The GARDENA gardening expert
Dear Mr. H., my response to your enquiry from September took a little while, because I wanted to consult a second professional as to their opinion – the damage pattern on your roses is indeed somewhat unusual. The result: As far as we can ascertain from your photos, this is in fact mildew infestation. This assessment has been confirmed from the Head of Cultivation at a large German rose breeding company. This Head of Cultivation states that such unusual damage patterns resulting from this fungal infestation occur because certain varieties or groups of varieties react in a specific way to the mildew. “The infestation can appear slightly different on different varieties; in some roses this conspicuous crimping occurs”, he said to me.
The deformation of the flowers can be related to this. At the same time, however, I would advise you to check for thrips (Frankliniella occidentalis, thunderbugs [Thysanoptera]). Thrips infestation can also cause flower petal deformations. However, you must look closely to detect thrips. From the outside, you can see tiny penetrations which later turn a dark colour. You can also see flower petal deformations, even on the buds. If you turn the opened flower onto its head and tap it forcefully onto your palm, pollen and thrips will fall out if your rose is infested. Look in the Internet using the search term “Frankliniella occidentalis roses”. However, this pest almost never attacks the leaves of rose plants – basically only then when it finds nothing else to eat.
You can combat mildew using the standard active ingredients and preparations from specialist stores, and the thrips using a suitable insecticide.