Here in Raleigh, NC, Rhododendrons die from Phytophthora root rot. Many live up to 8 years with some disfiguration, but in a recent study, ARS reports that R. x 'Caroline' Gable is totally immune to the disease. They think this is because it has a very different root structure than olher Rhododendrons. Has anyone experimented with grafting other varieties onto 'Caroline' rootstocks? Perhaps this would provide plants that are immune but offer a wide variety of flowers and habits. Breeding with 'Caroline' is another approach, but I won't live that long. So, if I could find an inexpensive source for 'Caroline' rooted cuttings (where I don't have to buy hundreds), I'd like to try grafting, assuming it hasn't been tried already and failed.
Been there, done that, and it doesn't work. It just slows the process and it's dreadfully expensive as well. Subdue is the recommended chemical. I've tried others as well. None really kill the Phytophthora, they just slow it down. As for cost. It'd almost be cheaper to replace the plants every few years. The other alternative is to plant all 'Caroline'. How boring. There are a few highly restant cultivars too. I'm trying them as well, but the grafting would let me have the delightful color of 'Dexters Champagne" or the georgeous foliage of 'Gomer Waterer' for a long time. Although my plants took nearly 8 years too die, they looked stressed, to say the least, after 3 or 4 years. I was really dissappointed the chemical treatments didn't work. But. I did find that "Messenger" treatments helped some cultivars really well, 'Chionoides' did the best. It actually recovered from the attack. Other cultivars didn't seem to respond as well. Trouble is, the minimum quantity you can buy is enough to treat a 40 acre farm and it cost well over $100. On top of that, it has a limited shelf life. Ray
The cultivar has been established to be immune to all Phytophthora that are known to infest rhododendrons, or just one or two? Phytophthora is an entire genus of water molds, your referring to them here as 'the Phytophthora' might imply you are actually talking about a single species. Buying some of the started plants and rooting your own additional ones from cuttings, as needed, might work out the best.
Not sure about your question. It was reported in the J. Amer. Rhododendron Soc.. I believe in the next to last issue, but I'm not sure. My understanding is that they injected the plants with the particular Phytophthora that causes Rhododendron root rot at a various dosages. Only 'Caroline' survived the highest dosage. But, ‘Brittany', ‘Ginny Gee' , ‘Ingred Mehlquist', ‘Normandy' , ‘Rocket' , and ‘Venus' proved to be highly resistant. ‘Anna H. Hall', ‘Bali', ‘Crete', ‘Hawaii', ‘Peter Tigerstedt', and ‘Samoa' showed resistance. Where "highly resistant" means no feeder root damage at elevated dosages and "resistant' means moderate feeder root damage. If you have more questions, I recommend reading the article. I'm not an expert in the field. Rooting my own cuttings is a possibility. I'll have to do this with the scion stock. I just hoped to get a head start on the root stock to avoid a year or two in growing them to a suitable state. Ray
Three species of Phytophthora causing root rot of rhododendrons are mentioned here. If 'Caroline' was found to be unaffected by all three that is markedly more useful and noteworthy than if it may only be able to fend off one of them. http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=968
Like I said, read the article. Then perhaps you can answer your own question for all of us. It would be interesting to know the answer. I'm no expert in this field.
hi where you are in NC you might want to have your soil tested for murcury. It can cause the the same type sympoms in the rhodo that area was mined for gold( yes that great red clay you have there)
Phytophthora has world wide distribution (i.e., not unique to NC). Phytophthora is a family of water molds that likes warm wet soil -- but they tend to not tolerate dry soils and doesn't like low PH (hint). There are three species of evergreen Rhododendrons native to NC, so if it was a NC problem, there would be none. But there are so it can be done. Resistant Hybrids doesn't mean they are immune -- provide poor warm wet soil phytophthora loving conditions and neglect and they will become affected and die. You are right that fungicides wont cure rhododendron in these conditions -- the conditions must change.
The natives do reasonably well in the piedmont, but the hybrids don't last more than 8 years for me. The plantings are above grade, well drained, full of pine bark & thus acid. "Subdue" is supposed to control Phytophthora and it does slow it down. "Messenger" actually reversed the destructive effect of Phytophthora in 'Chionoides, but not in 'Scintellation', 'Boule de Niege', or 'Gomer Waterer'. I understand that "resistant" doesn't mean "immune", but the high injected dosages had no effect on 'Caroline' whereas other "highly resistant" varieties in the tests showed modest root destruction. That's close enough to "immune" for me.
