CHAPTER XII. Diseases and Insect Pests

DISEASES AND INSECT PESTS

There is very much in the old-time advice, "Grow your plants clean," for a very large proportion of Orchid diseases and insect pests are due to errors in cultivation, more especially in the regulation of the temperature and the ventilation. Insanitary houses lower the vitality of the plants, and vegetation, like human beings, is a prey to disease when kept in unhealthy conditions.

Spot, or Orchid disease, exhibits itself in various forms. It is caused, as scientists say, by different micro-organisms, but in effect it is practically the same whether in the form known as "Spot," often seen in Phalaenopsis, Aerides, and Vandas, or in the decayed and blackened pseudo-bulbs of Cattleyas, especially C. Warscewiczii (gigas), which from an apparently healthy plant may develop a diseased condition of the pseudo-bulbs, and become useless in a few days. In all such diseases it will be seen that the tissues have collapsed, the result being brown or blackish spots on leaves or bulbs. Imperfect nutrition from lack of healthy roots is a frequent cause of this mischief, for Aerides and Vandas which have been affected with "Spot" recover in the new growth, for a time at least, if a satisfactory root action can be set up.

Propagation, by freeing the recently made parts of the plants from the old and worn-out back portions, which are not furnished with the roots necessary to support themselves is one of the best means of preventing Orchid diseases, and efforts should be made to keep the plants vigorous and, therefore, capable of resisting attacks by insect pests.

Plants are also benefited greatly by having their position in the houses changed, and that is one of the great advantages of the periodical inspection, for during this process the relative positions of the plants are altered.

It should be said that Cattleyas and other common Orchids badly affected by disease had better be burnt, for it is cheaper to buy a healthy young plant than to waste time in trying to bring the unsightly and diseased specimens back to health.

The Cattleya Fly (Isosoma orchidearum), first imported probably with Cattleya Dowiana, and frequently with other Cattleyas since, affects the new growths, the grubs causing them to swell and rendering the growth useless. The same species, or one closely allied, also attacks the young roots of Cattleyas, Laelias, and their hybrids, causing unsightly galls on the points of the roots. Fumigation, with some safe preparation to destroy the fly, should be carried out, and every young growth and root-point as soon as they are seen to be affected should be cut off and burnt. By adopting these remedies it is possible to get rid of the pest. In purchasing freshly imported plants, care should be taken to reject those which show signs of having been affected by the fly.

Thrips, Red Spider, and Aphides occasionally appear in every collection, and the remedy is fumigation and sponging with an insecticide, which some growers prepare for themselves, either by pouring boiling water over coarse tobacco tied up in a cloth and adding a little soft soap, or by making an infusion of quassia chips. But excellent insecticides can be purchased already prepared, which are guaranteed to be safe and effective, and being of uniform strength, they may be used with confidence if the instructions given with the preparations are observed strictly.

Avoid using paraffin and emulsions of paraffin, for it is dangerous, not only to the plants sponged with it, but to all the plants in the house, for it affects the atmosphere.

SCALE INSECTS

These appear much less in collections now than formerly, because the old large specimens are replaced by young and vigorous plants. Thirty or forty years ago, it was a usual thing to spend several days every year scraping the brown scales from tall plants of Aerides odoratum, Vanda tricolor, and other specimen Orchids, and what was called "cleaning" was going on all the year round. Now there is much less need of such work, although scale will appear in its various species on one section of plants or another. In the periodical inspections, all plants attacked by it should have the insects removed by a piece of stick blunted at the edge and point, sponging the leaves afterwards with some diluted insecticide. Syringing with an insecticide, or dipping the plants in the liquid, should be avoided, for the quantity applied is likely to saturate the material in which the plants are potted and to run into the centres of the young growths and cause injury. By means of a sponge, it may be applied lightly or heavily, but the operator has command in each case over what he is doing.

MEALY BUG

Fortunately this pest is rare in Orchid houses, but when it appears it is easily destroyed in the same manner as scale.

COCKROACHES

The first of these insects to be noticed should be the signal for the laying of poison. Search should be made for the breeding quarters, which are often in the stoke-hole, or in some hot, dry corner of the house. Various preparations are recommended, but the best still seems to be the old phosphor paste, which should be placed on pieces of paper in the haunts of the insects in the evening, and removed the next morning, a fresh supply being put down every two or three days so long as one of the insects remains.

SLUGS, SNAILS, AND WOODLICE

To combat these is more a question of diligence than anything else. The old remedies to attract them, such as lettuce leaves, or hollowed halves of potatoes, are still effective, and a walk round the houses with a light at night never goes unrewarded.