We were told the red leaves on this rosebush indicated disease and needed to be trimmed. Can anyone confirm or explain?
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Rose new growth is reddish from anthrocyanin pigments which protect the cellular growth from sunlight UV. It fades as the foliage matures. The new growth must be healthy without leaf distortion and the red pigment will fade out as the leaves mature.
The upper picture looks normal to me. The bottom one, I will leave to someone a little more experienced with the rose disease mentioned below to diagnose this early in the game. The leaves appear normal to me, personally I'd apply neem oil to all my bushes to kill any mites that might spread it and allow this one to mature out a bit more to see if it is normal growth.
The worry is whether it is rose rosette disease. When you have this in your roses, it is incurable. It is a viral disease spread by a sort of mite that inhabits wild roses. You uproot the bush and destroy it. Trimming it and then trimming other rose bushes will spread it to otherwise healthy bushes. You can only prevent it by controlling the mites with horticultural oils like neem oil.
Rose rosette disease is also known as Witches Broom disease because of the strange growth development. The bush starts putting up odd new, overly succulent water-sprout like growth that is highly pigmented. The leaf structure in the growth is wrinkled, distorted and in really bad cases, you have stems with what look like multi-shoot broom shaped growth sprouting out of the ends.
The following should be long term links to image content that will allow you to see what rose rosette disease looks like, it's really ugly, you'll know it when you see it.
Cross species comparison of anthrocyanin pigmentation in new growth.
Healthy rose, top new growth hardening and pigmentation fading, smooth, well-formed new leaves, bottom newly emergent and heavily pigmented:
10 feet away, a white oak showing the delicate pink found in their new growth:
And the most brilliantly pigmented I've come across are Blueberries (not shown) and Poison Oak. See this in early spring, stay away. They're soft, tender and full of oil. These have aged a little, earlier on, they were reddish purple with pigment.