PLANTS PESTS PESTICIDES PEOPLE

 
INTRODUCTION -


We all enjoy growing our favorite plants. While they are, as a general rule, hardy and disease resistant, like all plants, they are occasionally subject to attack and damage by various pests and diseases. This article will:

o look at some orchid afflictions
o consider how we can mitigate the ravages of those that do infect our plants
o look at the characteristics of the chemical pesticides we may have to use to control
their effects
o consider safety precautions associated with the use of these chemicals.

This article was written some years ago. Because materials can change, CHECK WITH AN EXPERIENCED GROWER OR YOUR GARDEN STORE OR SUPPLIER FOR CURRENT RECOMMENDATIONS. IF YOU CANNOT OBTAIN GOOD ADVICE, GO SOME WHERE ELSE WHERE COMPETENT ADVICE IS AVAILABLE.

Different countries have different products and regulations. CHECK THE LOCAL SITUATION APPLICABLE TO YOU.


PESTICIDE CLASSIFICATION


Pesticides are toxic chemicals which kill plant pests and diseases. There are different types of pesticides formulated for particular uses:

· fungicides - for the control of fungal diseases
· bactericides - for the control of bacterial diseases
· insecticides - for the control of insect pests
· acaricides - for the control of mites
· nematocides -- for the control of nematodes
· herbicides -- for the control of weed plants
· mulltiscieldes - for the control of slugs and snails
· fumigant gasses - for the control of a range of pests in soil or in enclosed spaces such as
· glass houses
· rodenticides -- for the control of rats and mice.

Some pesticides are selective in their action. Not all fungicides, for example, will affect all fungi; others will have a wider action, with even say bacterial control capabilities.

To understand the use and application of pesticides, some knowledge of how they act is useful. As orchid growers, we are primarily interested in the following:

Fungicides and bactericides have two basic modes of action:

o Protectants. The material forms a protective coating on the surface of the plant. Any fungus or bacteria landing on this coating is killed. Examples are Zineb, Rovral.
o Systemics. These are absorbed by the plant tissues and translocated inside the plant to all tissues. An example is Benlate.

Insecticides, acaricides may be either

o Contact/stomach poisons which kill when the insect touches the plant surface or are ingested with plant material eaten. Example Matathion,
o Systemic poisons. These are absorbed by the plant and circulated by the sap. They are especially effective against insects with piercing or sucking mouthparts. Example is Orthene.


CHEMICAL FORMULATIONS

Factors which can affect the performance of pesticides are:

* Formulations. Few of the materials available are soluble in water and must therefore be mixed with other chemicals to obtain an even dispersion on mixing with water. The principal formulations are:

o dusts - the active ingredient mixed with a filler e.g. tale
o water soluble powders - active ingredient dissolves in water to form a solution
o wettable powder -active ingredient plus dispersing agent which mixes with water to give a fine suspension
o emulsifiable concentrates -- active ingredient plus a solvent, pills an emulsifier. It mixes with water to form a milky white suspension.
o aerosols - pesticide and solvent stored under pressure
o smoke bombs - the active ingredient is incorporated in a pyrotechnic mixture and is vapourised when the mixture is ignited.

In general there is a greater danger of phototoxicity from an emulsifiable concentrate than from a wettable powder formulation. If two pesticides are to be mixed together, cheek their compatibility and avoid mixtures of wettable powders and emulsifiable concentrates.

Chemicals are not normally formulated specifically for orchids most being utilised in horticulture generally. Therefore the suitability of products for all types of orchids must be carefully ascertained. For the commonly grown genera cymbidiums, cattleyas etc.) it can be anticipated that they will be suitable is generally recommended. Those growing a diverse collection of genera, however, may find that some pesticides will be toxic to certain genera. Always test each new material on spare plants of the different genera grown or discuss with those who may have had experience with the material before using over the whole collection. These precautions could save you from the loss of valuable plants. Most companies, while giving general recommendations, place the responsibility for the use of their products entirely on the user; if the product kills your plants and there is no problem with the product specification, the responsibility for the use of the material is yours.

* Surfactants. Wetting agents reduce the surface tension of spray droplets, causing each spray droplet to flatten out to cover a larger leaf surface, and also reduce the opportunity of droplets falling off the leaf surface.

DISEASE PREVENTION


The above discussion presupposes that chemicals are always required. That is not always the case. so perhaps it is appropriate to consider at this point

o how a collection can be kept disease and pest free
o how to reduce or eliminate the opportunity for pests and diseases becoming introduced or established

These aims can be met by:

o Buying only disease free vigorous plants.,
o Observing good sanitation -- keeping benches and floor free from plant debris,
o Observing good culture - water when needed, preferably allowing the plant foliage to dry off quickly, fertilise appropriately, repot when needed every 2 -3 years, ensure temperatures and ventilation are adequate. It may be possible to vary your cultural practices to make conditions less suitable for any pests or diseases present or suspected.
o Quarantining new plants before introduction into your collection, inspecting. perhaps repotting, and giving preventative sprays.
o A preventative spray occasionally will reduce the chances of any undesirable visitor becoming established.
o Observing and handling your plants regularly. If anything unusual is seen, take the appropriate action as soon as possible if it is something undesirable. If it is a pest or disease unfamiliar to you, have it properly identified. and find out as much as you can about it, and seek the best available advice concerning its control.


*** REMEMBER, PREVENTION 15 ALWAYS BETTER THAN CURE. ***

CHEMICAL TOXICITY

Sometimes despite our host efforts, chemical control is appropriate, or even essential for the continued well being of our collections. Before looking at the specific use of chemicals. it is appropriate to remember, these chemicals are poisons, they are toxic not only to the pest. but in many cases, can be toxic, to you, the grower, as well.

People become poisoned by the material entering the body by the following means:

o dermally - by absorption through the skin.
o orally - by contamination of food. or swallowing the pesticide from the container.
o respirationally by breathing in the vapour or spray droplets.

The pesticides can vary greatly in the danger they present to the user. They are universally classified to the degree of hazard they involve to the operator. This is expressed as tile ID50 figure, being the amount in mg of the chemical necessary to kill 50%, of a population (of orchid growers!) per kg. of the recipient's weight. Remember, the figure represents tile amount killing 50'%; sonic people will remain unaffected at that dose rate, others can be affected by a lower level of contamination. The figures are therefore primarily for the standard comparison of the toxicity of materials. The toxicity classification generally acceptable is tabulated as follows:

TOXICITY HAZARD OF CHEMICALS

Classification

LD 50
(oral or dermal)

Theoretical amount required
to kill an "average" human
of 65 kg
(about 10 stone or 140 lbs) weight.

Very highly hazardous

Up to 50 mg/kg

One drop to one teaspoon full
taken orally or via skin or eye

Highly hazardous

50 - 100 mg/kg

About one teaspoonful

Moderately hazardous

100 500 mg/Kg

One teaspoon to 2 teaspoons

Slightly hazardous

Above 500 mg/kg

About 30 to 450 grams
(one ounce to one pound or more).

Examples are:
Very highly hazardous
Products of this toxicity are not available to amateur growers.
Highly hazardous Metasystox, Mesurol ®
Moderately hazardous diazinon, dimethoate (Rogor®)
Slightly hazardous captan, maldison (Malathion®), acephate (Orthene®), dicofol (Kethane ®) tetradifon (Tedion®)

® = Registered trade name for the product.


The LD50 figures for the main materials likely to he used by j orchardist may be printed on the packets. Lists are also published in some books such as Northen Home Orchid Growing. Remember that in an enclosed glasshouse chemical concentrations can more rapidly build up during spraying operations than is possible in the open air. Thus in glass houses greater care is required.


SAFE HANDLING PROCEDURES

The following of these basic rules will ensure the chemicals affect only the target. and not the person applying the material.

1. Read the label. Do this before opening the container and note the warnings and captions. Follow instructions carefully, and take particular note of what to do in the case of accidental poisoning-
2. Lock them away. Sprays. granules and dusts must be kept locked away, out of reach of children and pets.
3. Safe storage. Keep chemicals in their original containers. Never store any pesticide in jars, cans, or other unlabelled vessels, especially not in anything associated with human food or drink such as soft drink or beer bottles.
4. Wear protective clothing. The label will indicate what protection is needed, such as clothing and masks. Masks must he of the right type for the chemical being used, and must be well fitting, clean and in good order.
Toxic pesticides and chemicals can be absorbed into the body by inhalation, swallowing or direct skin contact. and all these make protective clothing essential at all times.
5. Wash thoroughly. The hands and face should be washed after spraying or dusting, and clothes should be changed for clean ones. Used clothing should be washed before reuse.
If a spill of chemical occurs, contaminated clothing should be removed and skin and clothing washed. A person working with any of these chemicals should not eat or smoke before washing thoroughly. Soap and water should be readily available so that if skin contact is made, the area can be washed immediately. In the case of chemical contamination of the eyes, they should he flushed immediately with copious amounts of clean water.
6. Protect food and foodstuffs. Make sure water supplies and food are not exposed to pesticides, and store the chemicals away from these. Make sure residues cannot accumulate in puddles, streams or ponds as they are toxic to animals, wildlife and humans.
7. Use chemicals accurately. Make sure the strength of the mixture is correct, and that the equipment has been calibrated accurately.
8. Dispose of empty containers, Cans or drums should be washed out, punctured, crushed flat and buried away from water supplies. The same applies to glass containers. Combustible containers should he burnt, and the ashes buried. Any leaks or spillage's should be cleansed up immediately.
9. Observe label directions. Make sure all application instructions are followed: this ensures the safe and economical use of the material.
10 Emergency action. If a user complains of nausea, headache, dizziness, double vision or other unusual sensations, medical help should he sought immediately.. First aid measures according to the label should he used until medical help arrives.
Anyone exposed to massive skin contact or inhalation of these chemicals should be rushed to a doctor or hospital. Take the chemical container with you.

It is appropriate to also remember that in the protected environment of a glasshouse, pesticide residues can remain for some time on the foliage; thus care in handling the plants after spraying must be exercised .

PESTICIDE INJURY TO PLANTS

Pesticides can injure the plant in some circumstances. This may arise from a failure to observe precautions with respect to temperatures, application rates, compatibility with other materials, and frequency of application. Injury varies from stunting, with no distinctive symptoms, to leaf-scorch, necrotic spotting, whitish banding on newly developing leaves and abnormal reddish or reddish-brown or purplish pigmentation in the blades. Necrosis may he superficial or it may extend through the blades with distinct collapsed margins to the affected areas and to rupture of tissues

PEST CONTROL PROGRAMS

This discussion will concentrate on some of the more significant problems experienced by amateur growers. The control measures will indicate certain chemicals but readers should note that these may not be appropriate in all circumstances. If you have a problem, talk it over with an experienced grower before taking drastic action as often there is more than one way of attacking a particular problem.

This article was written some years ago. Because material s can change, CHECK WITH AN EXPERIENCED GROWER OR YOUR GARDEN STORE OR SUPPLIER FOR CURRENT RECOMMENDATIONS. IF YOU CANNOT OBTAIN GOOD ADVICE, GO SOME WHERE ELSE WHERE COMPETENT ADVICE IS AVAILABLE.


· Insecticides

New products are constantly becoming available. Before using a product unfamiliar to you, try and ascertain from the manufacturer or supplier if it is suitable for orchids and if it has been tested on them. In any case, test on a few prints of each general grown before applying to a complete collection to ensure that no detrimental effects are caused.

Two pests, especially common on cymbidiums, are the two spotted spider mite, and the false spider mite.

The two spotted spider mite, more commonly just known as "red spider" obtains its name from two coloured blotches on either side of it,, body. It is a small insect just visible to the naked eye. These insects feed on the undersides of leaves. penetrating the plant cells and sucking out all the cell contents, producing a speckled stippled appearance on the foliage. They spin a fine web, although often this is not seen until heavy infestations become established. With a life cycle of two to three weeks, a rapid build-up in infestations can occur.

False spider mites are very small, being barely visible to the naked eye, and they do not spin webs. They cause damage to both the upper and lower leaf surface producing generally a silvering and pitting of the leaf surface. Once the leaf surface is damaged, invasion by fungal and bacterial pathogens often occurs.

Maldison (Malathion®) and Kethane usually control these pests. Give two sprays 7 to 10 days apart to ensure a complete kill. Specific miticides, however, may be more useful. as these products are effective against the egg stages. against which the two products above have limited effectiveness. Tetradifon (trade name Tedion) and acephate (Orthene.) are such specific miticides.
Red spider likes warm dry condition and the regular misting of the foliage especially under the leaves, and the maintenance of higher levels of humidity, will often prevent heavy infestations. reducing or even eliminating the need for chemical control.

Aphids are often found. especially on buds and flowers. Under good conditions, establishment of this pest can be rapid. Maldison will live good control. although because of continuing winged infestation and short effective life of some pesticides re-treatment may he necessary. On flowers, the use of powdered insecticides (e.g. Rose Dust) are generally considered safest.

Scale insects are sometimes troublesome. Not only do they suck the sap, but some species also produce a damaging toxin. They are difficult to control with sprays because they are generally well protected, and attack all parts of the plants. Several applications of a contact insecticide such as Maldison (with a wetting agent) may he effective, although a systemic may be more effective. Acephate (Orthene®) is a systemic product. Often on small collections hand treatment of affected plants with methylated spirits. painted on with a small paint brush, will give adequate control.

Mealybugs can often become prevalent. These soft bodied insects secrete honeydew on which a sooty mould grows. A thorough application of Maldison, especially with the addition of a wetting agent, will often be effective. A second spray application three to four weeks later may be needed (and perhaps repeated) for total control.


The use of one of the proprietary 'pest strips' utilising the chemical vapona can be effective in general insect control within a glasshouse. To be fully effective, ventilation has to be restricted to allow the build up of adequate vapour concentrations.


With all control programs, remember that the continued use of one material over a period of time can cause the build up of insect populations resistant to that chemical. Therefore the occasional use of another formulation is generally recommended to eliminate this problem.


· Fungal and bacterial diseases

There are a number of fungal and bacterial diseases which can affect orchids. They are generally dividable into two groups:

o those that attack the roots and stem base
o those that affect the foliage and flowers.

Because of the variety of organisms involved, if root and stem infestation is suspected, the following actions should be immediately implemented:

o Remove all infected tissue.
o Destroy all infected material.
o Repot in fresh (sterilised) media using new (or at least sterilised) pots.
o lower the temperature and reduce the humidity.
o Improve the drainage provided by the growing container. Excessive watering, old tight mixes, can exacerbate the problem.
o Apply a suitable fungicide, e.g. Benlate. Other materials may be required.

Foliage and flower afflictions can generally he controlled by the application of Zineb, captan or Benlate., applied at 14 day intervals. Petal blight may arise when cold damp conditions are prevalent; increasing, ventilation often satisfactorily controls this problem.


· Virus diseases

Viruses are small microscopic particles which multiply in living cells. The presence of a virus in a plant may not show visible symptoms: in fact the commonly considered virus symptoms are often caused by non-virus factors such as genetic abnormalities, insects, nutritional disorders, and chemicals. The viruses of orchids cause either or all of the following symptoms. mottling and streaking, necrosis and ring spotting of the foliate and colour breaking (abnormal colours) in the flowers.


Because of their small size, virus presence can only be definitely confirmed by the use of indicator plants, seriological testing or electron microscope examination.

The Cymbidium Mosaic Virus, and Tomato Mosaic Virus, orchid Strain, the two main orchid viruses, are spread by sap transmission when cutting, handling and leaf contact, and through vegetative propagation. The distribution by insect vectors (aphids, mites etc..) of the two commonest orchid virus" is not considered to be scientifically proven. The most important agent causing virus dissemination is man. Some of the less important viruses may be aphid spread.

Because of the total invasion of the plant by the virus pathogen, control of virus infections is difficult. This is complicated by the fact that the nuclear chemistry of the plant and virus is so similar to make chemical control almost impossible.


All plants obtained should be from virus free stock. It has been found that seed obtained even from virused plants, will be virus free, and then this gives one avenue for the obtaining of virus free plants.

Plants obtained by vegetative propagation (divisions or rnericlones) from infected plants, will in turn be infected. Thus source plants must be virus tested to ensure they are clear. The use of heat treatment and plant tissue multiplication with virus testing enables infected plants to be cultured in some cases.

In the growing area the aim must be at control to prevent mechanical and sap transmission. Virus is highly infectious, and to prevent spread to healthy plants the following precautions should be observed:

o Isolation. Known high health orchids should be kept separate from those of unknown health status.
o Sterilisation of cutting equipment by flame during use.
o Plant spacing. The keeping of plants from touching prevents spread, although may how-
ever he confined to just suspected plants in practical application.
o Hygiene. Protect potting tables by paper, cleaning up after each plant.
o Remove and destroy any infected plants.


First published March 1982


Site established 9th May 1998