Wednesday, January 30, 2013

Prionus larvae in crowns

In October 2010 and again in January 2013 growers reported 'Duke' highbush blueberry fields with severe damage to roots and crowns caused by grubs feeding on the underground parts of the plants.  The grubs appear to be the larval stage of a species of Prionus beetle.

Larvae removed from the crown of a dying blueberry bush

Prionus feeding damage to the crown of a blueberry bush

The larvae reportedly take three to five years to mature, and may be more likely to occur on stressed bushes.  Above-ground symptoms are hard to distinguish from drought stress or nutrient deficiency.  Affected bushes are weakened progressively as the larvae grow and inflect more damage.

The only way to confirm the presence of these insects is to dig bushes and examine the roots and crown for damage or larvae.  As pictured, the larvae are larger than most other grubs found in the root zone,  are widest at the head end, with strong jaws, and with bodies that are elongated rather than the typical "C-shaped" grubs of other beetles.

Friday, January 11, 2013

Mummy Berry

Mummy berry is a plant disease caused the fungus Monilinia vaccinii-corymbosi.  The fungus overwinters on the ground, and in spring produces a cup-shaped mushroom that releases spores.  These spores infect and kill emerging leaf shoots, causing the shoot blight (primary) phase of the disease.  Blighted shoots then produce a second type of spore that is carried by insects to the flowers, where fruit (secondary) infection 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.  Shown below are a few photos of mummy berry at different times of the year:  

 The October photo below shows infected fruit in winter.  All berry tissue has sloughed away, leaving behind a hard, hollow structure consisting entirely of dormant fungal tissue.

Dormant mummies

In spring, dormant mummies germinate and produce tiny brown cup-shaped mushrooms called apothecia.  These mummy cups (shown below) contain millions of spores (ascospores) that spread and infect new leaf shoots.  This stage occurs in February and March in eastern NC.

Three apothecia emerging from a single infected berry

Spores from apothecia infect new emerging leaf shoots.  This primary or leaf shoot infection occurs in March as shoots begin to emerge.  Infected shoots continue to develop and elongate, then quickly wilt and turn brown, first along the midrib of one or more leaves, then dying completely.  Infected shoots produce masses of the second spore stage, visible as a gray ash-like mass on the wilted shoot.  Infected shoots are usually visible in late March and April in eastern NC.
Wilting and browning along leaf ribs   

Infected shoot with a gray mass of spores

Multiple infections can defoliate a bush

 Fruit infection occurs when spores from infected leaf shoots are carried by insects to open flowers.  Spores germinate and enter the flower through the pollination pathway and infect the developing ovules.  Infected green berries appear normal until harvest, then turn a pinkish-tan color rather than blue, and drop prematurely.
Cutaway showing white fungal growth inside the fruit
Infected and healthy 'Croatan' fruit at harvest

Infected berries (mummies) fall to the ground and unless removed or buried, will emerge the following to re-infect the bush.
Enlarge to see infected fruit underneath this 'Gupton' bush

Sunday, January 6, 2013

Pruning Blueberrries

Cultivated blueberries are upright, deciduous, woody perennials, forming multi-stemmed bushes with maximum unpruned heights varying from 6-8 feet (highbush, southern highbush) to 10-15 feet (rabbiteye). All cultivated species require annual pruning to manage bush height and shape. Pruning also prevents over-cropping, increases berry size, and removes dead, diseased or insect-infested wood.
Pruning is second only to hand harvest in terms of annual labor expense. So how is pruning accomplished quickly and economically, and what is the easiest way to explain pruning goals to a crew of workers entering the field for the first time? The following teachable steps, in order, can be used at each bush to rapidly eliminate undesirable growth, selecting for flexible, upright, and productive canes. 

Tools and Techniques

Most blueberry pruning is done during the dormant (winter) season after the leaves have fallen. Mature canes can be up to two inches in diameter, so long-handled loppers capable of cutting large stems are essential. Smaller one-handed pruners are used for finish work and for shaping young bushes. Make flush cuts to avoid leaving stubs. Pruning cuts are not treated, though some authorities recommend timing standard fungicide sprays to occur immediately after pruning, especially when late spring and summer cuts are made on actively-growing bushes. 

Steps in Winter Pruning (November – March) 
  Step One: Define the crown. Pruning starts at the ground, not at the top of the bush. Visualize a circle 12 to 18 inches in diameter around the crown of the bush, and remove ALL shoots of any age that have emerged from the ground outside the circle. This narrows the base of the bush to facilitate machine harvest, but is also a good general step for hand-harvested fields as well. 

Step Two: Remove low-angled canes and crossovers. Low-angled canes that are too close to the ground are undesirable because the fruit is more likely to contact the ground, or to be contaminated by rain-splashed soil. Remove these low-lying branches, and also any canes that angle through the bush (crossovers). What remains is a narrower bush consisting of the most upright canes. 

Step Three: Open the center. If needed, remove one to three large canes from the center of the bush to reduce crowding, improve air circulation and phase out older canes. Old canes to target for removal are larger and grayer in color, and are more likely to be covered with a fuzzy growth of foliose lichens. The goal should be to move through the field rapidly by making large cuts close to the ground. 

Step Four: Thinning and heading back. As a blueberry cane ages, it branches repeatedly, resulting in smaller and smaller diameter lateral twigs in successive years. If left unpruned, this results in excessive numbers of unproductive, matchstick-sized shoots, each with a few tiny berries. To avoid reaching this stage, thin canes by making cuts to selectively remove clumps of twiggy, brushy-looking, matchstick-sized laterals. At this time also cut (head back) any long whips or canes that are too tall. 

About Flower buds 
Yield reduction via flower bud removal always occurs when winter pruning is done properly. This is often a sore subject with growers who are trying to maximize yields. Flower buds are readily visible during winter pruning, and it is tempting to leave too many. This is a mistake! Expect to remove at least a third of the flower buds during pruning. Why? Because overloading the bush in one year will cause reduced yields in following years, and will eventually require even more severe pruning to bring the bush back into production. 


These basic hand-pruning steps can be used with any blueberry bush. Every cultivar has a slightly different growth habit, and only experience will tell you how to manage each. Some cultivars produce too many new shoots from the ground and require a lot of thinning, while others are less prone to sprouting. Your goal should be to have a multi-trunked bush with strong canes of all different ages emerging from the ground, so that as each older cane is removed, a younger cane is already there to replace it.