Plants and causal agents of fungal diseases react in various modes to alterations of the natural light spectrum. Understanding these reactions can enable us to modify the transmission spectrum of the greenhouse cover so as to achieve optimal production conditions in terms of plant development, yield and disease control.

Examples of possible modifications will be given and discussed. The most prominent ones are:

1. . Ultraviolet radiation at waveband of 280–320 nm (U.V.-B), if followed by low leaf temperatures, induces petal blackening of red rose cultivars, causing considerable financial losses to the growers. By using the proper U.V. absorber at the required concentration in the plastic film this condition can be prevented.
2. . Sporulation of Botrytis cinerea is enhanced by U.V.-B radiation and inhibited by blue light (310 and 480 nm, respectively). Spore formation is correlated both in vitro and in vivo (on tomato and cucumber plants) to the U.V. -B transmissivity of the cover and negatively related to the ratio of transmitted blue/U.V. light.
3. . Growth rate of melon plants was accelerated using film containing fluorescent compounds which absorb light in the green waveband and emit it in the red waveband.
4. . Growth rate was accelerated and time interval between potting and flowering was shortened for Sainpaulia ionatha plants by using various photoselective covers.