We purchased 5 variegated red twig dogwoods (approx. 2 to 3 ft tall) about a month ago and planted them. In the last few weeks we have noticed black spots on most of the leaves for all 5 plants. We purchased a fungicide at the advice of the nursery where we purchased them and applied it. It seems to have helped a bit however we have lost plenty of leaves and there are still blackspotsspots galore. On one of the 5 plants
2 of the leaves have a purplish color? We live in upstate NY (near Syracuse) and the temps have been mid 70's to 80's since we planted them. We have been watering them and thought perhaps too much, however the nursery owner said "you can never over-water dogwoods" so we ruled out overwatering.
Any advice on what causes these blackspots and caring for them would be appreciated. Are these plants in danger or can they be saved/cured?
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I didn't even know redtwig could get blackspot. They do get powdery mildew, & that can indeed be worsened by too much dampness which wouldn't otherwise be anything but dandy for a redtwig. Fungicides do not fix leaves once they are infected; those have to be removed &discarded in such a manner that they don't spread the fungus (like burned, or wrapped in plastic & tossed in trashcan, not composted). If you let the leaves fall to the ground & remain, then next year the spoors will jump up on the shrubs all over again. Rigorous removal & clean-up of infected leaves will be much mroe beneficial than fungicides, which shouldn't be necessary at all if it's really only blackspot.
If it gets really bad you can cut redtwig nearly to the ground & it'll grow back fresh. Overwatering isn't likely to happen with redtwig, but if the shrubs are in perfectly draining soil, they could actually be UNDERwatered even if you get to them regularly. WIth brand new plantings, watering might not be getting in close enough to the as-yet short roots, or water is draining through too rapidly & the roots are dry within an hour, so the shrubs become too easily heat-stressed & drought-stressed hence susceptible to diseases they wouldn't ordinarily get. And in that case they will be much stronger next year when it gets more roots set out.
If the spots look too worrisome you might consider contacting a plant pathology lab (your nearest major university likely has a horiticultural extension, and someone there could point you to a lab that diagnose plant diseases for gardeners). It might be something nastier than blackspot that does need some aggressive treatment. Dogwood anthracnose (a death-sentence disease) does not afflict redtwig dogwood, but anthracnose has in the last couple years been observed jumping species, and will be found on increasing kinds of shrubs & trees in the future, alas. What a bitter thing it would be if you were one of the first to see the relatively short leap from flowering dogwoods to redtwigs.
But accepting that it's "just" blackspot, the other things to do is check out all rose-family shrubs growing too close by, prune to increase air circulation, clean up all fallen leaves & discard those completely, & starting late winter before spring spooring takes flight, spray the ground around the base of the shrubs with horticultural oil or neem oil, which pastes the spoors to the ground where they can do no harm. I'm in general no great fan of needlessly expensive & overly faddish "garden teas" -- but where mild funguses are concerned, use of those teas can shift the microorganism mix in the soil in a healthy direction so that useful microorganisms can better out-compete spoors of harmful funguses.
Thanks for the advice , perhaps I can offer some more details:
The red twig dogwoods are planted in our front yard which is sloped down to the street , about 5 to 10 degrees. They were planted with hemlock mulch around them. There is a bed of Hot Cocoa roses about
3 feet away from the dogwoods. I looked at the plant leaves close up and seems like the initial "blackspots" turn into pinholes with purplish/red borders over time. We do pickup the dead leaves and throw them away.
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