Treating Pest and Diseases in Multiple Plants

Group Treatment. It is not always necessary to know precisely what pest or disease is attacking plants in order to apply effective remedies. Frequently it is sufficient to be able to fit it into one or other of a few main groups. First decide whether it is a pest, e.g. insect, or other creature, or a disease, e.g. fungus, or other low type of vegetable organism. Pests must be treated with insecticides or acaricides (mite killers); diseases mainly with fungicides.

If it is an insect which is attacking the plant above soil level, e.g. leaves, stems, flowers, or fruits, it is probable that an insecticide such as BHC or derris will give reasonable control because these are poisonous to a wide range of insects, though not always as effective against particular insects as other chemicals which are more specific in their action.

Soil pests, such as cutworms, leather-jackets, wireworms, and millepedes, cannot be destroyed by spraying but must be attacked with a soil insecticide such as gamma-BHC. Slugs can be controlled by metaldehyde or methiocarb, either mixed with a suitable bait such as bran or in suspension in water.

Red spider mites, which are not insects, can be killed by using azobenzene in various forms or with malathion, derris, or dicofol.

If the trouble is due to disease, decide whether this is caused by a fungus, e.g. black spot, rust, mildew, etc., or bacterium or virus, e.g. streak, mosaic, etc. Fungi usually cause dark, dampish spots, or patches of decay or outgrowths of mould, rusty coloured spots, etc. Bacterial and virus diseases usually cause drier spotting or streaking without obvious outgrowth, but are in general more difficult to identify. Fungal diseases may be treated by spraying with reliable

fungicides such as Bordeaux mixture or other preparations containing copper, or with thiram, or by dusting with sulphur. Bacterial and virus diseases cannot as a rule be treated,.

Trapping. Rats and mice can be caught in spring or cage traps baited with cheese or fat. Moles may be caught with steel traps set across the burrows. Gloves must be worn while setting the traps to avoid leaving human scent on them. Soil removed to set the traps must be replaced so that burrows are dark. Traps should be set across main burfows and those leading to water. Cockroaches can be caught in proprietary traps baited with bran. Earwigs may be trapped in inverted flower pots stuffed with hay, hollow broad bean stalks, slightly opened matchboxes, or any other similar dark hiding place. Leather-jackets and slugs can be trapped under wet sacks or heaps of damp vegetable refuse laid on the soil. Millepedes and wireworms can be collected from sliced carrots and potatoes buried just beneath the surface of the soil.

Incurable Diseases. Not all diseases can be controlled as yet. In some cases the only way of preventing further damage is to remove and burn affected plants as soon as detected. Even with diseases that can be treated, it is generally advisable to remove and burn specially bad plants or portions of plants. On no account should these be placed on the compost heap or left lying about. This applies, among other diseases, to plum silver leaf, all collar and root rots, club root, aster wilts, and brown rot of fruits.

Virus Diseases. A large class of diseases caused by ultra-microscopic organisms which infect the sap. Symptoms vary from a slight mottling or rolling of the leaves to intense dry brown spotting or complete collapse. Such names as aucuba mosaic, bronze leaf, streak, leaf roll, yellow edge, etc., describe outstanding symptoms of different viruses. External applications have not proved very satisfactory in controlling these diseases. Infection is carried largely by sucking insects, especially aphids, e.g. greenfly, blackfly, etc., and these must be kept down by spraying with suitable insecticides such as derris, dimethoate, formothion, menazon or malathion. Virus may also be carried on knives, secateurs, etc. used in pruning, and these should be disinfected by dipping in a good household disinfectant. Badly infected plants should always be burned.

Some varieties of plants subject to virus disease are resistant to infection and some are tolerant, i.e. they show no adverse symptoms. Tolerant plants can become unnoticed sources of infection for sensitive plants.

Eelworms. Microscopic, transparent, eel-like creatures which often infest roots or stems of certain plants in great numbers, causing knots, goutiness, and distortion. Much larger, transparent nematode worms are frequently mistaken for them and are allied but are harmless, as they feed on decaying matter in the soil and are frequently found in manure, compost, or leafmould.

Eelworms are principally found in phloxes, chrysanthemums, narcissi, tomatoes, cucumbers, potatoes, and onions. Usually there is no satisfactory remedy and plants should be destroyed. Eelworms on potatoes form very small white cysts on the outside of the tubers and these can be washed off in plain water.

Infected narcissus bulbs can be cleared of eelworms and fly larvae by keeping the bulbs for three hours in water maintained at 110° F. followed by immediate cooling in cold water but special apparatus is required for this. Infected chrysanthemum stools from which all stems have been cut can be heated in a similar manner but for 20-30 minutes only. For strawberry runners the time is 5 to 6 minutes and the water temperature 115°. Clean stock of phlox can usually be

obtained from root cuttings. It is always advisable to sterilize the soil if practicable with steam or chemicals and.

Sterilization by Heat. In addition to treatment with cresylic acid or formaldehyde, soil can be sterilized by raising the temperature. Four methods may be employed, namely electrical heating, baking, steaming, and scalding.

ELECTRICAL sterilization requires special apparatus, and manufacturer’s instructions must be followed in the use of this.

BAKING can be done by spreading soil thinly in an oven or in trays in a special apparatus. The danger is that soil may be charred. This risk can be lessened if the soil is thoroughly moistened first. The temperature of the soil should be raised slowly until it is between 205° and 210° F. and maintained at this for 15 minutes.

STEAM sterilization must be done with special apparatus. Steam under pressure is forced through the soil until the temperature is raised to between 200° and 205° F. It is maintained at this for 1 hour.

SCALDING is a simple method for home use. Soil is placed dry in a sack and this is suspended in a copper containing a little water. The water is then boiled rapidly for 1 hour.

Soil that has been sterilized by heat shows a falling off in fertility for a few months. This can be counteracted by using the John Innes formulae for seed and potting composts,.

Spreaders. If water is sprayed on a leaf or other smooth surface it tends to run into globules instead of spreading evenly. This renders sprays inefficient. To overcome the difficulty, soft soap, or some other substance which lowers the surface tension of water, is added to the spray.

Soap is used at 1 to 2 oz. per gallon according to the hardness of the water. It cannot be mixed with lime sulphur or Bordeaux mixture, as it curdles them.

More efficient spreaders are now available which can be added to these fungicides. These include saponin, calcium caseinate, flour paste, and preparations of resin. Many are obtainable in proprietary brands ready for use according to makers’ instructions, or household detergent may be used.

Systemic Chemicals. Some chemicals when applied to plants remain on the outside, on leaves, stems etc. They are known as non-systemic chemicals in contrast to systemic chemicals, which are absorbed by the plant and enter into its sap in which they may be carried from one part of the plant to another. Systemic insecticides, fungicides and weed-killers are known. Advantages are that it is not necessary to cover the whole plant with the chemical to get a good result; that the chemical is not removed by rain; and with insecticides, that it is less likely to harm useful insects since these do not feed on the plant. Drawbacks are that systemic chemicals cannot be wiped or washed off, that some persist for a considerable time and may render a crop unusable until they have been dispersed or decomposed.

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