There is not much occasion to use fungicidal sprays on house plants, as most of the foliage diseases can be controlled by readjustment of the environment, and the root rots and wilts are not amenable to control by external applications. The control of insect pests, on the other hand, often requires spraying or dusting, but with materials designed for killing insects, most of which have little effect on fungi and bacteria. Occasion may arise when one can use, with benefit to house plants, certain mild fungicidal sprays, such as ferbam, wettable sulfur, or the slightly soluble "fixed" copper compounds, in concentrations considerably lower than those recommended for outdoor plants under summer conditions. Such occasions seldom occur with ordinary house plants under average home conditions.

Soil treatment to destroy disease-inducing organisms is often important in house-plant culture, and heating is not only the most adaptable method for this purpose but is also the most effective.

Heat treatments of the proper intensity and duration can destroy all kinds of soil-borne pests fungi, bacteria, nematodes, and insects. They leave no persistent toxic residues in the soil, as chemical treatments sometimes do. Soil that is to be heat-treated should first be prepared as if for planting, should be loose, free of lumps and coarse debris, and slightly moist. It can be spread in a pan, about 2 inches deep, and baked in an oven at 200 degrees for 1 hour. Another method is to place it in pots, which are either heated in a pressure cooker or placed on a rack above boiling water in a covered receptacle such as a kettle or wash boiler. The heating time for different sizes of pots by the pressure-cooker method, with the valve set at 10 pounds pressure, should be: 3-inch, 10 minutes; 4-inch, 15 minutes; 6-inch, 30 minutes. If boiling water, without pressure, is used, those intervals should be doubled.

After treatment, the soil should be stirred and well aired for several days to a week before replanting. With large soil masses, especially if heavily manured or fertilized, it is advisable to spread the soil in a layer and wet it down once or twice during the drying period, in order to remove toxic compounds that are produced in the soil by heating. Of course, one must be careful to avoid recontaminating the soil by contact with dirty surfaces or utensils, or replanting to diseased plants.

EXAMPLES of common maladies of house plants:

African-violet. The worst affliction is root knot, especially when complicated (as it often is) with fungus root rot and crown rot. Although not an infallible remedy, the propagation of new plants from leaf cuttings (which should be started in vermiculite, peat moss, or water, then transplanted to heat-treated soil) will usually eliminate this trouble. Gray mold (Botrytis) bud rot and mildew may attack plants that are chilled, overwatered, or kept in dull light and not sufficiently ventilated.

Aloe. Root rot caused by Pythium and Rhizoctonia may be troublesome if the plants are overwatered or are grown in heavy soil without proper drainage.

Asparagus-fern. The most prevalent ailment is shedding of the needlelike leaves as a result of too dry and too warm an atmosphere. Root knot caused by nematodes and a stem canker and wilt of fungus origin also may attack this plant; the remedy for this is to repot in new or heat-treated soil of light texture and provide good drainage.

Begonia. A victim of numerous diseases: Root knot, crown gall, bacterial leaf spot, nematode leaf spot, and gray mold bud and shoot rot; also oedema, a nonparasitic localized swelling of leaf tissue, followed by collapse and death in spots, and caused by excessive wetness of soil and dull light. Overwatering, frequent wetting of foliage, and lack of ventilation are the principal contributing factors in the foliage diseases; change or disinfection of soil is the remedy for the root troubles.

Cactus. Rotting of underground parts by fungi and bacterial leaf spot or rot of the tops are the commonest troubles. These dry-soil plants must be watered sparingly and uniformly, never soaked.

Calla. Bacterial soft rot and phytophthora rot of the rootstock or "bulb" are the worst enemies; both may be carried on these parts or in the soil. Affected plants are very difficult to cure. One must start with healthy rootstocks, planted in disease-free soil. Callas may also carry systemically the spotted wilt virus; affected plants are poorly colored and are weak growers. As there is no cure, they should be discarded.

"Chinese evergreens" (Aglaonema, Dieffenbachia, Philodendron). These are usually trouble-free plants, but may suffer from leaf spotting caused by bacteria or anthracnose fungi if the foliage is wet frequently and stays wet too long. Chilling may also damage them.

Ferns. Most house ferns are tolerant of a dry atmosphere but they prefer a soil that is uniformly slightly moist; extremes of wetting and drying are detrimental. Excessive wetting of foliage may induce leaf nematode attack and gray mold shoot rot. Scale insects are especially troublesome. One should not mistake the fern's own spore receptacles on the under surface or margin of the leaves for a rust or other fungus.

Fuchsia. Root knot and gray mold bud rot are the commonest maladies. Control methods are indicated under other headings, e. g., Begonia.

Gardenia. Very susceptible to root knot. Bud drop usually results from too sudden change of temperature, especially chilling, or from too dry an atmosphere. Overwatering and poor drainage are contributing factors, and may also promote stem canker. Chlorosis, a loss of green color or yellowing of the leaves, usually indicates a deficiency of certain mineral elements, as iron or magnesium, in the soil. It is most likely to occur in neutral or alkaline soils. It may also result from damage to the root system by overwatering or disease. The nutritional deficiency can be remedied by careful use of houseplant fertilizers, acidification of the soil with sulfur, or adding a pinch of iron sulfate to each pot.

Geranium (Pelargonium). Requires good light to bloom during the winter, and the plants become sluggish in weak light and short days, then are easily damaged by overwatering. Oedema (see under Begonia), gray mold rot, and stem rot may ensue.

Heliotrope. The common diseases are root knot and gray mold rot of buds and shoots.

Kalanchoe and other succulent (fleshy-leaved) plants, as Crassula and Sedum. Root and crown rot caused by fungi, crown gall by bacteria, are the commonest troubles; sometimes anthracnose on leaves and stems. These plants should be watered sparingly and given good light.

Oleander. Bacterial knot on stems, and anthracnose on leaves, may attack these plants occasionally. Scale insects are troublesome, and their attack may be followed by sooty mold.

Palms. Chilling must be avoided; may be liberally watered if the soil is well drained. A false smut, characterized by dark scablike spots with powdery brown or yellow centers, may occur on the leaves, especially of Phoenix (date palm). Heart rot or black scorch may attack the base of the fronds if the plants are in heavy soil and are overwatered. Sooty mold, following insect attack, may develop on leaves in a damp atmosphere.

Peperomia. A virus ring spot is the commonest disease.

Poinsettia. Yellowing and dropping of leaves is usually a symptom of chilling, or may be due to weak light and over-watering. Root knot and bacterial canker are the principal parasitic diseases.

Rubberplant (Ficus elastica). Anthracnose on leaves and stems is of common occurrence, probably often as a sequel to chilling and too frequent wetting. Also subject to root knot, and sometimes injured by bacterial gall (crown gall) on stems.

FREEMAN A. WEISS is curator of the American Type Culture Collection, 2029 M Street, N. W., Washington, D. C. He holds degrees from the University of Minnesota and Cornell University. From 1920 to 1950 he was a plant pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering and worked on the diseases of potatoes, ornamental bulbs, azaleas, roses, and other ornamentals.

Bacterial blight of cotton.