There is nothing more devastating than seeing your blueberry bushes dying off for no immediate reason. Diseases and fungus infections can be very tricky because they are fundamentally invisible until you start to see symptoms, and then you have to establish what the problem is quickly enough to save your plants.
Several dozen potential problems could strike your plants though some are more common than others. These are the most likely culprits:
Powdery mildew is a very common problem with any garden fruit or vegetable, and it is easy enough to take care of. It will look like a light dusting of white powder on the leaves and possibly the fruit as well. If they get enough mildew, the leaves will probably drop off. Unless it spreads all through the bush, your plant is not in any immediate danger. Most fungus sprays are ineffective so that probably won't help your plants at all.
Trim off any infected leaves and try to reduce the humidity around your plants. You can't control the weather but you can improve the air-flow by cutting away any interior branches that are not producing fruit, and not planting your bushes too close together if you already live in a humid climate.
With this or any of these diseases, you should always burn or otherwise cleanly dispose of any infected plant material. Diseased leaves or branches should not be stored anywhere near your fruit garden.
This is one disease that will kill your plants if left unchecked. This kind of blight will turn leaves and blossoms brown and twisted. If not removed, the infected leaves will then start to get furry with grey mold. It usually attacks leaves and blossoms rather than the fruit, but if your plants lose their flowers to blight then you will still have no berry harvest.
Like with other fungus infections, you should carefully cut out the leaves and branches that are showing any symptoms. When your plants are in bloom, use a general fungicide to help reduce any outbreaks if you have had Botrytis blight in your area before.
The first sign of this fungus infestation is wrinkled or sunken-in berries. On closer inspection, you will notice clusters of bright pink or orange spores on the fruit. They will occasionally also form on the leaves or branches as well. It will eventually kill off the entire plant, and certainly make any infected fruit unusable.
To treat, you should cut away any infected plant material right away to prevent it from spreading and treat the rest of your plants with a commercial anti-fungal spray. It will re-emerge in the next year if there is too much plant debris around your blueberry bushes (and that includes any mulch). Clear that away and replace with fresh, if you are using mulch.
The fungus also likes humid conditions just like the powdery mildew, so you should try to allow for as much air flow within and between your plants in the same way.
Not all regions have a problem with leaf rust, but you will want to watch out for it if you live in the eastern parts of the United States particularly. Dark brown spots will form on your blueberry's leaves, sometimes showing a bit of a rusty yellow fuzz to them. Once the spots start to spread across the leaf, it will turn yellow and drop off the plant.
This fungus also lives in hemlock trees, so if you have a problem with rust then you should see if there are hemlocks nearby. That will make it difficult to completely get rid of. Treat with fungicides as soon as you see the spots forming and remove the infected leaves. Dropped leaves should be picked up from under the bushes or it will just continue to spread the next year.
This can be a very big problem with fruit harvest because it directly attacks the berries more so than the other diseases. The first sign is usually a browning of the leaves, but only along the middle vein at first. It does eventually spread to the whole leaf so you have to check your plants regularly to catch symptoms like this right at the start.
Berries then get infected though it is not apparent while they are still green unless you cut one open. Inside, they have white masses of fungus. As they ripen, they are clearly misshapen and will soon shrivel up.
You must gather all the dropped “mummy” berries and dispose of them or it will return again the next year. Fungicides can also be used while the plants are in blossom to prevent any berries from getting infected while they develop.