Friday, June 10, 2011

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!