Hi Folks, Thanks for accepting me, I can't seem to find any site to help with rust on my rhododendron leaves, I live in zone five with cold winters and fairly warm and humid summers, other homes in my area have plants with this rust also, is it common and what should I use to stop it?? Also the new buds seem to look burned and dried out. Can anyone tell me what is wrong? Susieqmoose
The rust is a fungal disease..it spreads on the new growth. (it commonly attacks hybrids) Remove the affected leaves (they will fall off anyway). A fungicide containing bupirimate/triformine works effectively as a preventive spray. The bud blast ..not much you can do about it....sometimes praying with wiltproof before the harsh winter.
Actually, if the soil is highly acidic they will like it sweetened a bit. Already by the 1970s the local rhododendron society newsletter was recommending January applications of dolomitic limestone - and cultivated soils out here aren't generally as acidified as some in parts of eastern North America.
Rhododendrons need a soil PH of 4.5-6.0. They need very little calcium but on limy soils they take up large quantities of it causing poisoning. High PH tends to lock up elements necessary for the healthy growth. ..this is particularly true in the case of iron. The relationship between rhododendron health, the minerals they need and the soil PH is very complex.
Then there's the thousands of rhododendrons that grow wild on dolomitic limestone in China. Don't know if it's "very complex" but yes, there's more to it than them not being able to tolerate exposure to any level of lime in any form.
About the chalk thing.....I went to a lecture years ago by "an authority" (notice the quotes) on Rhododendrons. He was the one who made the statement about them not liking anyone to even walk past them with a piece of chalk in their pocket. I believed every word he said and now I suppose this "expert" was wrong. Well you live and learn...Thank you Ron B for enlightening me.
Limestone in China (and other soil) George Forrest often reported the phenomenon of rhododendrons growing on limestone, much to the surprise of gardeners at home. He reported that the limestone in the Lichang-Tali area of China has nothing chalky in it, but is apparently a strong magnesium limestone, greyish-white in colour, very hard and durable, such as in the Dolomites. Farther north towards Tibet (Tsarong) the strata, though still limestone, is much freer in composition, disintegrated more readily and is deeply stained ruddy-brown by the presence of minerals. The greatest concentration of rhododendrons is found here. Most rhododendrons in Yunnan grow on limestone according to Forrest, but in organic matter, even on cliffs or boulders. --Cox, THE LARGER RHODODENDRON SPECIES (Timber Press)
Interesting...I have killed many. I think that no home owner has the perfect soil for them..(like Kalmia and Skimmia). Again ...I have heard to use gypsum when planting Kalmia or they will die. Now I have also heard not to amend the soil too much when planting because they will love their soil for a few years and then hit the native soil and die. You have to amend clay soil if you want to plant a Rhodie. What to do?
In 1987 (1991) Carl E Whitcomb wrote in ESTABLISHMENT AND MAINTENANCE OF LANDSCAPE PLANTS (A Summary of Procedures, page 3) "Use no soil amendments except in very specific conditions of raised or amended beds for plants with very limited root systems. If the existing soil is very poor, remove and replace with good field soil or place at least six inches of good field soil on the surface. However, you should match soil types as backfilling with a good sandy loam in a heavy clay will serve as a collection point for water and the roots will suffocate. Soil amendments in a small planting hole do not assist plant establishment and growth. It is better to use the amendments as a mulch. The only exception is where the entire plant root zone for many years can be amended." For other, more recent comments from another source: http://www.puyallup.wsu.edu/~Linda Chalker-Scott/Horticultural Myths_files/Myths/Amendments.pdf
