Garden Pests and Diseases of Bulbs, Corms and Tubers

Garden Pests and Diseases of Bulbs, Corms and Tubers

Garden Pests and Diseases of Bulbs, Corms and Tubers  Bulbs, corms and tubers are liable to be attacked by quite a few pests and diseases, but, as with human ailments, the fact that they exist does not necessarily mean that they will be experienced.

I prefer to emphasise the importance of buying good stock (particularly important with this group of plants), providing them with congenial growing conditions and keeping a close watch on general garden hygiene, for these factors have a considerable bearing on whether or not one avoids trouble.

Garden Pests that affect Bulbous Plants

Ants. These can be troublesome in the garden and they can be controlled with BHC, or BHC/DDT applied direct to their nests or the ground where they are active.

Aphids. Numerous bulbous plants grown under glass or in the home are liable to be attacked by aphids, or greenfly as they are popularly called. These can be combated with various insecticides, including malathion, malathion/DDT, derris, pyrethrum, BHC, BHC/DDT, and menazon, applied as soon as the attack is noticed.

Birds. Even those of us who are bird-lovers must feel rage at times when the flowers of choice bulbs are ravaged for no apparent reason. Crocus, in particular, seem to attract such unwelcome attentions. There are bird repellents available nowadays, such as Morkit, or we can rely on rather unsightly criss-crossed weaves of cotton. Bulb Mites. These pests attack the bulbs of lilies, narcissi, tulips and hyacinths and the roots of dahlias. The mite, although very small, is visible to the naked eye and it is round and yellow-white in colour. It attacks plants which have already been damaged by some other means and the symptoms of attack are yellowish foliage and reddish scales on the bulbs or tubers. Burn all affected plants immediately, and sprinkle paradichlorobenzene chrystals among the other bulbs to deter further attacks from these most devastating pests of bulbs.

Eelworms. Bulbous plants like narcissi, irises, hyacinths, scillas and snowdrops may be attacked by Stem and Bulb Eelworm. Affected plants should be lifted and burnt immediately. The symptoms are withered or distorted foliage and stems, browning of the scale leaves of the bulbs and late flowering. If an affected bulb is cut open horizontally it will be seen that there are dark-coloured rings in the tissue. The pest itself is too small to be seen with the naked eye. Bulbs which are soft or badly damaged should be burnt, and the remainder can be given hot water treatment.

This treatment however, is not something which can be done without careful preparation, and it is difficult for the home gardener to do. The temperature of the water, 43.3° C (110° F), must be precisely correct and the timing, which varies with different plants, being three hours for narcissi and much less for some other bulbs, must also be accurate. A higher temperature than that indicated would destroy the bulbs, a lower one prove useless in controlling the eelworms. Tulips should not be given this treatment. Ground in which eelworm infested bulbs have been growing should not be used for hosts of this pest for at least three years.

Leatherjackets. The leatherjackets are the larvae of the Cranefly or Daddy-long-legs, soil pests which can cause considerable damage to bulbs and tubers. The larva is tough skinned, legless, a brownish-grey colour and about 1in. long and attacks the plants during spring and summer. There are various methods of control  — MC and DDT dust, applying Paris Green as a bait around the plants, or digging in naphthalene at 2oz. to the square yard.

Mealy Bugs. Bulbous plants grown in greenhouses may be attacked by the Mealy Bug, and the control of this pest is described under Plant Pests affecting Greenhouse Plants.

Mice and Rats. Mice, and perhaps less often rats, can cause considerable damage to bulbs in the garden and to those which are stored. A poison bait based on warfarin will control these pests.

Narcissus Flies. The Large Narcissus Fly, with a hairy body and looking rather like a smallish bee, is a serious pest of narcissi which also attacks amaryllis, hippeastrums, hyacinths, snowdrops, vallotas, lilies, scillas and galtonias. The larvae which are of off-white colouring enter the bulbs, one to each bulb, through the basal plates and if the foliage is in poor condition or distorted these pests should be suspected. Bum all badly affected bulbs. Subject others which may be infested to hot water treatment when dormant — immersion at temperatures of 43.3° C (110°F) for one hour. Alternatively, place dormant bulbs in a solution of lindane, with a wetter added, for three hours. Lindane may also be dusted around the plants from late April to late June at fortnightly intervals, for it is at this time that the eggs are laid in the ground around the bulbs.

The Small Narcissi Fly has a smooth body and is narrower than the Large Narcissi Fly. It also differs in that many larvae — up to 30 — will attack the same bulb. Narcissi, hyacinths and irises are worst affected. The control measures are the same as for the Large Narcissus Fly.

Red Spider Mites. The Red Spider Mite, a sap-sucking insect, is most likely to be found on bulbous plants under glass, for it needs warm, dry atmospheric conditions to thrive. Attacked leaves take en a yellow appearance and will fall early in severe attacks. Frequent syringing with water to moisten the atmosphere is a first requirement and fumigation with an azobenzene smoke. Against outdoor attacks use derris or malathion.

Slugs and Snails. Both these pests can be destructive if countermeasures are not taken. Metaldehyde and Paris Green in each case made into a bait with bran, are effective controls but Paris Green has the disadvantage of being poisonous to other animals and birds.

Springtails, see Plant Pests affecting Greenhouse Plants

Thrips. The small thrips, or Thunder Flies as they are called, are small, rapidly moving sap-sucking insects which attack cyclamen, begonias, hippeastrums, arums, gladioli (this plant has its own strain) and many other plants. Attacked plants have characteristic brown or silver streaks on their growths, suffer from arrested development of the flowers and may generally be deformed. Like red spider this pest thrives in a hot, dry atmosphere. Malathion, lindane, DDT, and derris are some of the chemical controls for use outdoors and under glass, and lindane and DDT can be used as smokes in the latter case.

To control Gladiolus Thrips spray or dust with lindane, DDT, or BHC/DDT.

Weevils. The Vine Weevil can be an especially damaging pest of cyclamen and tuberous begonias, attacking the plants in both the larval and adult stages.

The larvae are small, rotund, legless and of an off-white colour and attack the corms or tubers. An effective control is to water the soil with lindane if they cannot be repotted into new soil — the best procedure, of course. The larvae can also be removed by hand from the root balls of plants taken from their pots.

The adult weevils, which feed on the margins of the leaves, can be trapped in rolled sacking or paper strategically placed near the plants on which these nocturnal pests feed; or by spraying with DDT or lindane. These can also be used as smokes in greenhouses.

Wireworms. Wireworms, the larvae of Click Beetles, are soil pests which can cause considerable damage to bulbs, corms and tubers. The grubs are easy to recognise, having yellow segmented bodies with three pairs of legs near the head.

Pasture-land which has been dug over and used for ornamental plants for the first time is often badly affected with the main attacks coming in spring and early autumn. Site and DDT dusts can be used as a control measure.

Garden Diseases that affect Bulbous Plants

Botrytis. Dahlias, begonias and cyclamen are plants which are the most vulnerable to Grey Mould, Botrytis cinerea, but any bulbous plants may be infected through a wound, however small, particularly if the conditions are cool and damp, The common name comes from the grey spores of the fungus which form a mould on the leaves and stems of the plants. To keep down its spread improve the circulation of air, and try to prevent it from getting too humid. Chemical controls which may be used are thiram, captan, dicloran or quintozene.

Gladiolus Scab. This disease attacks both the corms and the top growths. On the corms quite shallow depressions form with raised edges and on the foliage brown spots appear, especially low down. In severe cases the plant will eventually topple over. As a precautionary measure immerse the corms in captan before planting.

Lily Disease. This is a trouble which must be watched for on lilies. The fungus, Botrytis elliptica, shows as brown, red-margined spots on the leaves or flower buds and stalks. Damp, cold sites with an absence of sunshine are conducive to the spread of the disease and it causes most trouble in wet seasons. Remove affected growths and burn badly damaged plants. Spray with colloidal copper at regular intervals.

Narcissus Bulb Rot. Fusarium bulbigenum is a disease to watch for when the bulbs are in store. The fungus invariably gains a hold on the base and spreads throughout the bulb. The bulb scales turn brown. Destroy immediately all affected bulbs and keep the storage conditions as cool and airy as possible.

Smuts. Various forms of Smut, Urocystis, attack scillas, Anemone nemorosa, colchicum and gladioli and all affected plants should be destroyed immediately. The black or dark brown powdery mould can easily be recognised on the leaves, flower stems, bulbs or corms.

Dahlia smut, Entyloma dahliae, has a different origin and begins as pale green spots on the leaves which later turn brown and may run together so that the entire leaf withers. Attacked foliage should be removed and the plants sprayed with Bordeaux Mixture or a copper fungicide.

Soft Rot. This bacterial disease, Pectobacterium carotovorum, attacks cyclamen, hyacinths, indoor-grown muscari and Zantedeschia aethiopica. The bulbs or corms become soft and slimy as the disease rapidly gains hold. Destroy affected cyclamen, hyacinths and muscari but with the corms, deschia cut out the affected parts of the corms, soak them in a 2 per cent mixture of formalin for four hours and replant in sterilised soil.

Tulip Fire. This disease, Botrytis tulipae, is another with a very descriptive name, for the plants do indeed look as if they had been exposed to fire. Scorched looking areas on the leaves and sometimes flowers spread rapidly. The bulbs are marked with brown spots. Destroy badly affected bulbs. Spray with captan or thiram when plants are making growth and until flowers are about to open. A well worth while precaution, too, is to dress the soil with quintozene when planting the bulbs.

Virus. Lilies, narcissi, dahlias and gladioli may be attacked by virus diseases and plants which are affected should be destroyed. As virus diseases are spread by sap-sucking insects, such as greenfly, measures should be taken to keep them under control.

02. December 2010 by Dave Pinkney
Categories: Bulbous Plants, Garden Management, Pests and Diseases, Plants & Trees | Tags: , , , | Comments Off on Garden Pests and Diseases of Bulbs, Corms and Tubers


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