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New Approaches in Virus Resistance Development

By Nancy Riggs




The NuMex Las Cruces cayenne pepper exhibits high heat and high yields, along with resistance to three curly top viruses.
Photo courtesy of Paul Bosland, NMSU.

U.S. consumption of bell and chile peppers continues to increase. Despite the volume of produce grown offshore, the 2011 U.S. chile pepper crop's value was $146.8 million. Diseases continue to be a significant concern for fruit and vegetable growers, particularly with the almost universal use of irrigation.

While arid regions of the Southwest tend to have fewer disease problems, disease issues are of concern in all growing locations, especially in irrigated crops, and peppers are high on the target list. With a continual increase in the number of insects, viruses are more likely to be transferred to crops. Developing virus-resistant varieties, along with hybrids that are not severely affected by viruses, and identifying varieties that are less attractive to vector insects are important approaches in pepper research to help growers avoid crop losses.

California leads the nation in chile pepper production, and other southwestern states produce the increasingly popular chile peppers. South Texas continues to grow peppers, though on less acreage than in the past. Kevin Crosby, associate professor, Texas A&M University (TAMU, http://www.tamu.edu), has done in-depth work on genetic pepper traits related to virus and insect resistance. New Mexico State University (NMSU) is home to the Chile Pepper Institute (http://www.chilepepperinstitute.org), founded and directed by Dr. Paul Bosland, NMSU horticulture professor. Chiles are considered a signature crop in New Mexico and are a major economic contributor.



Former Texas A&M chile breeder Ben Villalon in a field of TAM Ben Villalon chile peppers, which are resistant to multiple viruses.
Photo courtesy of Kevin Crosby, Texas A&M.

Breeding for virus-resistant varieties

Crosby specializes in breeding peppers for the South Texas growing climate, and research done at test sites throughout the state offers implications for peppers grown in other climates. TAMU released the Caro-Tex habanero pepper in 2012. Caro-Tex carries a gene resistant to tomato spotted wilt virus. TAMU previously released two virus-resistant varieties: TAM Dulcito and TAM Ben Villalon. Released in 2006, TAM Dulcito is a sweet jalapeno pepper, and TAM Ben Villalon is a mild green chile pepper that was released in 2010. Since both peppers are resistant to multiple viruses, they offer growers more options.

"We are continually breeding for virus-resistant varieties," Crosby said. "Some F1 hybrid cultivars exhibit hybrid vigor for traits such as rapid growth, heat tolerance and yield. These can sometimes compensate for the negative impacts of virus infection. This is most true with some tomato spotted wilt virus and tobacco mosaic virus infections, although I have even witnessed it with tobacco etch virus. Heat tolerance may play a crucial role, as most of these viruses are heat-sensitive and high temperatures impede their development within the plant."

Bosland specializes in chile pepper breeding and genetics in the NMSU chile breeding program. He said, "We have an increasing number of organic growers. Organic growers have very limited sprays that can be used. Anything we can do in our program to improve chile peppers for traditional growers is a great help to organic growers."

In 2010, NMSU released NuMex Las Cruces, a cayenne pepper bred for high yields and high heat. It also carries resistance to at least three of the curly top viruses. NuMex Vaquero jalapeno, also released in 2010, carries resistance to the fungus Phytophthora capsici. Bosland noted that while yields are of primary concern in the NMSU program, developing peppers that also carry disease resistance is extremely important to growers.

Vector insects increasing

"The number of vector insects that transmit viruses is increasing," Crosby said. There are various diseases of concern, and curly top viruses are high on the list. A curly top virus recently proved devastating to tomatoes in Fresno County, Calif. Curly top is spread by the beet leafhopper, which feeds on a number of wild host plants.



Seed company representatives view cayenne pepper field trials.
Photo courtesy of Kevin Crosby, Texas A&M.

Researchers are looking at methods to limit the ability of vectors to feed on plants. Several research programs have examined ways to forecast increasing populations of insects that spread the viruses as they feed on plants.

A decade of research at NMSU found that curly top viruses occurred more frequently in chile pepper fields adjacent to fallow land or roadsides with heavy weed growth. London rocket and wild mustard were identified as likely hosts for leafhoppers, the primary vector insect for curly top viruses. Dr. Rebecca Creamer, NMSU plant pathologist, led a project to track leafhoppers' flight patterns. Fluorescent dust was applied to leafhoppers, and a grid set up to track the locations of leafhoppers later caught in sticky traps. Creamer said, "Leafhoppers can fly good distances, but they prefer not to if they can find host plants." To help keep them from invading your crops, it is recommended that you avoid wild host plant growth adjacent to chile fields.

Vectors' preferences

While traditional breeding for resistance has been the norm and continues to be a significant focus, recent research has begun looking at the importance of vectors' plant preferences.



These pepper plants suffer from curly top virus.
Photo courtesy of the Chile Pepp er Institute.

Dr. Jit Baral, a plant breeder with Harris Moran Seed Co., Davis, Calif., was at NMSU for the release of the NuMex Las Cruces cayenne pepper. Baral emphasized that while resistance is sought in plants, in time the virus may overcome the resistance. In citing the newer focus on vectors' plant preferences, he said, "Now we mix both breeding for resistance and vectors' preference." Leafhoppers, for example, prefer light-colored leaves, such as yellow or light green, to feed upon. Some research has found that leafhoppers are less likely to feed on darker leaves, and thereby less likely to infect plants.

Interconnections of threats

Interconnections between locations and between various crops play a major role in the spread of viruses. Baral noted that outbreaks of viruses in locations such as Chile, Costa Rica and Mexico have implications for U.S. growers. Plant material is often imported to U.S. sites. With this material, vector insects are transported to the U.S. and begin attacking crops in the same manner as at the offshore sites. Additionally, he noted that most viruses that attack a crop, such as tomatoes, represent a threat to other crops as well, as has been the case with the curly top viruses.



Kevin Crosby discusses virus-resistant peppers and the advantages of newer hybrid plants.
Photo courtesy of Kevin Crosby, Texas A&M.

Much of the plant breeding and research into disease resistance and control have moved into the private sector, with few full-fledged plant breeding programs remaining. Work continues at universities, other public sector research institutions, and in the private sector to breed plants that offer not only higher yields, but also resistance to viruses. Determining insect feeding preferences and finding plants with leaves that are less attractive to vector insects continues to be a research focus. Ongoing funding is a major need in the public sector breeding and research programs to help assure profitable growing operations throughout the industry.



The Texas A&M Dulcito chile pepper carries resistance to multiple viruses.
Photo courtesy of Kevin Crosby, Texas A&M.

Nancy Riggs is a freelance writer from Mount Zion, Ill.