Pests and Diseases II
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: benomyl Benlate®, Tresan® carbaryl Carbaryl®, Septan 80®, Sevin®, Bugmaster® copper oxychloride Cop-ox®, Copperox®, Cobox®, Cupravit® demeton-S-methyl Metasystox® dicofol Kelthane® dimethoate Rogor®, Roxion®, Perfekthion® maldison Malathion®, Malathon® maneb Manzate D® metalaxyl Ridomil® metaldehyde Defender®, Snail&slug Killer, Blitzen®
<<< Previous Page
Extracted from: Palms and Cycads Beyond the Tropics, Keith Boyer.