moths:
Many butterflies and moths do not feed at all but lay eggs that develop into larvae and there are a large number of insect larvae referred to as caterpillars which cause damage to palm leaves or emergent cycad leaves by chewing or grazing on the foliage. Many can be picked off the leaves or controlled by spraying or dusting with carbaryl, maldison or pyrethrum.
Palms with a trunk growth form which are subject to caterpillar invasion can be protected by applying a tree band around the trunk and coating it in horticultural glue.
A strain of Bacillus thuringiensis (Bt) marketed by Yates as 'Dipel' provides targeted organic caterpillar control, killing caterpillars which eat leaves sprayed with the Bt solution while not harming beneficial insects.
Other Chewing Insects:
Grasshoppers, locusts, earwigs, beetles and their larvae can attack palms at certain times of the year. They can be controlled with an insecticide such as carbaryl or other types depending on the type of insect and the country. Snails and slugs will attack some species of palms, particularly Chamaedorea species, moving up the stems to feed on the leaves. They can be physically removed or prevented by the use of metaldehyde or methiocarb pellets.
Borers And Termites:
These insects are known to cause damage to palms and cycads. Borers can bore holes in the trunks of palms causing visible damage but do not threaten palms particularly if they are healthy. Termites can cause major damage to palms, attacking from below the ground and are difficult to control In areas where they are prevalent, prevention by destroying their nests is the best means before they affect a plant They will also invade cycads that are unhealthy, attacking the roots and trunks. Slaters behave similarly, attacking unhealthy cycads, feeding on their stems and any rot that may have developed.
Diseases are often difficult to identify and it is common to mistake a nutrient deficiency as a symptom of a disease. Cycads are relatively free from diseases and are mainly susceptible to root rot if they are grown without good drainage. Those diseases that commonly affect palms are usually fungal or bacterial in type although viruses are also known. Diseases can be spread by soil insects and human contact and when they do occur it is important to burn any infected parts of the plant once removed and avoid replanting where a soil disease is known to be present. If potting soil used to raise seedlings is infected because the seedlings died from obvious signs of fungal attack wilt or rot, that soil should not be reused for a new batch of seedlings because it is likely that the disease will invade the new seed or plants.
This disease can be common for newly germinated palm seedlings that are being raised in a greenhouse which is warm, moist and has insufficient ventilation. The seedlings may readily rot either soon after they germinate and before they emerge or after they have totally emerged. Clean pots, soil and seed will help to eliminate and prevent the spread of organisms responsible for damping-off. Good ventilation should be provided and in addition, fungicide sprays such as benomyl or maneb can be used to reduce fungi.
In palms, this term has also been used to mean Core rot and Crown rot. The symptoms include wilting, yellowing and finally death of the terminal bud of the palm, the growing point, therefore death of the palm. Bud rot is caused by fungus or bacteria entering the crown of the palm and the infected tissue becomes a gelatinous mass. It can be caused from physical damage as from hurricanes which damage the growing point of the palm or from excess water sitting in the crown allowing the entry of pathogens. It has also been caused by insect damage to palms, such as severe attack by mites that suck the sap from the plants, weakening their growth so that new growth is also weak and fungi can enter. It can be prevented after physical damage, or if it is detected in its early stage, by pouring a solution of a systemic fungicide such as benomyl or a general fungicide such as copper oxychloride into the growing point.
A fungal disease identified as Fusarium wilt (Fusarium oxysporum) a soil inhabiting fungus that enters the plant through the roots attacked 15% of the 3000 Phoenix Palms in San Diego, USA in the early 1980s, causing first the lower leaves to wilt and the palms to die. Other palms such as
Syagrus romanzoffiana and Washingtonia species were immune to the disease and not affected. Fungi of this type can remain in the soil for a number of years developing in warm weather and can be spread by tools and shoes. It can affect other palms planted in similar localities and it is transmitted by pruning; all tools should be sterilized between pruning trees.
Diseases in this class are caused by the fungi Phytophthora particularly in cold wet and soggy soils. Leaves of the plant begin to turn yellow and it may survive to warmer months to die in hot weather. Phytophthora cinnamomii or cinnamon fungus can attack Syagrus rornanzoffiana in soil that is too heavy and with poor drainage. If a plant has suffered and died from this disease, it is important not to replant in the same position unless the soil is treated with metalaxyl. All parts of a diseased plant should be removed from the site and buried or burned. Root rot has been known to affect Washingtonia filifera in Arizona during the 1930's causing a number of palms to die.
Cycads suffering from root rot should be dug out and washed to remove all the soil. Any areas of rot should then be removed with a sharp knife back to uninfected tissue and the damaged area soaked in a strong solution (5g per litre) of benomyl for at least 24 hours. The number of leaves should be reduced and the cycad placed in a dry shady place for around two weeks to allow it to callous and seal the wounds. It can then be planted in well drained medium, watered and allowed to dry out between each watering until signs of new growth appear.
These black fungi can cover the leaves of palms and cycads and have the appearance of soot The mould grows on excretions of aphids, mealy bugs and some scale and it is often called 'honeydew'. It may fall onto the plant from taller trees infected by these insects. Although it does not damage plants, it lowers their efficiency to use light and photosynthesise. The remedy is to remove the source of the insects producing the 'honeydew' and the mould will disappear.
Rusts are caused by fungi and are evident as yellow or orange spots on the surface of the leaves. They do not usually cause a more serious problem than to detract from the overall appearance and can be treated by spraying with copper oxychloride.
This disease was first reported in the 1800's and started to become widespread in tropical areas of the Caribbean and Florida in the 1970's when it affected Coconut Palms and had a devastating effect on coconut production. It spread to Texas in 1978 affecting Phoenix Palms in the Lower Rio Grande Valley then extended into Mexico. It was estimated that it spread at the rate of 1-3 miles per month. It is caused by mycoplasma-like organisms being carried from tree to tree by insects known as planthoppers who inject the microbes from their saliva, infecting palms as they feed on their leaves. New populations of insects then pick up the disease from infected palms and pass it on to others. Lethal yellowing generally kills a palm within five months. First the leaves turn yellow and fall until the bare trunk is left. The symptoms can be suppressed by boring a hole in the trunk of a palm and filling it with a tetracycline antibiotic. The treatment must be repeated every four months as the disease reoccurs as soon as the treatment is discontinued. The main method to prevent the disease from depleting palms from an area is to introduce species resistant to it. Coconut Palms are being replaced with species resistant to Lethal yellowing.
Chemicals For Pest And Disease Control
All chemicals for the control of pests and diseases should be handled in accordance to the manufacturers directions and should not be intermixed or stored mixed unless otherwise advised. Their toxicity warnings should be heeded and the appropriate protection used when they are being handled and sprayed.
The following table lists the chemicals mentioned in the section 'Pests and Diseases' giving some of the Trade names used by manufacturers.
|Chemical Name||Trade Names|
|carbaryl||Carbaryl®, Septan 80®, Sevin®, Bugmaster®|
|copper oxychloride||Cop-ox®, Copperox®, Cobox®, Cupravit®|
|dimethoate||Rogor®, Roxion®, Perfekthion®|
|metaldehyde||Defender®, Snail & slug Killer, Blitzen®|
Palms & Cycads Beyond the Tropics, Keith Boyer.