Exobasidium fruit and leaf spot

Symptoms caused by the fungus Exobasidium vaccinii have occurred sporadically in individual fields for years, but have not posed an industry-wide threat to blueberry production in North Carolina.  However, in 2011 we saw increased incidence of this disease, and symptoms on previously unaffected cultivars such as Legacy and Columbus.  The fungus causes spots on both leaves and berries. Spots on fruit are especially problematic because it is impossible for pickers to avoid harvesting affected berries, and nearly impossible for color-sorters and packing-line inspectors to remove them during the sorting and packing process.

Green spots caused by Exobasidium on 'Legacy', 1 June 2011

On berries, infection produces a green spot that fails to ripen normally. Affected berries do not leak or decay, but the green spot on an otherwise uniformly ripe berry is an unsightly defect that could lead buyers to reject the fruit when delivered.  On leaves, spots are pale green on the upper surface but pure white below, with a thin, dense layer of fungal growth on the underside of the leaf.  This fungal growth is most obvious on the underside of leaves, but can also occur on infected berries. 

Exobasidium leaf spot -- upper surface

Exobasidium leaf spot -- underside of leaves showing fungal growth

As spots on leaves age, they become brown and necrotic, although the white fungal layer is often still visible on the underside of the leaf.

Exobasidium on 'Legacy' leaves and fruit 7 Jun 2011

Eventually, the unique symptoms fade and the spots become indistinguishable from many other leaf-infecting fungi.

Symptoms on 'Legacy' upper leaf surface 20 Jul 2011

Symptoms on the lower leaf surface 20 Jul 2011

The fungus produces spores on both leaves and berries. As shown below, they are often a distinctive 'musiform' or banana shape, may be divided (septate), and measure roughly 4.0 to 5.2 μm wide × 13 to 15 μm in length.

Drawing of characteristic spores as seen under a microscope

 Little is known about the life cycle of this fungus on blueberry.  Infections appear in the spring on developing leaves and berries, but the fungus does not appear to infect later flushes of leaf growth.  Lesions have not been observed on other plant parts (stems, buds) and it is not known how this pathogen overwinters.  Visually, infections appear to be localized, distinct and limited to the affected berry or leaf, rather than systemic in the plant.  Since blueberries drop their leaves each winter,  there may be some quiescent infection stage on or in the remaining, bare dormant stems or buds that serves as the overwintering mechanism.

Fungicides have been shown to be at least partially effective in controlling this disease.  A study by David Ingram and John Braswell at Mississippi State University achieved measurable control on rabbiteye blueberries.  The combination of pyraclostrobin + boscalid (Pristine) was most effective in their tests.

Infections on the rabbiteye cultivar Premier, 13 Jun 2008

Why is this disease becoming more prevalent?  It may be due to changes in the cultivars we grow, the loss of key fungicides in recent years, or to changes in cultural practices such as the increased use of irrigation.  If you are experiencing problems with this disease, I would like to know about it.  It appears to be an emerging problem not only in North Carolina, but in other southern states as well.  The more we know about it, the sooner we will learn to manage it -- thanks for your help!

REFERENCES:
W.O. Cline, 1998. An Exobasidium disease of fruit and leaves of highbush blueberry.

Leaf Disease Symptoms

Leaf spot diseases caused by fungi are apparent in some fields this week.  Septoria leaf spot is a disease caused by the fungus Septoria albopunctata.  Infection results in small circular spots with dead gray centers. Often fungal fruiting bodies are visible as tiny black specks in the dead gray area at the center of each spot, as in the photo below. Spots first appear on oldest leaves near the ground. The cultivar Star is very susceptible

Septoria leaf spot on 'Star'

Another leaf symptom that is evident this week (21jul2011) is premature drop of yellowing older leaves.  The cause is unknown but is suspected to be a form of blueberry leaf rust.  Rust fungi cause yellowing and premature drop of leaves, and a characteristic symptom is the appearance of green spots on otherwise yellow leaves, consistent with the image below.  This symptom is especially noticeable on the southern highbush cultivar Legacy and on the rabbiteye Premier.  Rust diseases get their name from the appearance of orange spore masses on the underside of infected leaves, but so far we have not seen spore production on these yellowed leaves.  When viewed from the underside of the leaf, the symptoms resemble edema, so there may be some environmental stress, perhaps related to the drought, that is causing or contributing to the problem.

Yellowing and premature drop of 'Legacy' leaves on 20 Jul 2011

Leaf drop is a serious problem because it can severely reduce yield for the coming year. Twigs that defoliate prematurely will not set flower buds, and so will not bear fruit the following year.  There are several fungicides that can be used to control leaf diseases;  also, summer pruning immediately after harvest removes old infected leaves and forces clean new growth, greatly reducing the effects of leaf diseases.

New OSU budget for Oregon growers

Researchers James  Julian, Bernadine Strik, and Wei Yang at Oregon State University have developed a new budget (April 2011) for blueberry growers in their state entitled: "Blueberry Economics: The Costs of Establishing and Producing Blueberries in the Willamette Valley".  While North Carolina growers cannot use this budget "as is" due to the many differences in our establishment and production practices, this is an excellent budget and it is interesting to see their budget assumptions, costs, and especially conclusions regarding economy of scale.

Pruning Young Blueberry Plants

Young blueberry bushes are usually planted in late winter while fully dormant and leafless.  In North Carolina, this translates to February or early March. During the first year, flower buds are removed by pruning, or by stripping off flowers by hand after the blooms emerge.  In subsequent years, flower buds must be thinned to prevent overcropping and to promote the vegetative growth so vital to the establishment of a full-sized bush.

The diagrams below show growth of a single blueberry bush for the first three years, with "before" and "after" pruning comparisons each February.  The color red is used to represent one-year-old growth and dormant flower buds. Click on each image to enlarge.

 

 

Blueberry Necrotic Ring Blotch

Blueberry necrotic ring blotch symptoms were evident in some 'Star' fields visited this week.  The cause of this disorder is unknown, but a virus is suspected.  It seems likely that this disease was introduced, and is spreading, via propagation from infected plants.

Symptoms are round to irregular, brown to black spots or blotches.  Unlike the rings caused by blueberry red ringspot virus, these rings are visible on both the upper and lower surfaces of the leaf, as shown above.  As with other leaf diseases, affected bushes tend to drop leaves earlier in the fall of the year, and this can translate to fewer flower buds the following year.

Symptoms usually appear on the oldest leaves first, with younger leaves progressively less affected. These leaf spots might be confused with fungal leaf diseases, but unlike fungal spots that have dead centers, most of these spots remain green in the center.

There are no control measures known for this disease, although some cultivars are more affected than others. Cultivars most affected include V-1, Star and O'Neal. The disease is present in Florida, Georgia, North Carolina and South Carolina.  The vector, if any, is unknown.

Growers should be advised not to propagate from visibly infected bushes, and to buy plants from tissue culture or other sources known to be free of symptoms.

Blueberry cultivar ‘NEW HANOVER’

Released: 2007 (Patent date 12 May 2009)

Developed by:  Jim Ballington and Susan Rooks, NCSU 

Selection Number: NC 3103

Parentage: NC 1522 × 'O’Neal'

Species/type: Vaccinium corymbosum (Southern highbush blueberry)

Bloom Date: April 1 (or approximately 1 week before ‘Croatan’)*

Harvest Season: Begins May 25 (3-4 days later than ‘Croatan’)* 

  

Chill Requirement: Unknown; likely 800+ hours based on observations of leafing at the NCSU Ideal Tract farm in Castle Hayne.

Description: Moderately upright, sturdy, vigorous bush, flowers white, new leaf growth with reddish-orange tinge. Self-fruitful and very high yielding. Developed for hand harvest. Large, light blue fruit.

Good Characteristics: Excellent flavor, large, light blue fruit. Vigorous, very high-yielding and early ripening; easily propagated from cuttings.

Flaws: Low spreading branches require aggressive pruning as a young bush to force upright growth. Tearing of the berry skin around the stem scar at harvest (torn scars) has been observed on as much as 5% of the fruit. This tendency is worse on young bushes and when berries are overripe. 

Potential: Released for hand harvest, not considered suitable for machine harvest due to wide growth habit, rigid, non-bending canes and large berry size. Though slightly later ripening, 'New Hanover' is recommended as a replacement for 'Croatan' on lowland blueberry soils in the southeastern coastal plain of NC.  Not currently recommended for upland, piedmont or mountain sites.

*Dates are for southeastern NC

Scouting fields for blueberry red ringspot virus

Red rings on stems are diagnostic for BRRV

Blueberry Red Ringspot Virus (BRRV) is hard to see during most of the year, but becomes most visible in late summer and early fall. Symptoms include red rings on both stems and leaves. Circular blotches or pale spots may also be visible on ripening fruit, though yield is often not affected. The cultivar Ozarkblue in particular can have severely distorted berries.

Unlike fungal leaf spots, the rings caused by BRRV have green centers or pale centers, and usually do not show through to the underside of the leaf. Rabbiteye cultivars appear to be resistant to the virus, while highbush and southern highbush cultivars are more susceptible. Plants are infected for life, and cuttings taken from infected plants will also have the disease – so it is very important to scout fields that will be used for cuttings, and avoid or remove any suspicious plants before they are propagated. The vector (carrier) is not known, so the only control for this disease is to look for it in July-Sept and isolate or remove any infected bushes.

Spots on leaves are usually only visible on the upper surface. spots are red rings with green centers, unlike fungal leaf spots that have dead centers.

Infected plants may exhibit faint rings on ripening fruit that disappear when the fruit is fully blue.

Notes on softwood propagation

Blueberries are propagated by vegetative cuttings (not by seeds). Most blueberry cultivars are easily rooted under mist from either hardwood cuttings (collected in the dormant season and stuck in April), or from leafy softwood cuttings taken in summer.  The time for taking softwood cuttings will be here soon (around August 1).  Here are a few tips for increasing your rooting success and avoiding disease problems.

Softwood cutting ready for sticking in the rooting bed

Design a well-drained rooting bed – Failure of cuttings to root is often due to poor drainage rather than to disease.  Rooting beds, especially open-air beds exposed to rainfall, must have nearly unlimited drainage capacity.  This requires a coarse, open rooting medium (ground pine bark is preferred) over a well-drained base.  Build beds atop an 18-inch layer of coarse sand, with drain lines (sock pipe or tile) to ensure rapid movement of water through the bed.

A small outdoor rooting bed with a sand base for improved drainage.  Note windbreak!

Water quality – Before going to the expense of building a propagation bed, have your water tested by the North Carolina Department of Agriculture make sure it is of adequate quality for rooting cuttings.  Rooting is generally good with water from shallow wells in the coastal plain, even though some wells in southeastern NC have excessive levels of bicarbonates or iron that accumulate as a residue on leaves.  Some of the best wells for propagation have been deeper wells in Bladen and surrounding counties;  however, Sodium can be a problem in coastal counties, where saltwater intrusion has been observed in wells with depths of 160 to 200 ft.  On sites where saltwater intrusion is a problem, it may be necessary to use only shallow well water (from wells 50 to 60 ft deep) for propagation.

A large commercial rooting bed with pin-type mist nozzles

Bed design and rooting substrate – Two-by-eight-inch boards are used to construct beds of various lengths, 4 feet wide and 8 inches deep, filled with milled pine bark.  Wider beds are not recommended because they are difficult to reach across for sticking cuttings or for weeding.    Do not line the beds with plastic or weed barrier fabric, as this will inhibit drainage – if a bottom lining is needed, use 1/4-inch mesh screen wire (galvanized hardware cloth).   The pine bark rooting substrate should be in direct contact with the underlying sand.  One common mistake is re-using old bark.  Do not re-use old rooting media!  It is infested with fungal pathogens that will carry over and cause disease in the next batch of cuttings.

Scouting for disease-free cuttings -- Before taking cuttings, walk the field looking for disease symptoms, and avoid taking cuttings from infected bushes.  Blueberry red ringspot virus, Blueberry stunt, and Necrotic ring blotch virus are transmitted via propagation.  These diseases are visible in August when softwood cuttings are taken, so be sure to look for and avoid them.  Blueberry stem canker is a fungal disease that can be transmitted via infected hardwood, and to a lesser extent, by softwood cuttings.  Canker-infested fields can be identified visually and should also be avoided.  


Trueness-to-type – unintended cultivar mixing within a field can cause serious problems, especially when mixed-up cultivars have different ripening seasons.  While scouting for diseases, watch for and exclude any mixed or mislabeled rows.

Patented cultivars -- Propagation of patented cultivars without a license is illegal.  Patented cultivars include Star, Rebel, Emerald and many newer cultivars.  Non-patented cultivars include O’Neal, Legacy, Duke, Reveille and most older cultivars.

Collecting and handling – Softwood cuttings are usually collected around August 1 in southeastern NC, but optimal timing may vary by a week or two depending on growth stage, cultivar and location.  Cuttings should be five to seven inches long, leafy semi-hardened new growth, not fully mature but firm enough to be pushed into the rooting media without breaking.


Softwood cuttings are best collected in early morning and can be cut with pruning shears or broken by hand. If present, tender, succulent shoot tips can be pinched off to avoid wilting and decay.  Lower leaves are stripped off by hand, keeping the upper three to four leaves.  While collecting cuttings, do not allow them to wilt – sprinkle with ice water and bag cuttings in coolers in the field as soon as they are gathered.  Stick softwood cuttings about two-thirds of their length into the rooting media, and begin misting immediately.  Cuttings are usually spaced 1.5 to 2 inches apart in the rooting bed.

Softwood cuttings under mist

Mist timing and monitoring – The method used for watering cuttings during rooting is known as “intermittent mist propagation”.  Cuttings are misted for four to five seconds at a time, with the cycle repeating every five to ten minutes, using solenoid valves, timers and horticultural mist nozzles.  Daily misting in the summer begins around 9 AM and ends an hour or two before sunset.  Misting is not needed at night.  After the cuttings have been stuck in the beds, water management becomes critical.  The goal is to keep the cuttings from drying out, while at the same time not saturating the rooting medium.  If you can pick up a handful of rooting medium and squeeze more than a drop or two of water from it, it is too wet!

Water management for hardwood cuttings -- Misting of hardwood cuttings begins with leaf budbreak in April, and continues until cuttings are rooted in July.  After rooting, water is gradually reduced for the remainder of the summer to prevent drowning and avoid disease problems.  Hardwood cuttings are generally more forgiving of different watering regimes, and misting/watering protocols vary widely between growers.

Pin-type mist nozzles are easily maintained

Water management for softwood cuttings -- Misting of softwood cuttings begins immediately when the cuttings are stuck and continues for six to eight weeks until the cuttings are rooted, at which point watering can be gradually reduced. Leafy cuttings are very sensitive to drying -- during the first six to eight weeks, failure of the mist system for more than an hour or two on a hot day will result in complete loss of the cuttings.

Extending the propagation season -- some growers erect hoop supports and fully enclose late-summer softwood rooting beds with white plastic covers.  This high-humidity enclosure provides some shading, allows rooting without continuous use of intermittent mist, and keeps the beds warmer so that root development continues later in the fall.  Another modification is to use a greenhouse or high tunnel to exclude rainfall – this allows the use of less well-drained rooting media, and again extends the time of year during which cuttings will root.  Caution:  Covered or enclosed propagation beds can become too hot and “cook” the cuttings – proper timing and constant monitoring with careful use of shade and ventilation will be needed.

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. 

(This article previously published in the NC Blueberry News)