Why are Rhodo leaves growing poorly?

These are leaves from Rhododendron ‘Gomer Waterer’ which has been giving me grief for a number of years. While it has grown and bloomed well since I first planted it about 7 years ago, the leaves have been an ongoing problem. They come out fine but as the weeks go by, they lose the green colour between the veins and eventually become desiccated and develop brown patches.

Although this rhodo was growing in an area where 2 others grow very well, I moved it to a new spot this spring. The soil contained a generous amount of compost, amended with well-rotted steer manure.

Gomer Waterer is supposed to tolerate quite a lot of sun but I put it where it gets more protection from sun and wind. There was little if any root disturbance in the move so I let it bloom this May. This year’s leaves came out looking perfect but soon began showing the same netting pattern as before.

I don't think this is a disease problem but a nutrient deficiency of some sort. In late May I gave it a couple of gallons of Fish Fertilizer; waited, but symptoms persisted . . . then another gallon of FF; no change . . . then I tried about cup of Blood Meal around the drip line but have seen no improvement. At some point I gave it a good fist-full of Epsom Salts and sprayed the leaves with Chelated Iron. I try to wait a week or two between adding anything to allow time to see what effect the previous one might have.

Even rhodo experts in my local rhododendron society have no other suggestions.

As far as the garden goes . . . we’re in Zone 8a on the south side of a rocky hill with intermittent shade from large Garry Oaks and Arbutus trees. Rainfall is very low but supplemented by an irrigation system and hand watering. I have about 60 healthy rhodos and decades of experience growing them here and in other gardens.

Should I show Gomer Waterer the door?

 

Not an expert, but this sort of chlorosis between the veins and venules is possibly manganese deficiency. Is your soil generally alkaline? Sometimes our amendments, like iron, actually make it harder for plants to take up available nutrients. So it may be a combination of things that results in a deficiency that afflicts this particular rhododendron.

 

Thank you for your reply.

I have also begun to wonder if the pH is too high . . . maybe Gomer Waterer needs a more acid soil than most rhodos, because none of my other 60 rhodos show similar problems (although a nearby Euonymus alatus does). I had ruled out a manganese deficiency because our tap water, which I use for irrigation, is reputedly high in manganese .

Except for twice spraying the leaves with iron chelate, which made the green veins even greener, I've never amended the soil with iron.

Now I'm not sure what to do next. I don't want to waste money on chelated manganese or manganese sulphate which I may well not need.

Then, to add to my confusion, I found this on the American Rhododendron Society website:
Potassium & Iron: Potassium deficiency can be difficult to diagnose. A plant must have sufficient iron to utilize available potassium. For this reason the symptoms of potassium and iron deficiency are almost identical in the initial stages. Potassium deficiency begins with leaf yellowing, which eventually spreads between the veins. Leaf tips and margins show scorch and necrosis.

Perhaps a one-time drench of chelated iron would be worthwhile.