Friday, July 8, 2011

My Blueberries Look Funny...

Written by Bill Cline

In July, blueberries are ripening all across the state.  These excellent little berries can be grown in your back yard, but you may not know that they are also our most valuable commercial fresh fruit crop.  Growers in southeastern NC harvest hundreds of acres beginning in May each year, and each year the clinic receives diagnostic samples of berries affected by diseases, insects and weather.  Here are a few of the problems we have diagnosed recently:

Mummy Berry
Mummy berry is caused the fungus Monilinia vaccinii-corymbosi.  This fungus overwinters on the ground underneath blueberry bushes, and in spring produces a cup-shaped mushroom that releases spores.  These spores infect and kill emerging leaf shoots, causing the primary, or shoot blight phase, of the disease.  These blighted shoots then produce a second type of spore that is carried by insects to the flowers, where the secondary, or fruit infection stage takes place.  The end result is “mummies” – berries that do not turn blue, but instead turn pink or salmon-colored and fall to the ground.  Mummy berry can be controlled with fungicides, through the use of resistant cultivars, by removal of infected fruit, and by mulching to bury any mummies remaining on the ground.
Mummy Berries: Infected berries are pink (Photo by Bill Cline) 
Fruit rots
Fruit rotting fungi cause soft, leaky or “fuzzy” berries (visible spores or mold). Blueberries are more resistant to decay than other small fruits, but still need to be harvested in a timely fashion to avoid fruit rots.  Berries do not ripen all at once, but are picked over several weeks.  Highbush blueberries should be harvested every four or five days, while rabbiteye blueberries are harvested every seven to ten days (rabbiteye blueberries are the most common backyard species in North Carolina).  It is important to “clean” the bush at each harvest date by removing all blue fruit, to avoid having overripe, leaky berries in later pickings.  If the bushes are wet with dew or rain, wait until they dry before harvesting, since handling wet berries greatly increases post-harvest decay.  
Blueberry Fruit Rot (Photo by Bill Cline)
Exobasidium vaccinii is a fungus that causes a “green spot” symptom on blueberry fruit and can also infect leaves.  This disease is becoming increasingly common in commercial fields and has also been diagnosed on blueberries in other states.  Little is known about this fungus on blueberry, and no control measures have been established.  The disease occurs sporadically and seems to be associated with high humidity -- low or sheltered parts of fields where air movement is limited.
Berries infected with Exobasidim (Photo by Bill Cline)
Insects in berries
Blueberries are sometimes infested with larval stages of insects, including blueberry maggot fly, cranberry fruit worm, cherry fruit worm, Plum curculio and a new pest, spotted-wing drosophila.  All these pests are controllable, but must be properly diagnosed since control measures vary.  Cherry fruitworm and cranberry fruitworm are frequently encountered in unsprayed fields. The cranberry fruit worm produces “frass” that looks like a small clump of sawdust on the outside of a cluster of berries, while cherry fruitworm usually affects only two berries, “gluing” the two together and producing frass only inside the fruit.
Cranberry fruitworm frass (looks like sawdust) (Photo by Bill Cline)
Hail damage
There has been quite a lot of hail this year resulting in damage to blueberry fruit.  Affected berries are usually damaged in spring while the fruit is still green.  Initially only bruised or dented, hail-damaged berries develop scars and cracks as they swell and ripen, and these symptoms can be confused with disease or insect damage.  Where hail damage is suspected, look for tattered leaves as further evidence of a hail event.
Hail damage on blueberries (Photo by Bill Cline)