Botrytis Flower Blight

Botrytis flower blight and fruit rot caused by the fungus Botrytis cinerea is likely to be a problem this year in NC.  The disease can become severe when freeze injury to flowers is followed by cool, wet weather.  Over the past month we have seen repeated freeze damage to emerging blooms so the risk of flower blight is already high due to the amount of injured tissue.  The fungus infects through damaged flowers, may kill the entire fruiting twig, and can also cause a fruit rot at harvest.

Freeze-injured flowers are predisposed to flower blight

Masses of spores are visible on the infected flower bud at center

Post-harvest Botrytis fruit rot

 
The best control measure for this disease on blueberry is warm, dry weather!  With dry, sunny days any freeze-damaged flowers will quickly dry out and fall off.  However if cool wet weather is anticipated, the use of a fungicide may be warranted, and there are a number of labeled fungicides for control of  this disease.  If you are already spraying fungicides for mummy berry, this might be a good year to include a product that has activity against flower blight.  Recommended products can be found at smallfruits.org in the Blueberry IPM Guide.  Page 31 lists the relative effectiveness of fungicides against this disease.  Some good choices would be CaptEvate, Elevate, Pristine or Switch.  As always, read and follow the label.

Application for export to Canada

Just a reminder to commercial blueberry growers in NC who ship to Canada that March 1 is the deadline for participation in the state Blueberry Certification Program.  The blueberry maggot fly (Rhagoletis mendax) is a sporadically-occurring pest of blueberry in North Carolina, and shipment of blueberries to Canada is limited to fruit from fields enrolled in a state certification program administered by the NCDA&CS.  For more information contact Dr. Alonso Suazo, (919) 733 6931, ext.237, or email Alonso.Suazo@ncagr.gov

Blueberry Irrigation Methods

On most sites, irrigation is essential for survival of newly planted bushes, and for consistent growth and yield throughout the life of the planting. The types of irrigation  most often used on blueberry  in North Carolina are:

1.  Overhead -- impact sprinklers on 6 ft risers (12 sprinklers per acre) are used for both drought relief and for frost/freeze protection in the spring.  These systems use about 68 gallons per minute per acre (4080 gallons per hour on a 60 x 60 ft spacing) and usually exceed the continuous capacity of available wells,  so water is first pumped into a reservoir (pond), and the irrigation system pumps from the pond.  This is the most common system used on large farms in the coastal plain, where a single pond/pump setup may cover 20-25 acres.

Overhead irrigation can be used for drought relief or for freeze protection

2.  Micro-sprinklers -- Not very commonly used, these are small spray heads on stakes about 12 inches above the ground that wet a 3-ft to 5-ft circle around each bush.  Coverage is better than with drip lines, but like drip, micros cannot be used for frost/freeze protection.  Some growers who have tried micro-sprinklers have since replaced them with drip sytems, because micro-sprinklers are easily damaged by equipment or by farm workers stepping on them during harvest.

3.  Drip line (built-in emitters) -- a heavy flexible line with built-in, pressure-compensated emitters every 18 inches.  Output is usually about 0.5 gph per emitter, or 24 gallons per minute per acre (1452 gallons per hour per acre).  Different emitter spacings are available, and double lines can be used to increase the wetted area.  Water quality has to be good or the tiny emitters will become clogged with sand/grit or precipitated iron and carbonates.  Drip irrigation is the most commonly used system in the piedmont of NC.

Drip line with built-in emitters every 18 inches

4.  Drip line (punch-in emitters).  Similar to above, except the emitters are not built into the drip line.  Emitters must be inserted by hand using a special tool to punch holes in the drip line.  This system can be customized by only putting emitters at the plant, or by adding more emitters at a later date.

Drip line with punch-in emitters placed at every plant

5.  Lay-flat drip tape -- lightweight drip tapes are not recommended..  Most lay-flat tapes are less durable and can only be expected to last one or two seasons.  Occasionally used for temporary systems,  prior to installation of the permanent drip or overhead system.


Other methods of getting water to blueberries have included flood irrigation, water table management (by holding water in ditches around the field), and (in western NC) piping or channeling water by gravity from a spring or well further uphill.

Chill Hour Requirements

Most blueberries require a certain amount of cold weather during winter in order to leaf and flower normally the following spring.  Generally chill hours refers to hours below 45 degrees Fahrenheit, however for blueberry chill hour calculations in North Carolina we use a model developed by Dr. Mike Mainland to account for both (i) partial chilling at moderate temperatures and (ii) the negation of chill that occurs with warm temperatures in winter.  

The Mainland Model accumulates 1 point for average hourly temperatures below 45° F; 0.5 points for temperatures 45-55° F; -0.25 points from 55-65° F; -1 point for temperatures 65° F and above. The model begins each fall when there is a positive balance that is not negated by warmer weather. Once 800+ chill hours have accumulated, points are no longer subtracted for temperatures 55° and above. The model ends February 28 at midnight.

Chill requirements vary a lot by blueberry cultivar and species.  Gerard Krewer and Scott NeSmith at UGA wrote a nice article summarizing chilling hour requirements for cultivars in GA, many of which are also grown in NC.  For cultivars grown in the southeastern US, approximate ranges (chill hour requirements) would be something like this:

0 to 250 hours = Very low-chill southern highbush cultivars for Florida and other low-chill areas (Emerald, Snowchaser and others)
250 to 400 hours = Moderate low-chill southern highbush cultivars for southeastern NC, SC and GA (Star, Rebel and others)
350  to 800 hours = Rabbiteye cultivars for most of NC, such as Premier, Powderblue, Tifblue and others
400 to 900 hours = higher-chill southern highbush cultivars (Legacy, Reveille, O'Neal and others)
900 to 1200 hours = Most Northern highbush cultivars (Duke, Jersey, Bluecrop and others)

Some cultivars have different chill requirements for leaves compared to flowers.  O'Neal flower buds break dormancy readily after 400 hours or so, but the leaf buds do not seem adequately chilled until about 700 hours are reached.

Chill hours vary tremendously depending on where you are in NC.  To see current chill hour accumulation at a site near you, go to the excellent blueberry chill calculator hosted by the State Climate Office. For instance, today (18 Feb 2012) accumulated chill  hours at a mountain location (Asheville) are1578 hours, compared to only 772 in the coastal plain (Elizabethtown).  At these levels, blueberries in westen NC should leaf and flower normally, while in the coastal plain, higher-chill cultivars like Duke, Jersey or Reveille will experience reduced or delayed flower development.