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.
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.
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.