Plant Pathology: Extension: Plant Disease Library

Blueberry Diseases

There are a number of blueberry diseases that occur in Georgia. These include mummy berry, Botrytis blight, powdery mildew, various leaf spots, anthracnose of foliage and Botryosphaeria die back. The most important are mummy berry, Botrytis blight and anthracnose of leaves.

Mummy Berry
Mummy berry, caused by the fungus Sclerotinia vaccinii-corymbosi, overwinters as hard fungal bodies called sclerotia on the ground. These germinate in the late winter shortly before bud break to form small saucer shaped structures. The sexual spores, called ascospores, are produced in these saucer shaped structures and are blown to the buds about the time that they break. They infect leaf and flower buds and kill them. A second type of spore, called conidia, is produced on the dead tissue. The conidia infects the flowers during bloom, growing down the pistil to the ovary, and form sclerotia in the ovary instead of blueberries.

Control of mummy berry is accomplished by cultivation which buries the sclerotia prior to their germinating, and by spraying. There is no one fungicide which controls both spore stages. Current recommendations are to use Funginex at bud swell, prior to bud break, and repeat every 10 days until bloom. This will control the first spore stage, or ascospores. Benlate-Captan is used to control the second spore stage, called conidia. This needs to be started at 10% bloom and continued until the end of bloom (cap fall). Excellent control is available if all three of these control measures are used.

Botrytis Blight
Botrytis blight, caused by the fungus Botrytis cinerea, have caused losses up to 100% in blueberry fields in Georgia every year. Weather conditions favoring this disease are cool temperatures and wet conditions. The fungus overwinters as hard fungal bodies, called sclerotia, on the ground. In the late winter or early spring, these sclerotia germinates to form spores called conidia which is blown to exposed flower tissue at bud swell and bud break. The conidia infects the flower tissue, kills it, and moves down the flower stalk to the stem. It girdles the stem and moves down the stem for a distance of up to 6 inches. This destroys all of the fruit on the infected stem, and kills the shoots which would be fruiting wood the next year. Thus, this disease reduces yield for 2 years every time infection occurs.

Control of Botrytis blight consists of cultivation to bury the sclerotia. As in mummy berry, this reduces disease by about 50%. The best fungicidal control of Botrytis blight is a Benlate-Captan mixture sprayed starting a bud swell and repeated every 10-14 days depending on rainfall until the end of bloom (cap fall).

Anthracnose of Foliage
Anthracnose of blueberry foliage, caused by the fungus Colletotrichum, usually shows up in fields after harvest. It infects leaves by killing them and turning them rusty red in color. Frequently, it starts on one side of a leaf and kills that side before progressing to kill the entire leaf. The disease develops very rapidly, and can defoliate a bush within 3 weeks. Plants so affected go into the winter in a very weakened condition, and usually are winter killed. The disease can be controlled by spraying with Captan on a 10-14 day interval. Usually 2 sprays are required. Growers need to scout their fields frequently after harvest and spray at the first sign of the disease.

Powdery Mildew
Powdery mildew, caused by the fungus Microsphaeria alni, occurs frequently in Georgia. To date, the disease has not been severe enough to injure plants. The fungus infects leaves causing bright red streaks and blotches. No control is suggested.

Leaf Rust
Leaf rust, caused by the fungus Pucciniastrum myrtilli, was first observed in Georgia on southern highbush varieties. It has since been observed on most rabbiteye varieties. The disease builds up rapidly after harvest, and causes extensive defoliation. Plants so affected go into the winter weakened and frequently are winter killed. Since southern highbush frequently does not defoliate completely in the winter, the disease overwinters on foliage on the plants. This leads to early buildup and defoliation of plants in the spring. No control measures are currently available.

Leaf Spots
Leaf spots, caused by the fungi Dothichiza caroliniana, Phyllostictina vaccinii, Gloeocersporora inconspicua and others, frequently occur in Georgia. These diseases are not severe enough to cause injury to plants, and no control measures are suggested.

Botryosphaeria Blight
Botryosphaeria blight, caused by the fungus Botryosphaeria dothidea, is generally not a problem in vigorous plants. It enters plants through the dead twigs killed by Botrytis blight. It may extend the dead area on the twigs by a few inches. On plants that are not vigorous, it will kill the infected limbs to the ground, and may girdle the crown of the plants. It can also enter basal sprouts that have been killed by mechanical injury or herbicides. It is most commonly a problem on plants growing in soil with a high pH, high phosphorus and/or high calcium. On vigorous plants, control of Botrytis blight will also control this disease since the fungus will not have an entry point. On plants that are not vigorous, no control is available.