Blueberry Stem Canker

Blueberry stem canker caused by the fungus Botryosphaeria corticis is common on cultivated and wild blueberries in North Carolina.  The disease spreads by spores and through the use of infected cutting wood for propagation.  Canker is important because it can weaken and kill susceptible or stressed bushes.

'O'Neal'  canes dying out due to canker infections

Symptoms: Cankers are noticeably thicker than adjacent portions of the stem, forming raised areas with deep cracks.  Usually only short sections of the stem are affected, though canes of some very susceptible cultivars can have multiple infection points along their entire length.

Swollen area (at center) with deep cracks is a fully developed canker

Disease cycle:  Infection of stems is by spores (ascospores and conidia) that are released during wet weather and disseminated by wind from April through September.  Young, succulent, actively growing shoots are infected, and symptoms appear within 4 to 6 months after infection.  As the fungus continues to grow and invade the wood, large cankers with deep fissures and cracks develop that girdle, weaken and kill the stem.

Cankers on a young stem, exuding spores

Control:  Fungicides are partially effective but not practical. Control relies on maintaining general plant health,
pruning to remove canker-weakened canes,use of resistant cultivars, and use of disease-free planting stock.  Cutting wood should be selected from undiseased plants, or the plants sourced from disease-free nurseries.  In areas where canker is not present, the use of disease-free planting stock is critical to avoid introducing this pathogen to an new field.

Highbush blueberry:  Among the popular southern highbush cultivars (Vaccinium corymbosum) grown in eastern NC, O'Neal, Blueridge and Legacy are susceptible -- however all three are successfully grown where plant health is maintained via proper site selection, fertility, pruning and irrigation.

Cankers on a one-year-old 'Legacy' stem

Rabbiteye blueberry: Cankers can be numerous on rabbiteye blueberries (Vaccinium virgatum, synonym V. ashei) but generally do not cause yield loss on this vigorous blueberry species.

Collecting softwood cuttings

As discussed in a previous post, blueberries can be easily propagated in summer from leafy softwood cuttings by using an intermittent mist system to keep the cuttings from drying out during the six- to eight-week rooting period.  However, a specific stage of shoot growth must be selected for cuttings, because not all leafy shoots will root -- those that are too old will tend to form flower buds rather than roots, and cuttings from shoots that are too young and succulent will wilt before rooting can occur. The photos below illustrate when and how to collect and stick softwood cuttings.

Timing:  Collect cuttings when shoots are 10-12 inches long and semi-hardened.  In southeastern NC,  this occurs in late May and again in late July or early August:

Blueberry bush in early August with suitable shoots for propagation

Cuttings can be snapped off by hand or clipped with pruning shears

Pinch off the top of the cutting and remove the lower leaves

Age of cuttings:  Select cuttings with leaves that are intermediate in color between the darkest older leaves and the palest young shoots.  Softwood cuttings should be immature but not succulent.  Cuttings that snap easily when bent are too young -- the cuttings should be bendable and becoming woody.

Leafy shoots from oldest to youngest.  The two in the center are suitable for propagation.

Softwood cuttings of the proper stage are bendable but do not snap

Shoots that snap easily when bent are too succulent for outdoor propagation beds

Handling:  Collect cuttings early in the morning if possible, to minimize wilting.  Once collected, cuttings are susceptible to wilting and must be kept moist and cool. Sprinkle with water and store in a cooler with ice until ready to stick the cuttings in the rooting bed.  Caution -- direct contact with too much ice can damage cuttings -- keep them cool but not icy.

Take a cooler and water to the field to keep cuttings from wilting

Sticking cuttings:  Cuttings are stuck into six- to eight-inch deep rooting beds filled with aged milled pine bark. Stick the cuttings 1.5 to 2 inches apart in rows by cultivar.  Approximately half the length of the cutting should be in the bark, with only the leafy section above the bark. The mist system should be running during this process so that the cuttings are never allowed to dry out.

A measuring stick is used to space the cuttings evenly across the bed.

Cuttings are stuck approximately half their lengh in the bark

A commercial rooting bed partially filled with cuttings

Cuttings typically root within six to eight weeks.  Once fully dormant, the cuttings can be dug and either potted, or planted directly into the field.  Most nurseries and commercial growers pot rooted cuttings and grow them out for a year prior to selling or setting in the field.

For further reading: 
Krewer, G. K. and W. O.Cline. 2003. Blueberry Propagation Suggestions.
Bilderback, T. E., R. E. Bir and M. A. Powell 1993. A Simple Intermittent Mist System for Propagation.

Blueberry cultivar 'LEGACY'

Released: 1993 (not patented)

Developed by: USDA and NJAES, Rutgers University

Selection number: G-290

Parentage: 'Elizabeth' x US75 (Vaccinium darrowi,'Florida 4B', x 'Bluecrop')

Species/Type: Vaccinium corymbosum (southern highbush)

Bloom date: April 1 in southeastern NC (approx 1 wk before 'Croatan')

Harvest season: First harvest around May 30 - June 3 in southeastern NC

Chill requirement: Estimated to be 700 to 800 hours below 45F

Description: Upright, open habit with flexible canes. Highly vigorous, forming an abundance of flower buds on both longer canes and lateral fruiting twigs.  Often a few leaves from the previous season will overwinter, persisting until after bloom the following spring. Early-blooming with pure white,elongate flowers.  Fruit light blue, firm, with excellent flavor; large berries becoming medium in size with successive harvests, small dry stem scar.  Clusters are open and well suited to mechanical harvest.  Berries maintain their quality even when slightly overripe, and are less prone to decay than other highbush cultivars.

Good Characteristics:  Blooms exhibit cold hardiness during spring freezes. 'Legacy' is highly productive with high-quality fruit that can be hand- or machine-harvested for the fresh market.  Widely adapted to different soil types, 'Legacy' is a good candidate for highbush production on pine-bark-amended soils in the piedmont.

Flaws:  Early blooming may result in yield loss due to spring freezes.  'Legacy' ripens two to three weeks later than early cultivars like 'Star' and 'O'Neal'. 'Legacy' is susceptible to blueberry stem canker disease caused by the fungus Botryosphaeria corticis, and will require irrigation and selective pruning to manage this disease.  Purchase of disease-free propagation stock will reduce or delay the effects of stem canker.

Potential:  Rapidly becoming a leading cultivar in North Carolina due to high yields and excellent fruit quality, 'Legacy' is recommended for commercial planting on lowland blueberry soils in southeastern NC, and for trial plantings in amended upland soils in the coastal plain and piedmont.

Wind burn

Wind burn occurs when high winds coincide with the emergence of new, succulent leaf shoots.  The symptoms can be mistaken for insect injury, disease, drought or chemical injury.  Characteristics of wind burn are new shoots that first appear scorched and wilted, then become dried, dead and shriveled.  Older leaves with wind damage have necrotic areas and cracks at the leaf margins.  Symptoms appear immediately on new shoots in high winds.  The first photo below was taken on the day that injury occurred, while the wind was still blowing.  The second photo shows what the injury looked like one month later.

Browning of new shoots can occur very rapidly on a windy day

Wind burn one month after the injury occurred