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.