"Jean B." wrote:

Okay, so I see a lot of sickly rhododendrons around now. I am
wondering whether, instead of being a natural phenomenon that is
mysteriously spread through the air, these blankety-blank
landscaping companies are spreading these diseases with their
clippers?

Most causes of generaly poor behaviour of rhododendrons is cultural, not
disease. A healthy rhododendron is very resistant to disease. There
are diseases that spread in the soil, but very few that are spread by
pruning.

The biggest cultural problems are walnut roots or leaves, high soil
temperatures, drought, too moist a soil and over watering, lack of iron
available in the soil and alkaline soil, improper planting and root
stangulation, wind damage, sunburn, fertilizer burn, herbicide burn,
salt burn, etc. Rhododendrons are very sodium sensative, so winter use
of sodium salts is very toxic to them. On the other hand potassium
salts are beneficial of not used in excess.

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