Better Yields Through Science
Blueberries Diseases & Pests
|Diseases||Symptoms, Life-cycle and Pest Management|
|Anthracnose||Pathogen: Colletotrichum gloeosporioides Penz.
Blossom end of ripe fruit softens and orange spore masses seep out. Infected green fruit do not show symptoms until they ripen, and so the disease is nick-named “ripe rot”. The fungus overwinters on twigs. Spores are spread by rain splash to bloom and green fruit. Anthracnose is favored by prolonged periods of warm wet weather during bloom and just before harvest. Apply pre-bloom to reduce overwintering inoculum and twig blights.
|Fusicoccum (Godronia) canker||Pathogen: Fusicoccum putrefaciens
Fusicoccum canker appears as individual stems exhibit ‘flagging’ or wilting during the summer. Dark red or brown infected areas form at the base of canes, become covered with pycnidia. Older dead canes develop the sexual fruiting bodies (apothecia). This is the Godronia stage of the disease. Both Fusicoccum and Phomopsis canker can appear separately; however in some cases these cankers may occur simultaneously.
The fungi over winter on 1, 2, or 3-year-old wood, and produces sunken cankers encircling the stems. A mass of black tiny fruiting bodies, (pycnidia) are produced on canker surfaces the first year. During the spring to summer months pycnidia continue to produce conidia that infect susceptible tissue, spreading by rain or water contact. When the temperature is warm and fruit is present, leaves on cankered stems wilt. Precise environmental conditions for infection have not been determined, but the greatest activity for Fusicoccum canker is at temperatures between 5o –72°F. Dormant applications of lime sulfur have shown control of cankers.
|Mummy Berry||Pathogen: Monilinia vaccinii-corymbosi.
The first symptom that becomes visible is a blight of young developing shoots in spring. Occasionally, flower clusters also become blighted. The next symptom becomes visible as the berries start to ripen, at which time they become light brown to pink and start to shrivel up, turning into fruit “mummies”. Most of the berries fall to the ground, but some end up being harvested. The mummy berry fungus overwinters in mummified fruits on the ground, and in early spring produces spores that start the disease cycle anew. Mummy berry is more common in wet fields and poorly drained areas, therefore scouting should target those sites. Dry, sandy sites may not have any mummies at all. However, wild or escaped blueberries in nearby wooded areas can also be a source of infection.
Mummy berries can be found on the ground below blueberry bushes anytime after harvest and look shriveled and are purplish white in color. Mummies will start germinating in early spring and will show small brown, finger-like projections (stipes). Only a portion of the mummies germinate in any one year. The wetter the site, the higher the germination percentage will be. There can be anywhere from one to seven stipes on a mummy. The stipes have a hole in the tip that develops into a small mushroom cup, 1/16 to 1/3 of an inch in diameter. The mushrooms start shooting spores when the cup is about 1/8 inch in diameter. Over a million spores can be released per day by a fully functioning mushroom. The ascospores are windborne and can travel fairly long distances within fields and even between fields. Ascospore release continues until the cup collapses. Cups may last 1 week at room temperature to up to 4 weeks at lower temperatures.
There are two stages of infection. First the developing shoots are infected by the ascospores from the mummy berry cups. Blueberry cultivars are susceptible from bud break until shoots are about 3 inches long. Flower clusters also become blighted; these are called flower strikes. Secondary spores (conidia) develop in a grayish layer on blighted shoots and flower clusters and are rain splash- or insect-dispersed to the flowers. Bees are attracted to the infected shoots by their UV-light pattern and unwittingly pick up the spores, which then hitch a ride to the flowers. The conidia infect the flower stigma followed by growth of the fungus into developing fruit, which eventually mummifies and drops to the ground.
Lime sulfur is also approved for organic production and is applied when plants are dormant. Lime sulfur is sprayed on the soil surface very early in the spring to destroy the apothecia (as the first mummy berry cups appear) with lime sulfur at 16-24 gal/acre in 200-300 gal water.
|Stem Canker and Stem Blight||Pathogen: Botryosphaeria corticis and B. dothidea
Symptoms of stem canker first appear as small red lesions on succulent stems. Lesions become swollen and broadly conical within six months. Symptoms vary with the susceptibility of the cultivar. Numerous fruiting bodies of the pathogen occur in cankers of susceptible cultivars. Stems of current season growth are infected in late spring during wet weather and spread by wind throughout the planting. After cankers develop the stem can become girdled and will eventually die. In resistant cultivars, the fungus is restricted to outer portions of the stem, and those cankers are small raised lesions.
Stem blight is commonly known as dieback. Early symptoms will show up as yellowing and reddened or drying leaves on one or more branches. The most obvious clue is a dead branch among live branches. Infection near the ends of twigs can be confused with winter injury. Inoculum is present throughout the growing season in the south, with levels highest in June and July. This fungus enters the plant through wound sites. Most infections occur in the early part of
the growing season. Infected plants begin showing symptoms about 4-6 weeks after infection takes place. Dormant application of lime sulfur is recommended for control.
|Phomopsis Canker and Twig Blight||Pathogen: Phomopsis vaccinii
Phomopsis canker overwinters in infected plant parts. The primary symptom of twig infection is a blighting of one-year-old woody stems that have flower buds. As with other canker diseases, the most conspicuous symptom is “flagging”–during the summer, individual stems wilt and die while leaves turn reddish and remain attached. Under severe disease conditions, several individual canes may be affected on a single bush. The fungus enters the flower buds and eventually moves into the stem. Infected stems will wilt and die, and young twigs will die back from elongated cankers produced by the fungus.
Cankers on one-year-old stems become obvious by early summer and continue to progress downward, eventually encircling the entire shoot. In hot weather, leaves on infected twigs turn brown and remain attached to the stem. As canes mature, they become girdled by the diseased lesions. Fruiting structures of the fungus will form on dead twigs and leaves. These fruiting structures look like small, black dots, which are the spore-containing bodies (pycnidia) of the fungus. These spores are spread primarily by rain splash. Infected fruit are soft and often split and leak juice. To aid control apply lime sulfur at bud-break and in the fall.
|Blueberry Bud Mite||Scientific Name: Acalitus vaccinii
All four life stages of the blueberry bud mite live together in large clusters and reproduce rapidly within the scales of the blueberry bud. 15 days are required from oviposition to the adult stage. During fall and early winter, all four life stages are present in low numbers between the preferred large and succulent scales of dormant flower buds. The mites remain almost continuously in the protective confines of the bud. Here, they feed on the epidermal region of the developing leaf, floral parts, and the developing fruit, transferring a substance or toxin, causing the tissue to become roughened and blistered in appearance. Buds appear rosetted and may desiccate due to mite feeding and fail to open. Flowers and berries developing from buds with few mites usually have small blisters and pimples. Summer generations cause retarded leaf growth, reducing the vegetative growth that impacts the following year’s crop.
Although some blueberry cultivars are less likely to be infested, there are no fully resistant cultivars identified. Recommendations for the control of blueberry bud mite are limited to pruning of older and removing of infected branches, and post-harvest application of miticides. Unless the interior spaces of the bud scales are wetted, it is unlikely good control will be achieved. Use of a surfactant to improve the spreading and penetration of the spray increases control of bud mites.