Blueberry Leaf Scorch: Drought, Fertilizer Burn or High pH?

Brown, papery leaf edges on a blueberry look alarming, but they narrow the search quickly. On a potted blueberry the two usual causes are a root zone that has dried out and a root zone with too much fertilizer salt in it. A third problem, high pH, causes yellowing rather than scorch, and is often confused with the other two.

Start with the pot, not the leaves. How dry does it get, and what has gone into it in the last few weeks?

Drought or heat stress

Oregon State University Extension describes the blueberry root system as shallow and fibrous, which makes the plant susceptible to drought injury. Its home garden guide (EC 1304) shows drought or water stress as brown leaves with burnt-looking tips.

Signs that point to water stress:

• The pot feels light and the mix is dry below the surface.
• Scorch appears after a hot, sunny or windy spell.
• The plant wilts in the afternoon before the browning starts.
• Damage is worst on the most exposed side of the plant.

University of Maryland Extension notes that blueberries are very sensitive to dry conditions and that containers should be kept evenly moist, in its guide to growing blueberries in containers. It also suggests full sun with protection from late-afternoon sun where needed.

Pot-bound plants dry out fastest. If the pot dries within a day of watering, check the roots.

Fertilizer burn and salt injury

University of Georgia Extension explains in Suggested Blueberry Fertilization Timings and Rates that blueberries have a low nutrient demand. Overfertilization can cause salt burn to the roots, which shows as shoot dieback and leaf scorch.

Oregon State shows salt stress from too much fertilizer as blackened leaf edges. It also warns that liquid fish fertilizer is salty and will injure plants unless diluted 1:10 with water.

Signs that point to salt injury:

• Scorch appeared within a few weeks of feeding.
• Granular fertilizer was piled near the stem, or the dose was large.
• You used a general garden feed, manure or mushroom compost.

Oregon State notes that manures and mushroom compost often have high salt levels, and that blueberries are very sensitive to high salts. University of Georgia advises avoiding chloride sources such as muriate of potash.

The look-alike: high pH chlorosis

Yellowing is not scorch, but the two often turn up together. Oregon State says signs of high soil pH include yellow leaves with green veins, especially on younger leaves. Blueberries need a pH of 4.5 to 5.5.

In a pot, pH tends to creep up from two sources:

• Tap water. RHS warns that tap water is usually alkaline and, used regularly, raises the pH of the compost. It recommends rainwater.
• The wrong mix. Oregon State says a container mix should not contain lime or a high proportion of compost.

Clemson HGIC suggests ammonium sulfate where pH is slightly high, and iron sulfate if needed to correct yellowing, in its blueberry factsheet. Test the mix before adding anything.

A garden look-alike: bacterial leaf scorch

University of Georgia describes bacterial leaf scorch of blueberry, a disease of southern highbush blueberries in the Southeast. Its marginal leaf scorch looks like extreme drought or fertilizer salt burn. The distinctive sign is yellowed stems and twigs once the leaves drop, leaving a skeleton-like bush. Pruning does not cure it. This is mainly a problem of plantings in the Southeast, not of a well-watered pot elsewhere. Ask your local extension office to confirm it.

What to do

1. Check moisture first. Push a finger into the mix. If it is dry well below the surface, water until the whole root ball is wet. Virginia Cooperative Extension notes in its container watering guide that dry peat or bark mixes can repel water, and that two or three slow applications help rewet them.
2. Shade the plant in heat. Move it out of late-afternoon sun during hot spells, as University of Maryland suggests.
3. Stop feeding a pot you overfed. Skip the next feed and scrape away any granules sitting against the stem. Oregon State warns against piling fertilizer in one spot or against the crown, and recommends splitting the yearly amount into several small doses to prevent salt stress.
4. Feed less, and the right kind. Oregon State says blueberries use the ammonium form of nitrogen and that many all-purpose feeds supply mainly nitrate. Clemson warns that blueberries are easily damaged by excess fertilizer. Oregon State says slow-release products suit containers.
5. Test the pH if young leaves are yellow with green veins. Switch to rainwater and an acidic mix if it is above 5.5.
6. Judge recovery by new growth. Watch whether the next leaves come out clean rather than waiting for scorched edges to change.

The blueberry hub collects our other blueberry pages.

Related guides

Other blueberry problems we cover: root bound, poor pollination.

References

• Oregon State University Extension: Growing blueberries in your home garden (EC 1304)
• University of Georgia Extension: Suggested Blueberry Fertilization Timings and Rates (C 1163)
• University of Georgia Extension: Bacterial Leaf Scorch of Blueberry (C 922)
• University of Maryland Extension: Growing Blueberries in Containers
• RHS: How to grow blueberries
• Clemson Cooperative Extension HGIC: Blueberry
• Virginia Cooperative Extension: Properly Watering Container Houseplants (SPES-804)