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.

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).

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.

A. destructor

This may look like the toothy mouthparts of some sinister little animal,
but it's really the posterior end of an armored scale (Diaspididae).

UPDATE: After looking at new scales on an Aucuba recently (July 5 2016) it was determined that the scales identified in this post as Aspidiotus destructor, are most likely Aspidiotus hederae, the oleander scale. All information on the former species and on armored scales remains correct, but note that the images of the scales represent the latter species.

Armored scales (Diaspididae) are one of the most common insect pests of ornamentals. More like diseases than insects, these sedentary bugs (literally - they are in the Order Hemiptera) sit and suck the sweet fluids of plants, all the while taking energy from their host and replicating as big sacs of eggs. Their babies, called "crawlers", infest new areas and settle in for the long haul of motherhood (or the short, but free, life of a winged male).

Over the past month or so we received two samples of different plants that had an interesting armored scale infestation. The first was poet's laurel (Danae racemosa), that looked as if it was a variegated variety from the amount of chlorosis associated with the scales:

Poet's laurel (Danae racemosa) leaves with yellow and brown areas due to scale insect pressure. 

Looking under the microscope, I noticed two types of armored scales. The first, and less common, were some typical brown, oyster-shaped fern scales (Pinnaspis aspidistrae). However, the most noticeable scale was one I did not recognize. It had a very thin, translucent test that resembled delicate wax paper with a bright yellow scale underneath:

Although it may look like these scales are under the epidermis of the plant, they are really hiding under a thin test (the covering common to armored scales, to which they owe their name).

When lifted off, the scales underneath looked like this:

Scales with their test removed (top one is facing right and bottom one is facing left).

The scale certainly had the rounded shape characteristic of most members of the subfamily Aspidiotinae (as opposed to the elongate shape of most Diaspidinae, such as the previously mentioned fern scale)...but what was it? Well, I have to confess that our friend and former clinic member Dave Stephan took a peek during a visit and thought it looked like the genus Aspidiotus. Luckily he said that, because the scale book I most often reference first, Scale Insects of Northeastern North America by Michael Kosztarab, does not cover this genus which is primarily Southern in distribution. So I referenced another great resource Ferris's Atlas of the Scale Insects of North America. Lo and behold, Dave was right - the scale keyed out to the genus Aspidiotus and further to the species A. destructor, the coconut scale.

Luckily I had identified that sample, because the same day there was an image of aucuba (Aucuba japonica) that was submitted with a potential scale infestation. Although I was not able to ID the species from the pictures, when the sample came in I recognized the similarities with A. destructor:

The yellow spots on this aucuba leaf are intentional variegation - not chlorosis attributed to the scales. 

Close up of different-sized coconut scales (Aspidiotus destructor) oleander scales (Aspidiotus hederae).

After clearing some specimens I was able to definitively ID them as the same scale. Was this a coincidence? Probably, but who knows whether these scales are becoming more abundant. Our clinic records show that A. destructor was only submitted and identified four times in the 14 years prior to these two samples. Does that mean that we will be seeing more of this scale? I am not ready to conclude that, but if more are submitted this year we may have to investigate what's going on.

----------

A little more on the scale.

Aspidiotus destructor was described by Signoret in 1869 and goes by several common names including bourbon scale and transparent scale (I am assuming based on the thin test). The scale appears to be Southeast Asian/Pacific in origin, but has been spread throughout the world. Although mainly a pest of coconut and banana, it is extremely polyphagous being found on over 60 families of plants. The genus is characterized by the following traits that can be seen in the title image (Ferris, 1938):

• absence of paraphyses or intersegmental scleroses
• three pairs of lobes with no indication of a fourth pair
• plates long, flat and fringed (two between median lobes, two between median and second lobes, three between second and third lobes, and a variable number beyond third lobe)
• characteristic sclerotization on dorsum of pygidium

The scale can cause significant economic damage at high densities (which can be common), stunting plants and eventually killing them if enough of the leaves become unable to undergo photosynthesis. Treatments can include chemical control, but there is also a diversity of natural enemies known to attack the species including various fungi, ladybugs, thrips, mites and several parasitoid wasps, most of which are in the family Aphelinidae. In fact, after clearing the scales from the aucuba, I noticed some sinister-looking aliens inside a few of the scales that are certainly wasp larvae and likely a species of Aphelinidae (below "A"):

A cleared female coconut  oleander scale showing a parasitoid wasp larva (A), mouthparts/stylets (B), scale egg (C)
and pygidium (D). [Thanks to Mike Munster for helping take pictures of the scale under the microscope]

I don't know whether or not the wasps are able to keep these scales in check alone, but there were at least a few  being eaten by these tiny larvae. Every bit helps I guess!

References:
Ferris, G.F. 1938. Atlas of the scale insects of North America. Series 2. Stanford University Press, Palo Alto, California

Signoret, V. 1869. [Essay on the gall forming insects (Homoptera - Coccidae) - 3rd Part.] Essai sur les cochenilles ou gallinsectes (Homoptères - Coccides), 3e partie. Annales de la Societe entomologique de France (serie 4) 9: 97-104.