Controlling diseases on young trees growing in the greenhouse is a big concern. High humidity creates conditions favorable for disease infestations. That powdery mildew is a common problem on almost all plants. Host plants have their own specific species. It’s recognizable on foliage and stems as a white, powdery coating. It’s found on growing either the upper or lower leaf surface, both, and on twigs. Sometimes it is noticeable in small spots rather than over the whole leaf. And most often affects younger leaves. Light infestations don’t cause any real damage. Powdery mildew because of high humidity spreads rapidly in the greenhouse. Heavy infestations lead to defoliation and much reduced growth. Small plants can die from it. Chemical eradication isn’t usually possible, so the best control is prevention. Reducing relative humidity in and around the plant canopy is the best approach. We achieve these goals by increasing air movement with fans and with physical separation of the plants.