Scientists support southern producers

James Spiers, research leader and horticulturist, collects fruit from DeSoto, a new rabbiteye blueberry released by ARS breeders in Poplarville, Miss., while Geneticist Stephen Stringer selects propagation material from the plant.

Blueberries continue to offer good revenue for growers. As acreage increases in some portions of the south, the need for improved varieties and techniques emerges. Several research groups are developing better blueberry cultivars for the future, and tackling the threats to this lucrative crop.

Southern Small Fruit Research Consortium

The Southern Small Fruit Research Consortium (www.smallfruits.org) is a collaboration between several land-grant universities. By sharing the expertise of each institution, cost-effective projects to benefit growers are possible. Consortium grants support small fruit research, including several current blueberry projects.

One Consortium member, the University of Georgia, is investigating the use of soil fumigants and foliar-applied phosphates (common fumigants or a phosphonate material, ProPhyt) to overcome replant disorders in blueberries. As many plants in Georgia are about 20 years old, it’s time to replace them, but replant disorders may result. Fumigants have prevented symptoms such as slow growth and general decline in other plants.

Two fumigants, Telone C-35 and MBC-33, and the phosphonate fungicide ProPhyt were tested for suppression of replant disorder at the Alapaha [Ga.] Blueberry Research Center during 2007. Although the summer’s poor rainfall and high temperatures stressed the plants, fumigation did result in improved plant vigor and overall plant health. The ProPhyt didn’t appear to affect the crop. Scientists plan additional testing to determine if fumigation is an effective and cost-efficient technique for combating replant diseases.

Just as the extreme weather conditions affected the fumigant study, Mother Nature slowed several other Consortium projects. The University of Tennessee planned to investigate the effectiveness of applying acidifying agents, such as nitrogen-forming fertilizers, to soil for trickle irrigation versus traditional techniques. A study of ammonium sulfate was cancelled due to the April 2007 freeze and rescheduled for this year.

Other interesting projects to watch for include a University of Georgia study of mechanical harvesting. Although the technique is used widely in top blueberry states such as Maine, Georgia berries are primarily hand-picked. Georgia growers are expanding plantings of southern highbush varieties to capitalize on strong prices during the spring harvest period, but as acreage increases, prices may drop.

An additional wrinkle involves the scarcity of labor and high cost of hand-picking. It is estimated that mechanical harvesting would reduce the expense from about 55 cents per pound to less than 10 cents per pound. To pinpoint varieties that are firm enough to withstand mechanical picking, researchers planted several advanced selections last year. Harvesting will begin in 2009, with the entire study expected to run at least five years.

Geneticist Lisa Rowland, front, and Horticulturist Elizabeth Ogden collect blueberry plant leaf tissue for DNA analysis.

In addition to southern highlands, growers in the region produce rabbiteyes. As they ripen during the same time period as northern berries, the stiff competition results in lower prices. Thus, many are used for frozen products; when frozen, however, the skin becomes quite tough.

North Carolina State University is examining consumer acceptance of the frozen rabbiteyes. Berries harvested in 2007 were immediately frozen and are being subjected to taste tests at three-month intervals. Results may be announced later this year.

Agricultural Research Service

The Agricultural Research Service (www.ars.usda.gov) also plays a major role in enhancing blueberry crops. The Poplarville, Miss., center, for example, has helped establish the fruit as a viable crop in that region by developing early-ripening cultivars. Its latest offerings include two southern highbush varieties, Gupton and Dixieblue. Harvested in the lucrative window before other berries are available, both are well-suited to the Gulf Coast’s sandy soil. A new rabbiteye, DeSoto, reduces the need for top pruning, as the semi-dwarf bush’s mature height is about 2 meters. Its later blooming and harvest dates protect against spring freezes and enhance marketing opportunities late in the season.

New threat to southern berries

No group knows better than growers that with every positive development there is at least a bit of a downside. That’s the case with southern blueberries. Along with the improved cultivars and expanding growing region comes a new threat.

Over the last two years, University of Georgia scientists have uncovered a potential disaster for southern highbush growers in their state. Of course, the new disease, bacterial leaf scorch, may ultimately affect berries throughout the southeast.

The disease is caused by the bacterium Xylella fastidiosa. FL86-19 is the most susceptible variety, although Star and other varieties may be affected. Although study of the disease is still in its infancy, scientists postulate that its cycle may be similar to Xylella disorders found in peaches and grapes.

Bacterial leaf scorch is spread as insects eat affected plants in the spring and then come in contact with healthy plants. The glassy winged sharpshooter, common to south Georgia and Florida, may bear much of the blame by injecting bacteria into plant xylem, which transports water and nutrients from the roots to other parts of a plant. Ultimately, as the bacteria forms colonies, the xylem is too clogged to feed the plant. The plant begins to decline, generally with symptoms appearing one year, and death one or more years later. Bacterial leaf scorch also may spread when cuttings or root grafts are taken from affected plants for propagation.

The disease may appear as minor leaf burn similar to that caused by drought or fertilizer burn. Later, leaves drop and although the plant may seem healthy otherwise, the defining trait of new twigs turning yellow eventually occurs. Ultimately, the plant dies; often the neighboring plants show symptoms the following year.

Currently recommended cultural controls:

• propagating only from plants you have visually checked for disease;

• inspecting fields for diseased plants and destroying them immediately; and

• minimizing stress, as there may be a link between bacterial leaf scorch and drought stress.

As it seems that some varieties are more resistant than others, breeding resistant varieties may be possible in the future.

Chemical controls suggested by the University of Georgia include:

• controlling insects with chemicals, as there is no known product that kills the bacterium. Several insecticides are approved for blueberry kill leaf hoppers;

• using soil-applied neonicotinoid products as spring growth begins; and

• using foliar-applied pyrethroids and organophosphates in late spring, especially if glassy winged sharpshooters are detected.

Problem-solving the natural way

For Gulf Coast growers, the gall midge has been the Achilles’ heel. The pest has caused many in the region to abandon rabbiteyes.

Mississippi State University Entomologist Blair Sampson watches a Synopeas wasp insert eggs into a newly hatched gall midge larvae.

As larvae, gall midges feed on the blueberry plant’s buds, deforming them and endangering the fruition of up to 10 berries per bud. Entomologist Blair Simpson, working at the Poplarville lab, found that they have a natural enemy: immature wasps. Female wasps from the genera Synopeas, Inostemma and Platygaster sting midge larvae within blueberry buds. The wasps lay their eggs in the midges’ stomachs and brains. After the eggs develop into immature wasps, the insects battle each other for the right to feast on the hosting midge. Sampson speculates that a natural population of wasps in blueberry fields will kill 40 percent of midges present, controlling the pests for more than six months. His next step will be to investigate repopulating fields with wasps in areas in which prolonged insecticide use has wiped them out.

Although a southern state may never replace Michigan as the country’s top blueberry-producing state, Georgia is the leader for rabbiteyes and North Carolina is the fifth among highbush-producing states. With the support of consumers and solid scientific research, the crop is expected to remain a good choice for southern growers.

The author is a freelance writer based in Greensboro, N.C.