Cactus - Insects And Disease
( Originally Published 1942 )
THE pages of history do not record the number of examples of fine enthusiasm for cacti that have been wrecked on these twin rocks of tragedy, insects and disease. It is strange that so little has been written to guide the adventurer in pursuit of his chosen hobby. It may be that men who do most of the writing live in the colder sections of the country where they have only a limited number of plants and where the natural enemies of the cacti do not thrive as they do in the warmer or the more humid sections.
The most dreaded disease is the one which attacks the roots of the plant causing them to decay and the plant to die. The disease works underground so that its presence is usually not suspected until it has reached such an advanced stage that little can be done for the afflicted plant except a surgical operation. When diseased roots are even suspected, the plant should immediately be taken up and examined. All the diseased roots should be cut away. The disease follows up the tissues of the roots and is generally present considerably in advance of the point where it is apparent to the unaided eye. It is, therefore, well to sever the root above the point where the disease can be detected. Do not hesitate to sacrifice seemingly healthy roots—you are contending with a serious disease and vigorous measures are demanded if you are to check it. If the disease has reached the tissues of the stem itself, continue the cutting process. Do not worry about how little of the plant you are going to have left, because no matter how much or little it is, it is utterly worthless so long as it contains the least fragment of infection. It must be remembered that the knife blade becomes contaminated whenever it touches an infected tissue and will transfer the infection to clean tissue unless it is first sterilized. Before making the final cut in the tissues, the knife should be dipped in some disinfectant such as a solution of carbolic acid or Semesan and then rinsed by pouring clean water over it. It is good practice to dip the entire plant in a moderately strong solution of Semesan or spray Semesan over the plant after the operation is completed.
If roots only were removed, the plant may be repotted, but a clean or sterilized pot must be used and fresh soil in which cacti have not been previously planted.
The plant that has had the tissues cut away may be treated either as a cutting or as a scion for grafting, and the directions as outlined under those headings should be followed.
It is much work and considerable annoyance to fuss with a diseased plant, and few of us really 'enjoy it. It is more logical to so care for the plants that they do not contract a diseased condition. A few simple precautions will be found very helpful.
Any broken tissue, especially if the tissue comes in contact with the soil, is a source of possible infection. Such wounds, either intentional, accidental, or due to the trimming of roots, should be dried in the sunlight or disinfected with Semesan before planting.
Trouble with disease is often the result of improper soil and indiscreet use of water. Soils have already been discussed and need not be reconsidered here except to emphasize-the importance of proper composition in avoiding disease infection. The greatest loss occurs among the smaller species—the But-ton Cactus is especially susceptible to root diseases. Large and rapidly growing species of cacti thrive on more water than is generally suspected, and respond with increased growth and flower production, but these plants that are still only tiny things when mature need little water for further growth, as they can absorb almost enough from the air to maintain life and produce their flowers and fruit. These smaller species have reached such a stage in their adaptations to desert conditions that they do not need and cannot stand the amount of water that they are generally given. If you have had difficulty with diseased roots on your smaller cacti, you would probably help to check it if you reduced the water allowance for the mature plants except for a short period prior to blooming.
One of the most dreaded diseases is the shot hole disease so called from the appearance of the pads of the Opuntia (prickly pear) group when the disease is advanced. The disease is an anthracnose and is particularly active in the more humid months. It is first a brownish spot. This later turns gray with black spores underneath the dried skin. Probably the most effective remedy is the removal of any pads showing marks of the disease, or if the plant is a rare one, cut out the diseased portion leaving the sun to sear the cut edges, or else disinfect.
The larvæ of several insects attack cactus plants when they are growing wild, but they seldom venture into the cultivated garden. You are most likely to encounter them in cacti that come to you directly from the field. Such plants should always be care-. fully examined whether they are collected by yourself or purchased from others. These larvæ feed on the inside of the plant, and usually the evidence of their presence will be found about the base of the plant where they usually make their entrance. Their presence often goes undetected until the plant is so far destroyed that little can be done for it. The entrance to the burrow may be closed, and one must look very carefully indeed to locate it. Often a little webbing is about the entrance. Some of the more serious species, however, have the opening to their burrows at or near the base of the plant and sometimes under the ground.
These larvae are very difficult to control when they once become established. Each addition to the cactus garden should be examined minutely to see that it contains none of these destructive pests. It is not especially difficult to kill the larvae in the burrows, but when they are left undiscovered in the plant the plant decomposes and causes a rotting of the tissues which is really worse than the larva itself. The larvae may be dug out, but often there is little left of the plant after this is done. Dr. Arthur D. Houghton in the August, 1930, number of the Journal of the Cactus and Succulent Society of America, suggests a most novel and interesting solution of this problem. Doctor Houghton opens the tunnel, inserts a tiny glass funnel, and fills the burrow with metallic mercury (quicksilver) . The larvae floats to the surface, whence it is removed, and the mercury recovered for use another time. The burrow is then washed out with a solution of Semesan.
Practically all of the insects that attack the outside of the plants have mouth-parts developed into tubes which are thrust into the plant. Some of these insects often be-come so numerous that the loss of sap taken from the plant is very serious. Even though the plant may not die, its appearance is ruined by unsightly spots due to the drying of the cactus skin where the insects have been feeding. Some of the insects that travel from plant to plant are known to carry diseases, and so they become a double menace.
It is useless to treat the surface of the plants with "stomach poisons" for these in-sects suck only the juices beneath the surface of the cactus skin. Solutions that kill by contact are the only ones that are effective against these sucking insects, and one must be very sure that the solution used will not injure or kill the plants as effectively as it does the insects. Some species of cacti are much more susceptible to spray injury than are others. Therefore test carefully in a small way any unfamiliar spray material to assure yourself of its effect on your plants before using it extensively.
Plants should never be sprayed while ex-posed to hot sunshine. The solution may collect on the plants in drops which act as burning-glasses and cause burned spots on the plants. The more force with which the spray is applied, the more effective it will be on the insects since it is driven into the pores of the body through which the insect breathes.
The cochineal insects are rather a serious pest on Opuntias in Southern gardens. The females are very small and they cover them-selves and their egg masses with a white fluffy material which makes them very conspicuous. This covering is of considerable protection from sprays since the solution penetrates to the insect and eggs beneath only with difficulty. Wood alcohol, applied with an atomizer, has been extensively used for the control of this insect. A general spraying of the plants is hardly necessary. The plants or portions of plants infested can be located and given a local treatment.
Scale insects are not so easily seen. The tiny scales, often smaller than the head of a pin, generally escape the attention of the amateur until his plants are suffering severely. The insect itself is hidden under a shell-like secretion or scale which protects it from many spray materials. The newly hatched insects run about much like very tiny mites, but after they once start to feed, they remain in that position.
Oil emulsions are most effective against scale insects. There are a number of such emulsions on the market which may be both safe and efficient in controlling scale on cactus, but the author has had most experience with Volck. This emulsion, developed in California, seems to be harmless to the most tender vegetation, has no disagreeable odor, does not leave any greasiness or otherwise affect the appearance of the plants. It seems to have some germicidal value and will also help to control the early stages of the cochineal insect.
The "stink-bugs" are medium sized in-sects. They are very active and have a disagreeable odor when crushed. The older insects are very resistant to contact sprays, so that control should be directed toward the younger and more tender forms. The older ones are often picked off by hand. The solutions containing nicotine as the killing agent are fairly satisfactory if the spray mixture contains plenty of nicotine and is used with considerable force. Extractives of pyrethrum will also give very good results.
Ants are sometimes very annoying. When the nests of the small ants are found, they may usually be easily fumigated by pouring into the entrance of the nest a table-spoonful or two of carbon bisulphide, or, use Cyanogas according to directions on can. If the nest cannot be found, some of the ant poisons such as Antrol or Kellog's Ant Paste will give relief when used according to directions.
Where the nests are under the cactus plant, place crystals of paradichlorobenzine in a circle in the dirt around the plant. Paradichlorobenzine will not burn the roots.
The most serious mistake that can be made is to permit insects to become thoroughly established in the garden before any attempt is made to control them. It is extremely difficult to eradicate an infestation that has once reached serious proportions, but incipient infestations may be blotted out by frequent and timely application of proper solutions. A clean, healthy garden is a delight and a prideful pleasure; but one foul with insects and diseases is a distressful thing.