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I have two types of Southern Highbush blueberry bushes, Misty and O'Neal. This spring, I noticed that their leaves are yellowing as shown in the photos below:

Misty:
enter image description here

O'Neal:
enter image description here

They are in 100% peat moss with pine bark mulch on the surface. What typically causes their leaves to turn yellow?

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The typical answer to yellowing leaves in an acid loving plant is chlorosis due to a lack of available iron caused by a soil ph of over 7.

However I find here that soils in Tennessee are mostly acidic and tend to remain at that level due to sulfur from air pollution and existing soil composition. You have already done the right thing by planting in peat moss.

Here are some diagnostics you can do:

  • spray a dilute solution of chelated iron on the foliage
  • do a pH test on the soil to ensure it is less than 6, preferably less than 5
  • examine the plants closely to see if there are other causes. Look for:
    • after frost damage branches look like upside down hooks
    • small reddish spots on the branches are Canker: Fusicoccom (Godronia
    • is the soil waterlogged due to poor drainage? If you dig a hole in clay based soils and back fill with peat it is like making a bathtub. Water goes in and stays in.
    • are the stems still healthy?
    • is there any sign of green veins and yellow leaf indicating chlorosis?
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    Looks like it was frost damage that caused a temporary yellowing. After a few weeks, they were green again. – ROFLwTIME May 22 '13 at 17:32
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Almost as crucial as soil pH is the moisture level. If the pH is right, and the moisture is good, blueberries are easy. If moisture or pH are wrong, blueberries are impossible.

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If I may humbly add something to check: the grass strip under ground and under obstacles with long rhizomes, to find another exit point. Even when it can not exit to the surface, because of the mulch.

Sometimes it finds an exit point along the way, but even if not, continues its path. So, for a blade of grass that you see, there can be dozens of underground rhizomes that have no outlet. To rip just the crest visible is not enough, because every piece of rhizome produces a new plant.

In the first picture, examining the maximum magnification, bottom left, just behind the leaves of the bush, you can see the blades of grass very close. Perhaps grass rhizomes are crawled under the mulch strangling the roots of blueberry.

If so, remove the mulch (and perhaps also the soil below) and check where they are rhizomes. If necessary, clean up the area and cover it again.

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    it's always a good idea to check what's going on in the soil. Grass roots will take compete for nutrients but are unlikely to cause the yellowing we see in the photos. – kevinsky Apr 14 '13 at 13:42

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