Monday, August 18, 2014

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.

Friday, August 15, 2014

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. 

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.

Friday, July 18, 2014

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.

Tuesday, June 17, 2014

Bark & Ambrosia Beetle Academy

An Ambriosiodmus beetle excavated from its tunnel in a dead maple tree. The beetle is very small, only a few millimeters long.

Beetles are hard to avoid. They are the most diverse group of organisms, making up a whopping 25% of all multicellular species described to date. With a grab bag of over 350,000 species you're bound to have many that are interesting biologically or important to human endeavors. Scolytinae and Platypodinae (or Scolytidae and Platypodidae for the old schoolers) are two groups of highly specialized weevils (Curculionidae) that fit both of those profiles. Thus, if any insect groups merit their own week-long course it's these amazing beetles.

Luckily, a group of experts had that in mind when they developed the Bark & Ambrosia Beetle Academy, hosted and organized by Jiri Hulcr and his team at the University of Florida. I was fortunate enough to attend the first ever (and surely not the last) class in early May, and although I could tell many stories I am going to just mention some of the informative tidbits I learned through fun facts and photographs I took.

  • "Bark beetles" and "ambrosia beetles" are not taxonomic groups. They are not even completely black and white definitions. Some of these beetles feed only on bark and phloem (former), others grow fungi on which they feed (latter), and still others use different strategies like finding fungi or supplementing their wood diet with fungi. There are even "bark beetles" that bore into cacti or herbaceous plants - truly a diverse group.

Galleries under the bark of a loblolly pine (Pinus taeda) made by the southern pine beetle (Dendroctonus frontalis). Dark streaks of Ophiostoma blue stain fungus can be seen which compete with bark beetles and can cause mortality. Life under bark is a diverse ecosystem.
Bore holes lined with fungus grown by beetles. These ambrosia beetles can gnaw away the wood, but need to feed on the fungus to survive.

  • Fungus farming has evolved independently at least 11 times in these two groups (once in Platypodinae and several times in the Scolytinae). The beetles have special pockets (mycangia or mycetangia) on various body parts to carry the spores of their fungi. What was even more surprising to learn was that some ambrosia beetles actually "steal" fungus (called mycocleptae) from other beetles by tunneling close to the true farmers and letting the fungus grow into their tunnel!

A composite image of two different ambrosia beetles tunneling in a dead maple. The one on the left is a female Ambrosiophilus while the one on the right is a male Ambrosiodmus (see female in title image). The two upper right tunnels are in such close proximity that the fungus grows between them, a phenomenon that lead some beetles to evolve a fungus stealing strategy (including some species of the genus Ambrosiophilus).

  • Like wasps and some other animals, many of theses beetles have unusual sex systems (namely haplodiploidy) whereby males are produced from unfertilized eggs, while females come from fertilized eggs. What's more, in many of these beetles a foundress mother beetle will create a tunnel and lay many female eggs and one male that will end up mating with his sisters. The males in many cases are much smaller, cannot fly and are mainly used for one task - reproduction. In one extreme case, the species Ozopemon uniseriatus, the male is larviform (paedomorphic) and very different from a typical adult beetle. This sexual system along with living in a domicile with other members of the family is likely the reason one species, Austroplatypus incompertus, has become the only known eusocial (i.e. truly social) beetle, with a reproductive "queen" that has many offspring that do not reproduce and instead take care of their brothers and sisters. Colonies can last as long as 37 years (as does the queen) in a single eucalyptus tree!

The black twig borer (Xylosandrus compactus) is a species that exhibits haplodiploidy. Here a tiny, pale male (center) can be seen with his more massive sisters. He will fertilize them before they go colonize other twigs.

  • In the South, pines (Pinus sp.) are a prevalent tree that have their own groups of bark beetles, mostly members of the genus Ips and Dendroctonus. Several species inhabit trees just under the bark and are usually located at specific heights depending on the species. Ips are likely to invade unhealthy/dead trees, stumps and logs but can cause damage to healthy ones in certain situations. They are easily identified by the scooped-out rear (elytral declivity) with several spines surrounding the concavity (see below). Dendroctonus have a head that's visible from above and a gradual, even declivity. Though the black turpentine beetle (Dendroctonus terebrans) is large and infests healthy trees, it is rarely ever of concern. The southern pine beetle (Dendroctonus frontalis), on the other hand, aggressively attacks healthy trees en masse until tree death occurs. There are other Dendroctonus species that attack pines out West, with similar consequences (the mountain pine beetle Dendroctonus ponderosae, for example, is extremely damaging). The main difference between identifying D. frontalis and D. terebrans entrance holes and pitch tubes on pine is that the former's pitch flows are larger and nearer the base of the tree, while the latter's are smaller, more numerous and about breast height (see below).

Resin flows from a loblolly pine (Pinus taeda) under attack by southern pine beetles (Dendroctonus frontalis). Note that the "popcorn" (i.e. resin flows) is found in the crack between the bark, unlike Ips which will often bore right through the bark plates.
Close up of the small, hardened resin flow created by a southern pine beetle (Dendroctonus frontalis). These are made when the beetle enters the tree, which uses the resin to push the beetles out. When they exit, the beetles leave tiny dry holes because the tree is dead and does not produce the protective resin.
An adult southern pine beetle  (Dendroctonus frontalis). The total length of the beetle is ~3 mm. Note the head is visible from above as in all Dendroctonus
The resin flow of a black turpentine beetle (Dendroctonus terebrans) is much larger, as is the beetle itself  (about 5-8 mm).
Ips, like this Ips grandicollis, are frequently found under pine bark, but rarely cause problems for healthy trees. Their presence may mean that a tree is under stress or unhealthy in some way. This beetle is about 4 mm long and has a head hidden below the pronotum and spines circling the tip of the elytra. 

  • As most people know, some species of these beetles are extremely economically important. The classic example is the European elm bark beetle (Scolytus multistriatus), which along with a few other species transmits the causative agent of Dutch elm disease (Ophiostoma sp.). I learned that it's actually the maturation feeding of adult beetles (i.e. when they feed externally on new trees to gain nutrition) that partly helps to spread the disease. Other beetles are as or more destructive. Closer to home, the redbay ambrosia beetle (Xyleborus glabratus) is an exotic species from Asia that attacks healthy members of the family Lauraceae. It brings with it a fungal pathogen (Raffaelea lauricola) that causes laurel wilt, a disease that leads to mortality in these plants, many of which are important for forests and commercial use (e.g. avocado and cinnamon). Lastly, a commodity close to many people's heart is also under attack around the world: coffee. The coffee berry borer (Hypothenemus hampei) threatens the supply of the world's favorite caffeinated drink by boring into the seeds (the very beans we love) and living out their existence inside. Their small colonies significantly reduce the quality of the product and are difficult to control. All of these beetles are being studied to understand the best ways to manage and prevent their destructive, however unintended, nature.

The redbay ambrosia beetle (Xyleborus glabratus) is a tiny beetle that prefers infesting healthy trees in the family Lauraceae. Though not a pest in its native Asia, outside of its home range it transmits Raffaelea lauricola to trees, causing a wilting disease by interfering with the plant's vascular system.
The effects of laurel wilt (Raffaelea lauricola) can at first be seen in the dying leaves atop redbay trees. Upon further investigation, dark streaks can be seen under the bark of trees with the disease.

  • It's an uphill battle: every year we get more species of insect pests. Bark beetles, due to their cryptic nature and small size, are among the most commonly imported species. Over 60 species of bark beetles have been introduced and established in the US and the number will surely climb. Many are not important pests, relying largely on dead trees for their homes and food. Those that attack healthy trees, and especially those that bring with them pathogenic fungi, are the ones we should and do worry about. It should be noted, though, that these beetles in general are important decomposers of dead trees in natural systems and play a key role in forest health.

Xylosandrus amputatus was recently (2010) found in Florida and has now been collected in Georgia; its home range is East Asia. Luckily this species attacks dead or dying trees, so it will likely not become a pest.

  • Even though it may seem futile, we are developing new methods for controlling them. One researcher is even using the sounds these beetles produce to communicate with each other as a potential source of control, by confusing or repelling them. We are also not in this alone: many species of insects and other animals are predators or parasites of bark and ambrosia beetles. Knowing the good from the bad is important, as is implementing control strategies that reduce pest beetle populations while encouraging the livelihood of these beneficial insects.
Darkling beetles (Tenebrionidae) in the genus Corticeus (like this C. thoracicus) are often found among bark beetle galleries where they feed on many things, including the larvae of bark and ambrosia beetles. They can contribute to the control of some pest beetles.

A cylindrical bark beetle (Zopheridae: Colydiinae: Colydium lineola) is the perfect shape to fit into bark and ambrosia beetle tunnels. They are predators of the wood boring beetles.

Checkered beetles (Cleridae) are predators as larvae and adults. Here the larva of one is found under pine bark, likely feeding on the many Dendroctonus larvae located underneath.
A clown beetle (Histeridae) is yet another predator of bark and ambrosia beetles. Though this one was a moderate size (~4 mm), I have seen tiny Plegaderus (~1.5 mm) that were mistaken for bark beetles because they were found inside  tunnels.

It's not just beetles that feed on bark and ambrosia beetle. These maggots of the long-legged fly Medetera (Dolichopodidae) inhabit wood-boring beetle galleries and feed on their young. 

The course was excellent and I advise anyone interested in the subject to attend the next time it is held. I enjoyed meeting all of the participants, and the presenters were very helpful and had a wealth of knowledge. It's great to see so many people are interested in not just controlling these beetles, but understanding their lifestyles and evolution.

Participants signed a copy of one of Stephen Wood's tome on bark and ambrosia beetles.

For more photos, please visit my Flickr album from the trip (including photos of some other insects and nature I encountered).

Tuesday, April 15, 2014

Jack Frost Does Not Work Alone

Dead canes of a flower carpet rose
As March was going out like a lamb, a nursery submitted four container-grown shrubs to the PDIC: three rose cultivars and a lilac. Very young shoots on these plants were withering and dying. At least in the case of the lilac – and possibly with the roses, too – the new flush of growth had been hit by the last freezes of the spring. While you’d expect the tender shoots to be blasted by the cold, in this case the woody stems were also dying. Bacterial streaming was seen in much of the stem tissue. We don’t see fire blight on rose or lilac, so what was happening?

The grower suspected Pseudomonas blight. He was right.

A bacterium and its victims

Bacteria were cultured from the stem tissue of the affected plants. Since only Pseudomonas species were of interest, only colonies fluorescent* on a special agar medium were chosen for further work-up. Unfortunately there are a lot of nonpathogenic (non-disease-causing) Pseudomonas species in this world, so it took a little time to sift through the isolates and confirm the diagnosis as Pseudomonas syringae.

Wilting new shoot of a container-grown lilac.
Although Pseudomonas syringae is named after lilac (Syringa), it is capable of causing cankers and dieback in a wide variety of plants. Besides lilac, we’ve found it on the following woody ornamentals: cherry-laurel, flowering quince, Indian hawthorn, Yoshino cherry, and multiple varieties of rose.  In addition, we’ve recovered it from leaf spots of hydrangea and Japanese holly. Bacterial canker caused by Ps. syringae can be a serious problem in peach orchards, but with woody ornamentals we almost always see it in nursery situations. One exception came in last year, on the twig of a weeping willow from a home landscape. As the weather warms up and cankers become inactive, this disease becomes more difficult to detect. According to the PDIC's records, almost every case of Pseudomonas bacterial canker on woody ornamentals since 2008 was diagnosed between February and May. The bacterium is still present on and within plants during the summer, but what I believe is happening is that the hotter temperatures slow down this particular bacterium at the same time as they're (up to a point) invigorating most plants, thus shutting down the disease process temporarily.

Note: We occasionally find Ps. syringae causing leaf spots on ornamentals in the greenhouse, and there are variants – called pathovars – that cause certain very specific problems such as bacterial speck of tomato and angular leaf spot of cucurbits.

How Pseudomonas syringae does its dirty work

Blighted shoots and a Pseudomonas stem canker on rose
Like many bacteria, Pseudomonas syringae is able to live and multiply on plant surfaces. This is known as its epiphytic (“on the plant”) phase. In the recent case, the bacteria were almost surely present before the spring flush occurred, and so were able to strike quickly. These bacteria enter plants following injury, in particular frost damage. What’s more, the bacterial cells actually promote freeze damage through a process known as ice nucleation. How this works is succinctly expressed by Sinclair and Lyon:

"Ice-nucleating strains of P. syringae and certain other bacteria can trigger ice formation in plant tissues cooled to between -2 and -5ºC [28 to 23 ºF] but not acclimated to low temperature. Ice then disrupts cells, causing symptoms of frost damage. In the absence of an ice-nucleating factor, frost-sensitive plants may tolerate brief cooling to these temperatures because water in their tissues remains in liquid form, supercooled." (Diseases of Trees and Shrubs, 2nd Ed. 2005. p.368)

For more information, see this review by Gurian-Sherman and Lindow. You might also check out this laboratory video of ice nucleation by bacteria added to supercooled water.

As if this ice-nucleation trick were not enough, Pseudomonas syringae also produces a toxin that damages plant cells.

How to reduce your losses

The most important way to minimize damage to woody plants from Pseudomonas syringae is to limit the stressors that predispose plants to infection. Stress factors include pruning injury and frost injury. Bacterial canker of stone fruits caused by Pseudomonas syringae can be reduced by pruning in the early summer, instead of the fall or winter. Sanitize shears or knives frequently, and avoid working the plants when wet. Don't overfertilize plants, especially when they need to harden off for the winter. Protect plants during cold snaps. Don't allow plants to undergo stress from too much or too little water. Keep foliage and stems as dry as possible by changing irrigation methods or reducing overhead irrigation, which favors and spreads the bacteria. If you’ve already had this problem, Ps. syringae is probably present as epiphytic populations on the surfaces of much of your nursery stock and even the surrounding weeds. There are few chemical options that hold any promise, at least not enough to make a recommendation.

As I write this, winter is getting ready to take one last shot at North Carolina, with freeze warnings up for the western half of the state. It's another opportunity for Pseudomonas syringae, too.

*Chemist’s Corner:

Colonies of fluorescent pseudomonads photographed under UV light.
The fluorescent pigments of Pseudomonas species are seen by shining a long-wave UV lamp on the cultures. These compounds belong to a class of chemicals called siderophores. If you know Latin, you might think that siderophore means “star bearer”, but in this case the root is the Greek word for “iron”. (A big thanks to Roland Wilbur Brown’s 1956 book Composition of Scientific Words for setting me straight.) Iron is an essential element for microbial growth, and siderophores have the important task of scrounging precious iron from the bacteria’s environment.