NC State Extension

Tomato Spotted Wilt Management in Tobacco

Tomato Spotted Wilt (TSW) is a topovirus that infects a wide range of plants in this area, including many native weed species. The virus is spread by a tiny, almost microscopic insect called a thrip. Damage to plants can be minor or result in death depending upon the strain of the virus, the growth stage of the plant and time of infection.  There are multiple species and high population of thrips with a huge range of weeds, field crops and ornamental plant hosts within Craven County. Thus, susceptible plants have a great risk of infection.

With greater risk of TSW incidence, it seems reasonable to aim to predict the timing of potential TSW infection and seasonal intensity of this disease.   However, there are many factors that contribute to TSW incidence. First, and often overlooked is the wide range of host weed plants found naturally within this area. These weeds serve as host to the thrip. If these weeds have low viral infection TSW incidence in tobacco may be low even when the thrip population is very high.  NCSU faculty regularly monitors the TSW infection level in weeds to assist in prediction.

The thrip population feeding on these weeds is also important.  As weeds become desiccated due to herbicide applications, cultivation or increasing temperatures, the thrips migrate from the weeds to other hosts.  Thus, timing of weed control and natural weed desiccation affects the timing of the thrip migration from weeds to other susceptible plant host.

Lastly, rainfall affects TSW incidence. Heavy rainfall events kill immature thrips thus decreasing the thrip population. Conversely, frequent but light rains will postpone the natural desiccation and dieback of winter weeds. Thus, thrips will continue to remain on these weeds and increase in number.  As a consequence, thrip migrations occur much later in the year and the thrip population is often very high.

Even with these many variables affecting thrip migration and TSW incidence, NCSU faculty has partnered with other agencies and groups to provide a reliable prediction of thrip migration based upon local conditions. This prediction and historical database is targeted specifically for tobacco producers and is found at http://www.nc-climate.ncsu.edu/thrips/index.php.

Control of TSW in Tobacco Production

Three basic strategies guide management of TSW in tobacco.   The first involves the use of Admire Pro (or similar materials) applied as a drench to greenhouse transplants. NCSU data shows this treatment to be effective for areas of historically low TSW incidence.  Typically, within Craven County, growers following this management option have TSW incidence ranging from 6-30%. However, when circumstances suggest high TSW incidence, using Admire Pro alone may result in much higher TSW incidence.

The second and proven option for this area is to include the product Actigard as a supplement to Admire Pro. Actigard applied as a greenhouse drench treatment 3-4 days prior to transplant has shown to reduce TSW incidence greatly even in years with high TSW incidence.  Producers choosing this options should simply aim to transplant within 4 days of application of Actigard. If this is not possible, plants should be removed from the floatbed and placed into a non-treated waterbed to reduce any potential adverse effects to plants.

The rates of Actigard chosen will determine the length of control.  While higher rates increase the length of control, they may also increase the probability of slow plant growth. This is especially true if treated plants are left within float trays too long or cool weather follows transplanting. Growers should choose a rate based upon transplanting time and expected length of control needed. If the major thrip migration occurs during or shortly after transplanting, a long period of control is not necessary. Conversely, if plants are transplanted early (mid-April) but the major thrip migration is not expected until early May, higher rates should be used to provide longer control. It should be noted that rates of 25 ppm or higher have resulted in severely slow growing plants in NCSU research trials.

Table 1, below, shows a conversion of the label recommended rates of parts per million to ounces per floatbed based upon floatbed size. Typically,  producers utilizing rates of 10-15 ppm have adequate TSW control compared to non-treated plants within Craven County. Care should be taken in estimation of floatbed water content since this greatly influences concentration of Actigard. It is advisable to measure the water level in several places. In spite of best efforts, floatbeds are rarely uniformly level.

Table 1. Conversion of Actigard Rates (ppm) to Ounce Per Floatbed by Floatbed Size

Actigard Rate (ppm)

Gallons Per Bed

10

15

20

25

3000

4.0

6.0

7.9

9.9

3200

4.2

6.4

8.5

10.6

3400

4.5

6.8

9.0

11.3

3600

4.8

7.2

9.5

11.9

3800

5.0

7.6

10.1

12.6

4000

5.3

7.9

10.6

13.2

4200

5.6

8.3

11.1

13.9

The third management option for TSW is to apply a foliar application of Actigard to tobacco fields prior the major migration of thrips from weed hosts.  While shown to be effective, this requires that growers monitor the thrip migration at least twice weekly via the previously mentioned website and have the ability to apply Actigard to all fields in 1-2 days. This Actigard application MUST be applied at least 3-5 days PRIOR major thrip migration to be effective. This is not always possible given the unpredictable weather conditions. Furthermore, model prediction of thrip migration do not account for activity in adjacent fields. Thus, thrip migration from adjacent fields may occur prior to predicted movement should adjacent fields receive cultivation or herbicide applications destroying weed hosts. Given that thrips can easily migrate miles on wind currents, this may result in high incidence of TSW in isolated fields for grower choosing this option.

To download additional information for TSW prevention, refer to pages 142-147 of the 2013 Flue-Cured Tobacco Guide HERE.

 For more information on tobacco production, visit the website, The Tobacco Grower Information Portal at http://tobacco.ces.ncsu.edu/ 

The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the N.C. Cooperative Extension Service nor discrimination against similar products or services not mentioned.  Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conform to the product label. Be sure to examine a current product label before applying any product.

Written By

Photo of Mike CarrollMike CarrollArea Agent, Agriculture (252) 633-1477 mike_carroll@ncsu.eduCraven County, North Carolina
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