Pest Alert: Boxwood Blight on Holiday Greenery

Note to the reader: This blog was originally published in December 2014. The first paragraph was updated in 2016 to reflect changes in the intervening years.

Boxwood blight has been a problem in North Carolina since it was first found here in the fall of 2011.  At first it was primarily a problem for the nursery industry, but it has been found in an increasing number of landscapes across the state. Now, according to NCDA&CS plant pathologist Leah Roberts, there are widespread reports this month [November 2016] of boxwood blight on holiday greenery being sold at retail outlets. For this reason we are re-publicizing this 2014 blog post.

Boxwood blight on tips from a holiday wreath. Photo (2014) by Matt Bertone, NCSU PDIC.

The situation took a new turn this month [December 2014] with the discovery by the North Carolina Department of Agriculture and Consumer Services of boxwood blight in boxwood holiday greenery sold at retail stores in a few North Carolina locations. We do not know how widespread this disease is on boxwood tips sold for holiday greenery, but this alert is being distributed to avoid potentially irreversible damage to established boxwood plantings. It is written for purchasers of boxwood wreaths or other holiday greenery made from boxwood. Tip growers and nursery operations should consult with their local County Cooperative Extension Service or the NCDA&CS Plant Protection Section.

Dark leaf spots (left) and dark stem streaks and defolation (right) are typical boxwood blight symptoms.

Affected boxwood show three main symptoms:

• dark leaf spots
• dark streaks on green twigs
• leaf drop. 

In some cases leaves will lose their luster and dry up without the typical spots, but this can happen for reasons other than boxwood blight. American and English boxwoods are particularly susceptible and are rendered unsightly by the disease, although they do not die.  Sarcoccoca (sweet box) is also affected. The fungus can infect Pachysandra, too, causing leaf spots that could go unnoticed.

Boxwood blight is caused by a fungus known scientifically as Calonectria pseudonaviculata. It also goes by the names Cylindrocladium pseudonaviculata and Cylindrocladium buxicola. The fungus is harmless to other kinds of plants, to animals, and to people.

Defoliation typically begins near the base of the boxwood plant and moves upward. Photo by Kelly Ivors.

Calonectria pseudonaviculata can be spread long distances on infected plant material and could be moved on contaminated clothing, bags, footwear, tools, vehicles, etc. used by workers moving from field to field. Short-distance spread is by splashing water and potentially by animal activity. The sticky spores do not easily become airborne except by water splash. Infected wreaths and roping that are exposed to rains could be a source of the fungus for nearby boxwood, sweet box, or Pachysandra.  Greenery that is hanging in a sheltered area will pose little immediate risk, but leaves falling from them could be a source of contamination later on.

What should you do if you have boxwood greenery for the holidays? As a precaution, we are recommending the following:
(1) Inspect boxwood greenery for blight, and immediately discard suspicious material, including fallen leaves.
(2) If there are boxwood shrubs in your landscape and if any boxwood greenery has been placed in an area where it is exposed to rainfall, relocate the greenery or bag and discard it. Do not handle the material when wet, because you could easily spread the spores.
(3) At the end of the holiday season, bag up all boxwood greenery and dispose of it in a landfill. Do not place it in the compost.
(4) Monitor nearby boxwood plantings for symptoms of the disease.

More information about this disease, including more photographs, can be found on NCSU’s Plant Pathology Portal. The Virginia Boxwood Blight Task Force web page is also a good reference and includes lists of best management practices for different situations.

Almost-plague-level Liriope Munchers!!!

The red bricks of NCSU's campus are a nice resting pad for some grasshoppers with full bellies. 

As I walked back from lunch the other day, I stopped by the library to take a peak at a single, large grasshopper on a rail. It was a differential grasshopper (Acrididae: Melanoplus differentialis), and although I don't usually look at these common insects it was too conspicuous not to stop and admire. The next day our good friend Dave Stephan came to me to ask if I had good pics of the species. I said no, but figured "Why not?" So Dave, being the great person he is, collected some for me. The odd thing though was what they were feeding on:

A differential grasshopper sitting among the destruction it and its friends caused.

That's right, liriope or monkey grass (Liriope sp.)! This widely planted, grass-like ground cover is rarely ever attacked by pests. The most frequently encountered insect feeding on this ornamental is the fern scale, Pinnaspis aspidistrae. Otherwise I could only find reference to snails and slugs as animals that feed on this host. These grasshoppers must not know that they are not supposed to feed on it, because there were hundreds in the large patch on campus! But why? We really don't know. I am also unsure if they will feed on the related lilyturf or mondo grass (Ophiopogon sp.).

Male Melanoplus differentialis, the differential grasshopper.

Female differential grasshopper.

Differential grasshoppers are typically pests of field crops like soybean, corn and cole crops, but also feed on many grasses and other plants. They can become serious pests in some areas during certain years. Their large size (about 2" long) also makes them conspicuous and also very hungry! Keep an eye out and let us know if you notice them feeding on liriope (or mondo grass) - perhaps this is an isolated event, but we don't know for sure.

Attack of the Armyworms

The edge of the battlefield.

Spodoptera frugiperda (its species name meaning "fruit ruiner") was always a fun name to say when learning insect trivia for Linnaean games. The common name, fall armyworm, was also visually interesting, evoking marching hordes of munching, wriggling larvae. Apparently those descriptions aren't too far off. This year looks to have been a good year for them and a bad year for homeowners with lawns and crop producers. As evidence, there are now many reports about these pests from across the country (TX, OK, AL, PA, and more).

Fall armyworms feed on a variety of plants - at least 80 species. Hosts like apples, strawberries, flowers and many weeds may be eaten by these caterpillars. They are even common in many field crops like alfalfa and soybeans. However, fall armyworms prefer to feed on various grasses (Poaceae), including some of our favorite foods (corn, wheat) and turf grasses (fescue, Bermudagrass, etc.). The larvae grow rapidly by chewing holes in leaves, or completely devouring entire grass blades. At first, larvae are less than a centimeter long and may be a bright green:

Young fall armyworms are more green than brown, but even older ones can vary in color. Specific traits other than color are best used to identify this species.

As they age through six instars (larval "stages"), they normally change to a darker green-brown color:

Mature fall armyworms can be close to 1.5" long and olive green-brown, giving them a military appearance further adding to the "army" moniker.

It takes about 16-30 days for the caterpillars to mature (depending on temperature) at which time they burrow into the ground where they pupate. Pupae are reddish brown and may be found in high densities in the soil of infested areas:

Pupae of fall armyworms are typical of many moths - reddish-brown, wiggly and buried in the soil.

After a little more than a week in warm weather, adult moths emerge from the pupae, take flight, and mate to make more armyworms. Eggs are laid on vegetation around grassy areas. Adults are readily sexed due to their strong color dimorphism. Females are a drab brown with subtle markings, while males have much more bold patterns and are actually pretty attractive:

An adult female fall armyworm, exemplifying the typical brown moth appearance (length ~2 cm).

Male fall armyworms are pretty nice looking for a pest (length ~2 cm).

The best way to tell larvae of fall armyworms from other armyworms in the genus Spodoptera (as well as other owlet caterpillars in the family Noctuidae) is through a combination of traits. The most often cited characteristic is the inverted yellow "Y" on the head (extending up the pronotum) as seen here:

Mature caterpillar's head and thorax, showing the inverted "Y" found on this species.

However, some other members of the genus have a similar "Y" and some fall armyworm instars lack it (later ones show it best). Thus Wagner et al. in Owlet Caterpillars of Eastern North America have a key that includes a character that is more reliable and separates out S. frugiperda from its relatives in the first couplet: the dorsal pinnacula (top-most black spots with an associated seta) are as big or bigger than the spiracles:

The black dots (pinnacula) on the tail end reveal the fall armyworm's identity: the dorsal ones (square of four shown by orange arrow) are each larger than the spiracles (blue arrow).

Fall armyworms can be found during warm parts of the year throughout the eastern United States (east of the Rocky Mountains). However, they can only survive winters in places like Texas and Florida (as well as Mexico and the Caribbean). Adults are strong fliers and migrate up through the states, sometimes with help from storms that blow them part of the way. Here in NC they may be present for several generations from spring through fall, while there is usually only one generation up north. The generations around late summer and early fall are usually the largest, thus the "fall" in the armyworm's name.

From Sparks (1979)

So what can be done and why is this year worse? Fortunately, most years do not see mass amounts of these caterpillars. Parasitoids and pathogens kill many of the armyworms in the overwintering areas, reducing the size of the resulting northern migrants. However, as Sparks (1979) describes below, some weather conditions in their year-round range can cause mass outbreaks:

From Sparks (1979)

If you fear you may have armyworms in your lawn or corn crop, monitoring early can help detect the caterpillars. For homeowners, the presence of birds in a lawn can indicate an abundance of larvae. However, by the time large, mature larvae are seen, control may not be helpful as they will soon pupate. Thresholds for crops and potential control methods can be found through the links below.

Helpful Resources:

• A Review of the Biology of the Fall Armyworm, Sparks, A.N. (1979) 
• Fall armyworm information at NCSU (1) (2) (3) (4)
• Fall armyworm at UFL
• Bugguide: Spodoptera frugiperda

Box Blight Confirmed in Wake County

Box blight has been confirmed in boxwood plants originating in a nursery in the NC mountains and offered for sale at the North Carolina State Farmers Market in Raleigh. The disease also has been confirmed at the Raleigh home of the vendor. A small number of customers may have purchased infected plants between the beginning of July and mid-August 2014.

Box blight is a destructive fungal disease of boxwood leaves and twigs. Symptoms include brown leaf spots, dark streaks on twigs, and extensive leaf drop. Sarcococca (sweetbox) and Pachysandra can also become infected. A fact sheet is available with additional information about identification and management of this disease. Note that sanitizer information is currently being updated. For most bleach formulations the correct ratio of bleach to water is now 1:14.

Personnel from the North Carolina Department of Agriculture and Consumer Services are attempting to trace the sales of these plants from the Farmers Market. Careful removal and destruction of all infected shrubs may help keep losses to a minimum and prevent further local spread.  If believe you may have purchased one of the plants in question, please contact the office of Phil Wilson, Plant Pest Administrator for the NCDA&CS at 919-707-3753. Other parties with questions about box blight should direct them to their local County Cooperative Extension Service office.

Soybean Disease Update from Steve Koenning

Physiological Scorch – Is it SDS, Stem Canker, Black Root Rot (CBR), Brown Stem Rot, or something else? 

We are receiving soybean samples in the Plant Disease and Insect Clinic (PDIC) that have symptoms of Physiological Scorch (Figure 1).  Most of the scorch in these samples is due to SDS or Sudden Death Syndrome, but numerous diseases can cause similar symptoms.  Regardless of the cause, this symptom is indicative of a problem with the vascular system once soybean has shifted to the reproductive phase.  Usually “Scorch” is the result of a root-rot such as SDS, CBR, dectes stem borer, or Phytophthora root rot.  Fungicide sprays will not impact these problems at all and should be avoided. Below are links to disease notes that will explain how to differentiate these diseases and what action to take in the future. 

http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy007/soy007.htm

http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy005/soy005.htm

Physiological scorch symptoms

Frogeye leaf spot, Target spot, and Stem Canker

Target spot of soybean and frogeye leaf spot have both been identified in North Carolina this year. Many cultivars are resistant to these diseases so there is no cause for alarm at this time.  If the disease is detected, a fungicide should be applied.  If target spot is identified, it warrants an application of a strobilurin fungicide.  If frogeye is identified, then a combination fungicide (StrategoYLD, Fortix, Quadris Top, or Affiance) may be warranted since resistance to strobilurin fungicides was identified last year in Beaufort County. See the North Carolina Agricultural Chemicals Manual for more information.  

Soybean Stem Canker has been found in the Piedmont and in Martin County.  Soybean stem canker must be controlled with varietal resistance. Fungicides rarely impact this disease, especially at this point in the season.

TSWV in Chrysanthemum

A greenhouse-grown chrysanthemum was received in the Plant Disease and Insect Clinic on July 10th and diagnosed with Tomato spotted wilt virus (TSWV) by Emma Lookabaugh. Symptoms consisted of dark leaf spots, lateral curling of the leaves at some of the spots, and at least one stem lesion.

TSWV symptoms on Chrysanthemum

Although TSWV is the most commonly diagnosed viral disease here in the PDIC, it has been a long time since we've detected it on chrysanthemum from North Carolina. We have no records of it during the current millennium, but if memory serves there was at least one case back in the late 1990s. The current case does not constitute an outbreak, but should serve as a reminder to growers to take measures to prevent this disease.

A different sort of TSWV symptom on mum, from a different sample.

Tomato spotted wilt occurs on hundreds of field and crops, including peanut, tobacco, tomato, pepper, and potato, as well as on a wide range of ornamentals. In the last 6-1/2 years we have diagnosed it on the following ornamentals from commercial sources: African marigold, angel-wing begonia, calla lily, Cyclamen, Gaillardia, Gerbera, Senecio confusus, Lisianthus, Lobelia, Madagascar periwinkle, Sedum, and Stoke's aster. Its sister virus, INSV, is a frequent problem on many ornamentals.

Mottling and ringspot symptoms on TSWV-infected Senecio (left) and Stokesia (right)

Both TSWV and INSV can cause a wide range of symptoms, including mottling, ringspots, stunting, and necrotic leaf and stem lesions. Both are members of the genus Tospovirus and are transmitted by minute insects called thrips*. One curious fact about this transmission is that the virus is acquired by the insect during its larval development, but then the insect itself becomes permanently infected. Of course the virus can be brought into a greenhouse with infected plants, and could be perpetuated through vegetative propagation.

A thrips compared to the tip of a pin.

These strategies against TSWV (and INSV) are recommended for greenhouse flower production:

• Avoid growing vegetable transplants and flowers in the same greenhouse, and avoid growing plants of different ages together.
• Screen greenhouse vents and air intakes to exclude thrips from entering the greenhouse.
• Control weeds in and around the greenhouse. Many weeds are susceptible to tospoviruses and can serve as reservoirs of virus and thrips.
• Monitor greenhouses for thrips activity using blue or yellow sticky cards, with the top 2/3 of the card placed above the plant tops.  Use two cards per 5000 sq. ft. of greenhouse area.
• Use insecticides to manage thrips populations when necessary. Remove flowers from plants before treatment since the interior of flowers rarely get adequate coverage. It is important to note that some thrips populations have developed insensitivity to commonly used insecticides. In addition, no insecticide can completely eliminate thrips. Utilize the most effective chemistries wisely by rotating insecticides by mode of action (IRAC class) with each application, or at least with every generation of thrips. Always follow label directions and check that products are labeled for the intended crop. Details on insecticides for thrips management can be found in the NCSU Information Note on Western flower thrips and the University of Florida's thrips management information.

TSWV symptoms on Lobelia

If you suspect you have infected plants, we recommend having the diagnosis confirmed by a laboratory. Large growers with recurring problems may want to keep a supply of the simple lateral-flow ELISA tests on hand. Suppliers** include AC Diagnostics and Agdia. There is no cure, so all infected plants must be removed and destroyed. The potting mix of these plants should also be discarded, as this is where the thrips vectors pupate. Eliminate old stock plants as these are often sources of thrips and viruses.

More information about TSWV in the following crops is also available:
- peanut 
- tobacco
- tomato

Mike Munster and Steve Frank

*Grammatical footnote: The word thrips is both singular and plural.
**Mention of trade names and companies does not imply endorsement by North Carolina State University or the Plant Disease and Insect Clinic.