More information about the 2013 strawberry virus outbreak

Many of you probably have seen an article in today's Raleigh News and Observer about virus problems in this year's strawberry crop. The Plant Disease and Insect Clinic has received several strawberry samples with virus problems in the past few months.

Here are some comments from Dr. Frank Louws, Extension Specialist for Strawberry Diseases in NCSU's Department of Plant Pathology about the strawberry virus situation:

This link will take you to an excellent article from Dr. Chuck Johnson at Virginia Tech about this year's strawberry virus outbreak. All the information for Virginia is true for NC and other states with this problem. The main question growers ask is: “What can we expect in yields?”. We have only limited experience with these viruses in our annual systems. However, we had a case of Strawberry Mild Yellow Edge Virus (SMYEV) in 2004, as shown below in Sweet Charlie plants photographed at full harvest. About 25 to 30% of the plants were affected by the virus and the virus dramatically affected plant growth and yield, as seen in the picture. This year, two viruses, SMYEV and Strawberry Mottle Virus (SMoV), have been diagnosed from problem fields. We do not know how these viruses will affect crop yields in 2013, but some reductions are expected. Specific recommendations for dealing with this problem can be found in Dr. Johnson’s article. With good management, this problem should not recur in future years. 

Rose rosette hits close to home.

Several roses in this bed are showing symptoms of rose rosette.

Those of you living in the Raleigh, North Carolina area may have read the article in the News and Observer on Saturday August 25, 2012 about the removal of several rose bushes from the Raleigh Rose Garden and from a traffic circle on Hillsborough Street. The reason: they had been diagnosed with rose rosette. This disease has been known in North America for decades, but it seems that it has become more common in our area over the last two years. The author of the N&O piece, Bruce Siceloff, did a good job of gathering and presenting the pertinent facts about this disease. Let me review some of them here and expand on what he provided.

Witches' broom and leaf deformation

Symptoms can vary depending on the variety of rose involved and may include elongated flexible shoots, proliferation of shoots leading to a “witches-broom” appearance, excessive development of thorns (soft or not), leaf deformation, retention of juvenile red coloration in shoots, flower abnormalities, decreased cold hardiness, and plant death. There is a rather elaborate molecular test that can be used to confirm the presence of the virus that causes rose rosette, but we do not currently offer that service at the NCSU Plant Disease and Insect Clinic. If you see the “hyperthorniness”, then you can be confident in the diagnosis, but some cases are not clear-cut. Not all symptoms may be present in any given plant. Shoot proliferation and leaf deformation can also be caused by accidental exposure to low doses of the herbicide glyphosate (Roundup), so if you observe this symptom do some sleuthing to see if drift might have occurred.

The shoot on the left retained its red color.

Rose rosette was only recently proven to be caused by a virus, but it has been long known to be transmitted by the microscopic eriophyid mite Phyllocoptes fructiphilus. These are not the same as the more familiar spider mites. Small size makes up for their lack of wings, and these mites can be carried about on air currents. I’ll leave it to my entomology colleagues to comment on whether mite control is of any benefit. There is no chemical control for plant virus diseases. Since viruses become systemic in their hosts, pruning may not be effective. Removal of infected plants is the safer bet. You should bag them before digging, to reduce the chance that the mites will scatter on the wind and take the virus to nearby plants. Remove enough of the roots so that the infected plant does not re-sprout. Also remove any nearby weedy multiflora roses that may be serving as a reservoir of the virus. Fragments of small roots left in the soil should pose no risk. I could find no studies proving the spread of Rose rosette virus through natural root grafts, although this has been demonstrated for other rose viruses. For this reason and because of the mite vectors, planting rose bushes next to one another should be considered a risky behavior. Of course propagating from infected plants or grafting onto infected rootstocks is a no-no.

Research has shown that the incubation period for rose rosette can vary from 17 days to 9 months. Incubation period simply means the length of time it takes for a plant to show symptoms once it has been infected. We don’t have set recommendations about quarantining plants you get via purchase or trade, but some period of isolation and observation may be a good idea.

Extreme thorniness and flexibility are often seen in canes with rose rosette.

Is there a bright side to this story? It’s cold comfort to rose growers that this disease does not affect other kinds of plants. More encouraging is that some rose species are resistant. According to the second edition of Sinclair and Lyon’s excellent book, Diseases of Trees and Shrubs (2005, Cornell University Press), resistant species include the native Rosa setigera and Rosa carolina. No doubt some of these will be exploited in breeding programs trying to bring resistance into garden roses. Until then, vigilance and a shovel are our best tools against this serious problem.

Attack of the Killer Tomato (Spotted Wilt Virus)

TSWV fruit symptoms
Photo by F. J. Louws

Lately, we have seen a lot of tomato samples with tomato spotted wilt  (TSWV) come through the clinic.  TSWV is a virus that is spread by at least seven different kinds of tiny insects known as thrips.  It takes only 15 minutes of feeding for a tomato plant to become infected, and once TSWV is acquired, there is no cure.  TSWV is sporadic in nature with heavy disease pressure in some years relatively little disease pressure in others.  Due to this unpredictable nature, and the broad host ranges of thrips, controlling TSWV can severely challenge traditional pest management practices. 

TSWV can be a major problem both in greenhouses and in the field and can affect field crops like tomato, peanut, and pepper, tobacco along with many ornamental plants.  On tomatoes, symptoms may be expressed on leaves, petioles, stems, and fruit.  Early symptoms include cupping and off-colored bronzed foliage.  Later, leaves may show small, dark spots and eventually die.  Dark brown streaks can be seen on stems and petioles.  Plants may be severely stunted and new growth can be deformed.  Sometimes the plant may exhibit one-sided growth.  The tops of the plants may turn yellow and wilt.

TSWV foliar symptoms
Photo by E. C. Lookabaugh

Fruit symptoms are very distinctive.  Immature fruit have mottled, light green rings with raised centers.  Mature fruit has a unique red/orange mottling that can make the fruit unmarketable.

TSWV immature fruit symptoms
Photo by F. J. Louws 

TSWV mature fruit mottling
Photo by E. C. Lookabaugh

In North Carolina, tobacco thrips (Frankliniella fusca) and western flower thrips (F. occidentalis) are the two most common vectors.   Tobacco thrips are able to spread the virus from nearby weed hosts before they can be killed by insecticides applied to the crop.  Early infections usually result in the greatest impact on yield because early infections can prevent flowering and fruit set.   Secondary spread within fields is very uncommon but may occur by large populations of western flower thrips.  Western flower thrips are extremely difficult to control because they are highly tolerant to insecticides and prefer to reside deep within the blossoms where it is difficult to reach with insecticides.  In addition to spreading TSWV, thrips alone can be damaging to crops because of how they feed.  Thrips feeding causes the collapse of plant cells which leads to deformed plant growth, flower deformation, and silvery areas and flecking on expanded leaves. 

Two common TSWV vectors
Photo UGA

Controlling this disease is very difficult.  In home garden settings, there is usually little secondary spread after the first wave of infections in the spring when virus-bearing thrips are moving from winter weeds to garden plants.  You may wish to remove infected plants, especially those that were infected before fruit set, because they will not recover.

TSWV field symptoms (left plant showing severe stunting)
Photo by F.J. Louws

In a field setting, it is important to manage weeds adjacent to the field because these weeds harbor both the thrips vector and the virus during the winter.  Infected plants should be removed and destroyed as soon as symptoms appear.  TSWV resistant varieties are available and can be effective.  Organic growers and other larger acreage growers may want to consider reflective mulches to cover their beds rather tha the traditional black mulch.  In greenhouses, thrips populations should be eliminated so they, along with TSWV, are not spread to the field when seedlings are transplanted.  

More information on managing this disease can be found here, under "Viral Diseases"

Special thanks to Dr. Louws for helping put together this post and for supplying images!