What is causing those yellow leaves with dark green veins?
Blueberries are a wonderful fruit, tasty, healthy and the bushes make attractive landscape shrubs. They are easy to grow once you get the soil and water right, but if they have a problem they let you know! Some of my blueberries have begun to exhibit classic signs of iron chlorosis, that is iron deficiency. A blueberry with iron deficiency has yellowing leaves with dark green veins, the new growth will be affected by this first.
Iron deficiency in blueberry plants
Now, before you run off and bury iron nails next to your blueberry bushes one of the biggest culprits of this problem isn't that the soil doesn't have enough iron, but that the plants can't use the iron in the soil. Blueberries are unable to use the iron in soil when the soil pH is too high. Blueberries like their soil pH to be between 5.2 and 4.0 with the optimum being 4.5 to 4.8. Another problem that can cause iron chlorosis is too much water, this can happen with wet springs or irrigation that is set to water amounts that appropriate for the summer heat but not a cool spring.
How to test for iron deficiency in the soil
The first step to address this problem is to do a pH test. This simple, inexpensive pH test kit is perfect for this type of monitoring. You'll need to prepare a soil sample and the kit contains instructions, you can also watch a video of Tricia showing you how to prepare a soil sample. Once your sample is taken, dried, and crushed put a cap and a half full in in the test tube and 4ml of reagent, shake for 30 seconds and you're in business. 
This is the soil test for my Reka blueberry which seems to be the most unhappy of my five bushes. It looks like the pH is about a 5.0 which is ok for blueberries, looks like my problem might be a bit of a wet spring!
How to cure the blueberry plant
If your test comes out with a high pH you can add iron sulfate or elemental sulfur in the recommended amounts. Other helps to lower the pH is a pine needle mulch and the addition of peat moss. You can also fertilize your blueberries with an acidic fertilizer like Cottonseed Meal or Acid Mix. If your blueberries are going into containers, a mix of half potting soil and half Peat Moss works well.
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I have a shallow raised bed with six different blueberry cultivars growing in it. I prepped the soil a year in advance and made sure to have adequate ph and drainage. Only one of my plants (alapaha) had leaves that are yellowing. All of the others are bright green, bushy and full of fruit! Could there still be a problem with the soil ph and/or too much water, or do I maybe just have a weak plant?
Lily, you probably do not need to do anything until spring. But once you start to see new growth in the spring and the leaves still look yellow, you will want to check the pH of your soil If it is too high then your plants cannot take up the iron in the soil. If the pH is in the range the plant likes, then you can feed with some iron.
My blueberry tree has all of the leaves yellow. What does that mean and what should I do? Thank you so much! Lily
JB, blueberry leaves will turn colors in the winter and depending on where you live, might drop all their leaves over the winter. In the spring you can give them a feeding of acid loving plant fertilizer. You really should not fertilize this time of year. The plants are getting ready to go dormant.
My leaves are yellow and red. It appears the plant is dying. How do I save it ?