Greenhouse Plants Pests and Diseases

Greenhouse Plants Pests and Diseases

greenhouse-plants-pests-and-diseases In my comments about plant pests and diseases affecting chrysanthemum plants, I have stressed the importance of general greenhouse hygiene as a significant factor in plant health. This advice is just as applicable to other greenhouse plants and should, I feel, be kept in mind at all times.

Garden Pests affecting Greenhouse Plants

Leafminer, see Plant Pests and Diseases affecting Chrysanthemum Plants

Mealy Bug. This pernicious greenhouse pest belongs to the same family as the scale insect and is related to the aphids. Like tiny wood-lice, these insects are covered with white, waxy, wool-like material which protects them against water. Though capable of movement they usually stay in one place when adult and they feed by sucking the plant sap. Breeding is continuous and multiplication very rapid. The root mealy bug is similar in appearance and is a common pest of cacti.

Deal with small infestations by hand, using a stiff paint brush dipped in insecticide. Spraying with derris, malathion, nicotine or white-oil emulsion is effective but repeated applications are necessary to ensure that no young appear from eggs which have escaped treatment. Where root mealy bug is identified, the soil must be shaken from the plants roots, the worst infections cut away and the roots dipped in insecticide before re-potting in fresh compost.

With vines all loose bark should be removed in winter and the rods and spurs painted with petroleum emulsion insecticide. In addition, all woodwork should be scrubbed with hot, soapy water.

Red Spider Mite. This pest attacks many plants and is usually most troublesome in a hot, dry atmosphere. Frequent syringing with clear water is, therefore, the most effective method of combating it in greenhouses and frames.

The mites can just be detected with the naked eye and they congregate on the underside of the leaves, usually in the angles of the veins. The leaves develop a mottled yellow appearance. Fumigation with azobenzene is effective, and spraying with white-oil emulsion or malathion is also effective.

Scale Insects. These small insects attach themselves firmly to leaves and stems and suck the sap from the tissues. This seriously weakens the shoots. The adult scale insects are covered in a hard protective shell. There are numerous species, but the greyish Mussel Scale is possibly the most common and attacks many plants. The Brown Scale is also fairly common. Plants may be sprayed with malathion or diazinon whenever an attack develops; small numbers of scale insects can be removed with a knife or piece of wood. I also find white-oil emulsion an effective spray to use.

Springtails. These are very small white insects which hop when disturbed and sometimes attack seedlings and young plants in considerable numbers in the greenhouse, particularly bulbs and the roots of plants. They can cause quite a lot of damage They are also found in dead and decaying vegetation. Water the soil with lindane, provided none of the plants being grown is any kind of crop plant, as this may taint the soil for as long as two or three years. Dust with BHC or DDT. Remove decaying vegetation — here we come back to my point about hygiene — and try to improve the drainage of soil in pots or borders.

Thrips, see Roses Pests and Diseases

Weevils. The Vine Weevils can do a great deal of damage, both in the larval and adult stages, especially to cyclamen and tuberous begonias (see Garden Pests of Bulbs, Corms and Tubers for description of this pest and its control).

White Fly. The adults of this pest give it its name, being very small insects with white wings. The young live on leaves and suck the sap from them, and the adults may be present in such quantities as to produce a dense cloud as they fly away when disturbed. They can cause havoc in greenhouses  — they are particularly troublesome on tomatoes — and are difficult to control, but fumigating with DDT or BHC, or spraying with DDT or malathion at least twice with a 14-day interval in between will help. Repetition is needed because the eggs continue to hatch over a long period and the young are fairly resistant.

Woodlice. Under glass these nocturnal pests may cause considerable damage by eating holes in leaves, attacking young roots and feeding on seedlings. Dust the plants and the soil with DDT or BM, or trap them in inverted flower pots stuffed with paper or chopped hay. Again, keep the greenhouse scrupulously clean at all times and throw out dead and dying plants and vegetation of all kinds.

Plant Diseases affecting Greenhouse Plants

Botrytis and mildew are the two fungal diseases which cause endless trouble in the greenhouse, particularly when the weather is warm and the atmosphere moist. In this respect, it is vitally important to keep the greenhouse clean, and free of dead and dying plants. Do not leave dead flowers and leaves lying about. This will do a lot to prevent disease spreading, and will also prevent pests from breeding as fast as they might otherwise do.

Make sure that the greenhouse is adequately ventilated at all times, even in cold weather (you can heat the house to keep the air dry and circulating); it is dank stagnant air which results in the proliferation of fungus spores. Where mildew appears — it is identified by white or greyish patches on the leaves or stems which often appear to be mealy — dust the leaves with flowers of sulphur or spray with colloidal sulphur or dinocap.

If plants are badly affected by botrytis, a group of fungal diseases of which by far the most common is Grey Mould, destroy them, but in the case of less severe attacks spray with thiram or captan or vaporise sulphur, as directed by the makers. Spacing plants out will encourage the free circulation of air, and so decrease the chances of these diseases causing trouble.

02. December 2010 by Dave Pinkney
Categories: Garden Management, Greenhouse Gardening, Pests and Diseases | Tags: | Comments Off on Greenhouse Plants Pests and Diseases


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