I moved into a home with several mature rhodos approx 5 years ago and have been trying to mend them ever since. They are certainly infested with weevils (based upon the notched leaves), and I've tried Nematodes, nocternal hunting (although I didn't find that many - I'm not sure how many I should expect to see) keeping the areas around the trunk clean of debris, and even pesticide one year. most of them continue to bloom beautifully almost every year. I've been careful to water well, and the soil is almost certainly very well draining and acidic (north shore) - so I don't think its a root rot problem. However, they look like they are getting sicker and sicker (or at least not improving); in addition to the notched leaves they lose a lot of them to browning / yellowing - sometimes I'm surprised that they bloom as well as they do and continue to show new growth. Recently I spotted tiny red mites on black sprinkler heads at a close but other location in the garden (I don't know if these are new, recently had 25 yards of soil delivered; and the sprinkler heads are also new) - I believe these are a variety of mites (although there doesn't seem to be mite problem evidence on the rhodos) - any thoughts on further investigation / saving methods - they are probably approx 25 years old and I don't want to lose them! thx
Sounds like same as many others in the region your rhododendrons have become infested with powdery mildew. http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=969 Strongly red mites large enough to be noticeable are beneficial predator mites.
Re: weevil / rot cure? (no, probably mildew) thanks Ron, I hadn't considered a mildew, it makes more sense than a root problem in my circumstances; I hadn't realized the diversity of effects that the mildew would cause; good link. I'll try to get some photos up here of the problem in the next couple days; I took another look and noticed that what I had considered normal bark may also be showing signs of disease - these poor Rhodos - to top it all off they got beaten up real bad this winter by falling limbs from above.
With your weevil issue, when my weevils were at their worst, my nocturnal hunting could pick off 40-50 per night (and I too am on the North Shore). I think I got over 300 last year total. You have to have patience and a good eye (but your friends and neighbors will think you are a bit nuts...) I also applied the nematodes, but ended up giving up on 1 rhodo that was particularly infested badly and ripped it out. It seemed to always be the problem and it never looked particularly good because of the continuous chomping. good luck
