As might be supposed, pests traveling from one plant to another can spread diseases, so by controlling the pests you simultaneously safeguard your greenhouse from certain diseases of plants. Here are descriptions of the most common diseases in greenhouses:Damp-off is the most common of the fungous diseases and the one most likely to be encountered first by the home gardener. It attacks seedlings particularly, and is evident when healthy plants suddenly rot off at the soil line. Sterilized planting mediums have pretty well brought damp-off under control, but if you must use unsterilized soil for planting seeds, you will do well to drench it first with a fungicide, such as Pano-Drench.Leaf Spot causes purple, brown, or black spots on the foliage. Many of the favorite plants of the hobby gardener are susceptible: roses, caranations, cyclamens, and other flowering plants. Leaf-spot is spread from one plant to another by splashing water on foliage. Spraying or dusting with ferban (Fermate) will help to control this problem. Infected foliage and blossoms should be removed from the plant and destroyed, preferably by burning.
Mildew appears on leaves and stems as powdery white areas, and affects both upper and lower leaf surfaces. The spores of powdery mildew are transported through the air when the relative humidity is extremely high. Dusting with sulfur is usually effective, but proper air circulation must be maintained.
Rot (stem and root) attacks plants at the surface or just below the surface of the soil. The fungus that causes this troublesome problem may travel through the soil from an infected plant to healthy ones. Soil drenches will prevent the disease from traveling to healthy plants, but those already affected must be destroyed, as there is no known cure.
Rust appears on snapdragons and carnations especially, in blisterlike areas which, when broken, spew forth a rusty-looking powdery substance that is spread to unaffected areas by air currents or water splashed on the foliage. A dusting of Fermate will prevent development of the spores.

Aerosol Sprays
While there are numerous pests and diseases to which plants are susceptible, the hobby gardener will probably have only one or two to contend with, according to the kinds of plants he grows. Sprays or dusts applied in a greenhouse remain effective for a longer period than they do outdoors as they are not washed away by rain or blown away by brisk winds. Aerosol sprays are effective in small hobby greenhouses, say 10 by 10 feet or so, and the convenience of not having to mix several ingredients to make an effective pesticide is a boon to the amateur gardener. Aerosol cans may seem to be expensive, but several applications would stretch the cost over a period of time, and the convenience is well worth the few extra pennies. When the greenhouse is closed up for the night, a quick spray with an aerosol usually provides adequate control or prevention, and no time is lost from the growing schedule.

Systemic Poisons
A radical change in control of insects and diseases is being effected with the development of the new systemic poisons. These are applied to the soil and assimilated by the plant throughout its entire structure in quantities sufficient to kill insects and destroy bacteria and fungi. Chewing and sucking pests can be effectively controlled with systemics for periods of several weeks with one or two applications. Most systemics are extremely toxic to humans and animals, and the amateur gardener must handle them with respect and caution. Professional growers have access to several systemics, but amateurs should confine themselves to Cygon 267 or Scope, and never re-use the containers. Such are the advances being made in this field, however, that other systemics may be available for general use soon.

Spraying
Be sure to spray the undersides of foliage as well as the upper surfaces, because some pests prefer the hidden areas. A small amount of soap or detergent powder added to most spray solutions acts as a "spreader" to insure complete saturation and adherence. Sprays and dusts contain chemical elements which in some instances are incompatible, so thoroughly clean your spraying equipment after each use, and don't use the same equipment for sprays as for fertilizers. Prepare only enough spraying solution to take care of your immediate needs; throw out any that is left over. An amateur gardener should especially avoid using parathion.

CAUTION: If your greenhouse is attached to your home, be sure the spray material is one that will not penetrate cracks or other openings between house and greenhouse. When you are ready to spray or dust, post a "No Admittance" sign on the greenhouse door face the sign to the outside to warn anyone approaching the greenhouse, but hang it on the inside of the glass door so it won't be knocked off or blown away. Close the ventilators to contain the spray or dust in contact with plants. Wear a mask or respirator if it is recommended for the product you are using. Start the operation at the end of the greenhouse opposite the door, and work toward the door so you can step outside and close and lock the greenhouse door immediately.

Fumigating
A smoke generator (not a bomb) is the only fumigator recommended for use in a hobby greenhouse; the generated smoke carries the pesticide to every part of the greenhouse, insuring complete coverage and providing treatment for most of the pests likely to appear in a home greenhouse. Two fumigators recommended are Plantfume 103 for greenhouses of 5,000 cubic feet or larger; and Fulex Smoke Generators, which come in sizes suitable for small greenhouses, as 1,000 cubic feet or 2,000 cubic feet. Fulex Smoke Generators can be divided safely, to take care of even smaller areas, whereas Plantfume 103 cannot be thus reduced. Other fumigators available to commercial greenhouse operators should not be used by amateur growers. Fumigate on a regular schedule as a preventive measure.

CAUTION: Follow the manufacturer's directions exactly. Post a conspicuous "No Admittance" sign where it cannot be blown away or removed, close the ventilators, and be prepared to step outside and close and lock the greenhouse door the instant you activate the smoke generator.

CAUTION: No kind of fumigator, including smoke generators, should be used in a greenhouse attached to a residence or other building that houses people or animals, as the smoke penetrates even the most minute crack or opening.