Pocket Scouting Guide revised in 2015

The popular Extension publication A Pocket Guide to IPM Scouting in Highbush Blueberries has been revised (April 2015) and is again available from Michigan State University. This third edition includes information on new pests that have emerged since the previous revision in 2008, most notably Spotted Wing Drosophila.The cost is $19.00 each, with discounts for volume purchases. For more information, see: http://shop.msu.edu/product_p/bulletin-e2928.htm

 

Mummy berry emerging this week

Mummy berry is now active in southeastern NC -- the photos below were taken yesterday.  The disease begins each spring with the emergence of the fungus from last year's infected fruit (mummies) that have overwintered on the ground.  The first sign of activity (and potential for infection) is the appearance of apothecia or "mummy cups" in spring.  Spores from these small brown cup-shaped mushrooms infect emerging blueberry shoots.  Yesterday at the NCSU Horticultural Crops Research Station in Castle Hayne, the spore-producing (infective) stage of this disease was visible on the ground underneath bushes in wet areas of some fields (click on images to enlarge):

Apothecium at right center of photo is less than an inch in height

Growth stage of bushes in the wet area where apothecia were found

Several mummy cups (apothecia) emerging from old infected berries

Mummies germinating through leaf litter 4 March 2013

The emergence of this fungus coincides with emergence of new shoots on blueberry bushes, so the disease will appear later in colder areas of the state where bushes are still dormant.  Fields in southeastern NC with a history of this disease should be sprayed with fungicides as soon as possible, especially in early-blooming cultivars where flower and leaf shoots are beginning to emerge. For further descriptions, and images of other stages of this disease, click here.  For specific recommendations on control, go to the NC Ag Chem Manual or the Southern Region IPM Guide.

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:  

DORMANT

 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

SPRING EMERGENCE

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

PRIMARY (LEAF) INFECTION

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

SECONDARY(FRUIT) INFECTION

 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

 

AFTER 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

 

Mummy Berry on 'Brightwell'

Mummy berry disease was widely reported during the 2012 season in North Carolina, including the first really severe outbreak of the fruit infection phase on 'Brightwell' rabbiteye blueberry in the coastal plain.  Usually the primary phase of the mummy berry disease causes severe leaf infections on rabbiteyes, but little or no secondary infection of fruit, at least in the coastal plain (rabbiteye fruit infection is more common in western NC).  NCSU research specialist Benny Bloodworth took these pictures in late June in Bladen County.

Infected berries (at center) turn pinkish-white, shrivel and drop

Abundant mummies on the ground under heavily infected 'Brightwell'

Mummies that fall to the ground are filled with fungal tissue and serve as the overwintering structures for the fungus.  In spring, the infected berries (mummies) on the ground produce a specialized cup mushroom that releases spores, and the disease cycle begins again.  Destroying or burying the mummies helps to reduce disease carryover from one year to the next.

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.

Small Fruit Viruses

Dr. Ioannis Tzanetakis with the University of Arkansas has written an excellent article illustrating the status and implications of emerging small fruit virus diseases in the US.  This APSnet post is one among many fascinating on-line articles published by the American Phytopathological Society (APS).

Evaluating leafspot effects

Late September is a good time of year to evaluate the effects of leaf spot diseases.  If most of your leaves are still attached to the bush at this time of year, then most likely flower bud formation has occurred and the crop potential for next year is good.  If, however, your leaves have already detached by the end of September, then chances are that some flower buds did not form due to premature defoliation, and crop potential for next year has already been reduced.

The image below shows three twigs from the cultivar Star, collected on 30 Sep 2011.  Star is susceptible to Septoria leaf spot caused by the fungus Septoria albopunctata. These are older twigs from the first flush of leafy growth in spring, and by now should have formed visible flower buds at the base of each leaf.

The shoot on the left is from a bush that was protected with a fungicide, retained its leaves, and has formed 5-6 flower buds.  The unprotected shoot in the middle defoliated early, and by comparison formed only a couple of flower buds at the tip.  The shoot on the far right was also defoliated early, not by leaf spots but by hurricane Irene.  It too, has only one or two flower buds.  Note that the shoot defoliated by hurricane winds still has the leaf petioles (bases) attached -- the leaves did not abscise normally, but were torn away:  This also happens (leaves missing, but petioles still attached) with caterpillar feeding damage. 

Regardless of the cause, when the leaves are taken away too early, flower buds do not form, and the potential crop is reduced for the following year -- because fewer flower buds means fewer berries.

The two main fungal leaf spot diseases that cause defoliation on blueberries in North Carolina are Septoria leaf spot caused by Septoria albopunctata, and Anthracnose leaf spot, caused by Gloeosporium minus. The two diseases have very different symptoms, as shown below:

Anthracnose leaf spot (Gloeosporium minus)

Anthracnose leaf spot, shown above, consists of large lesions that often occur at the edge of the leaf. Examples of susceptible cultivars are Pender (shown) and Duke.

 

 

 

Septoria leaf spot (Septoria albopunctata)

 Septoria leaf spot, by comparison, produces dozens of small lesions on each leaf.  The southern highbush cultivar Star (shown) is very susceptible.

By evaluating leaf drop at this time each year, growers can get a good idea of how much disease pressure they have on their farm, whether fungicide use is warranted, and which cultivars are susceptible.

Cylindrocladium blight

Cylindrocladium blight is a disease caused by a plant pathogenic fungus.  Infection results in leaf spots, brown, dead stems, and often dead plants.  On blueberry it is a disease of propagation and nursery beds, and is rarely seen on plants in the field. Symptoms commonly appear in blueberry rooting beds in the late summer and into fall. The disease may also occur in potted plants that are spaced too close together in pot yards.  One species of this particular fungus also infects peanut plants in the field, so blueberry propagators in North Carolina often call the disease "peanut blight".  There are many species of Cylindrocladium that infect plants, and there may be more than one species of this fungus causing disease in blueberry rooting beds.  At present the only species reported on blueberry in North Carolina is now considered to be Cylindrocladium parasiticum, hence the common name of the disease, Cylindrocladium blight.

Cylindrocladium blight in a bed of softwood cuttings in late fall.

Closeup of orange fungal fruiting bodies on an infected cutting

Irregular-shaped spots on lower leaves of older rooted cuttings

The disease is most often seen in propagation beds and pot yards that have grown vigorously and are too dense to allow proper air movement between plants.  Spores of the fungus infect leaves and stems, and eventually kill entire plants.  Often the root system is the last healthy part remaining.  The phenomenon of plants dying in the center of a dense block, while those at the edge (with better air circulation) survive, is characteristic of this disease in dense beds of plants.

 

Large area of dead plants in a propagation bed late in the season

In pot yards and late-stage propagation beds where the plants have already formed roots, this disease is best managed by not over-watering or over-fertilizing plants, and by maintaining proper plant spacing so that the leaves and stems can dry between waterings.  Cull to remove and discard any dead plants, and space the remaining plants out as much as possible so that there is good air movement between the bushes.  Do not re-use any old used potting media, as this disease can carry over from one year to the next in contaminated soil or potting mix.  Clean up as much debris as possible and sanitize the bed area and any pots that you plan to re-use.

Blueberry Stem Canker

Blueberry stem canker caused by the fungus Botryosphaeria corticis is common on cultivated and wild blueberries in North Carolina.  The disease spreads by spores and through the use of infected cutting wood for propagation.  Canker is important because it can weaken and kill susceptible or stressed bushes.

'O'Neal'  canes dying out due to canker infections

Symptoms: Cankers are noticeably thicker than adjacent portions of the stem, forming raised areas with deep cracks.  Usually only short sections of the stem are affected, though canes of some very susceptible cultivars can have multiple infection points along their entire length.

Swollen area (at center) with deep cracks is a fully developed canker

Disease cycle:  Infection of stems is by spores (ascospores and conidia) that are released during wet weather and disseminated by wind from April through September.  Young, succulent, actively growing shoots are infected, and symptoms appear within 4 to 6 months after infection.  As the fungus continues to grow and invade the wood, large cankers with deep fissures and cracks develop that girdle, weaken and kill the stem.

Cankers on a young stem, exuding spores

Control:  Fungicides are partially effective but not practical. Control relies on maintaining general plant health,
pruning to remove canker-weakened canes,use of resistant cultivars, and use of disease-free planting stock.  Cutting wood should be selected from undiseased plants, or the plants sourced from disease-free nurseries.  In areas where canker is not present, the use of disease-free planting stock is critical to avoid introducing this pathogen to an new field.

Highbush blueberry:  Among the popular southern highbush cultivars (Vaccinium corymbosum) grown in eastern NC, O'Neal, Blueridge and Legacy are susceptible -- however all three are successfully grown where plant health is maintained via proper site selection, fertility, pruning and irrigation.

Cankers on a one-year-old 'Legacy' stem

Rabbiteye blueberry: Cankers can be numerous on rabbiteye blueberries (Vaccinium virgatum, synonym V. ashei) but generally do not cause yield loss on this vigorous blueberry species.

Wind burn

Wind burn occurs when high winds coincide with the emergence of new, succulent leaf shoots.  The symptoms can be mistaken for insect injury, disease, drought or chemical injury.  Characteristics of wind burn are new shoots that first appear scorched and wilted, then become dried, dead and shriveled.  Older leaves with wind damage have necrotic areas and cracks at the leaf margins.  Symptoms appear immediately on new shoots in high winds.  The first photo below was taken on the day that injury occurred, while the wind was still blowing.  The second photo shows what the injury looked like one month later.

Browning of new shoots can occur very rapidly on a windy day

Wind burn one month after the injury occurred

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.

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.

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.