Growers should know that powdery mildew infection of some varieties of plants does not reveal the ‘tell-tale’’ presence of the white spores. Attacks on Begonia  can cause greasy looking spots on the undersurface of leaves in the winter or pale yellow spots on Poisettia leaves in the summer. On Kalanchoe it the disease manifests itself as scabby patches on the leaves. Powdery Mildew  attack of Petunia and Verbena plants  makes them look weak and chloritic. The most commonly found genera found on greenhouse crops are Erysiphe and Podosphaera. Many are host specific, only infecting a single family of plants. Different genera, species and variety within a given family show different levels of susceptibility. Look out for these variations.  To help combat the disease, air circulation should be improved,  humidity reduced and suitable fungicides applied. Note however that since there are so many strains of the fungus some fungicides will work well while others less well or not at all.  Marjery Daughtrey, GrowerTalks  www.growertalks.com