Washington State University

Dahlia research

In Washington, dahlia (Dahlia variabilis) is an increasingly important crop for small farmers who sell cut flowers at local markets. There are also a large number of display gardens and festivals such as the Swan Island Dahlia Festival, which add millions of dollars to the economic vitality of communities through agro-tourism.

Viral diseases on dahlia

Dahlia growers often leave tubers in the ground or dig and replant each year to reduce the cost of obtaining new planting stock. The vegetative propagation of this crop can lead to a buildup of viruses. The most common viral diseases of dahlia are caused by Dahlia mosaic virus (DMV), Tobacco streak virus (TSV), Tomato spotted wilt virus (TSWV), and Cucumber mosaic virus (CMV) (Albouy 1995). DMV is considered to be one of the most important disease constraints affecting dahlias (Brunt, 1971).  Previous studies by Lobesntein et al. (1995) and others on dahlia viruses included a description of symptomatology, the propagation of virus within various hosts, and role of various insects in transmission (Albouy et al., 1992; Brierley, 1951; Brierley and Smith, 1950; Brunt, 1971). 

 

Advances in research

Pappu et al. at Washington State University have made considerable progress in understanding the molecular biology of the DMV including genome structure and organization and the development of reliable detection tools for rapid diagnosis of infected plant material. They have also shown that besides DMV, there are at least two more caulimoviruses associated with dahlia. Dahlia common mosaic virus (DCMV) was found to be a distinct species within the genus Caulimovirus. An endogenous plant pararetrovirus sequence, tentatively referred to as DvEPRS, was found to be prevalent in dahlias besides DMV and DCMV (Pahalawatta et al. 2008, Pappu et al. 2008).  They have reported on the widespread prevalence of these viruses in US dahlias (Pappu et al., 2005), documented symptomatology (Pappu and Wyatt, 2004; Pappu, 2009; Pappu, 2004) and developed ELISA and PCR-based tests for rapid detection of DMV (Pappu et al., 2003; 2004) and characterized new viruses and virus strains in dahlias (Eid et al. 2009 and 2011; Pahalawatta et al. 2005, 2007, 2008a, b; Pappu et al. 2008).

 

Virus survey

During the past two years, the WSDA Specialty Crop Block Grant (WSDA SCBG) program has supported a project at WSU Puyallup to survey dahlia farms that supply cut flowers to local farmer markets in the Puget Sound region of western Washington to determine the incidence of viral diseases on this crop. Samples from plants exhibiting virus-like symptoms and asymptomatic plants that appeared healthy were collected from 10 farms in 2011 and 5 farms in 2012. Each sample was tested for the following viruses: CMV, DCMV, DMV, DvEPRS, Impatiens necrotic spot virus (INSV), Potato spindle tuber viroid (PSTVd), Potyvirus group test, TSV, and TSWV.

 

Figure 1. Viral-like symptoms observed on dahlia in 2012 included a) mottled yellow-green leaf discolor, b) distorted/crinkly leaves, c) and d) discolor along the mid-vein of the leaf, e) flower deformation, and f), g), h) various severities of ringspot.

Sampled plants were found to be infected with five viruses (Table 1). Overall, 73% of the plants sampled in 2011 and 91.6% of the plants sampled in 2012 were infected with one or more viruses. The most common virus was DvEPRS, which was detected in 49.5% of the samples in 2011 and 86.7% of the samples in 2012. Other viruses detected included DCMV, DMV, TSV and TSWV. None of the samples were positive for CMV, INSV, PSTVd, or the Potyvirus group test. Frequently, the incidence of infected asymptomatic plants was as high as the incidence of infected plants with virus-like symptoms (Table 1). This makes it almost impossible for growers to effectively minimize the buildup of viruses in their planting by roguing out symptomatic plants.

 

Table 1. Frequency of viruses in symptomatic and asymptomatic dahlia plants

 

% samples positive

 

2011

 

2012

Virus

+Sym1

-Sym2

Total

 

+Sym

-Sym

Total

DvEPRS

40.7

73.3

49.5

 

86.4

88.2

86.7

DCMV

19.8

20.0

19.8

 

48.5

64.7

51.8

DMV

7.4

6.7

7.2

 

13.6

35.3

18.1

TSV

23.7

33.3

26.1

 

10.6

11.8

10.8

TSWV

32.1

10.0

26.1

 

19.7

11.8

18.1

CMV

0.0

0.0

0.0

 

0.0

0.0

0.0

PSTVd

nt3

nt

nt

 

0.0

0.0

0.0

INSV

0.0

0.0

0.0

 

nt

nt

nt

Potyvirus group test

0.0

0.0

0.0

 

nt

nt

nt

Samples with 1 or more viruses

69.1

83.3

73.0

 

92.4

88.2

91.6

1+Sym = samples from plants with symptoms, 2-Sym = samples from plants with no symptoms, 3nt = not tested.

 

Results from these and previous surveys suggest that viruses are a widespread problem in commercial cut flower dahlia farms. As part of the WSDA SCBG project, we have also collected grower feedback relating to their virus management programs. In general, very few growers utilize Best Management Practices (BMPs) to minimize the potential impact of virus diseases on dahlias. In fact many of the growers have questioned the importance of virus disease. While virus infections can enhance the aesthetics of some ornamental plants (Valverde et al. 2012), most viruses have significant adverse effects on productivity and quality. Growers’ questions during our surveys have indicated a need for information relating to the impact of specific viruses on production and quality of cut flowers, including effects on vase life.

Moving forward

Unfortunately, this type of data is not currently available for dahlias. During our surveys, symptoms on virus infected plants ranged from severe stunting, to plants that appeared healthy. Obtaining information on the impacts specific viruses have on dahlias has been hampered by the lack of virus-free (VF) plant material to work with. The use of disease-free plant material is also a key BMP relating to the management of viruses in vegetatively propagate plants (Byther and Chastagner, 1993; Chastagner et al., 2012; Daughtrey and Benson, 2005; Lawson,,1990; Lobenstein et al; 1995; Spiegel and Lobenstein,1995). Tissue culture techniques have been developed to eliminate viruses in a number of ornamental flower bulb crops, including dahlias (Albouy, 1992; Bach and Sochacki, 2012; Kamo et al, 2012; Kim and DeHertogh, 1997; Lawson, 1990; Mowat, 1980; Mullen and Schlegel, 1978; Spiegel and Lobenstein,1995; Wang Wen Cai et al., 1988). We are pursuing funds to use these techniques to produce VF dahlias. This will provide an opportunity for future research to obtain a better understanding of the impacts specific viruses are having on grower stocks and allow for the development of improved best management practices targeting these viruses.

Contact: Gary Chastagner, 253-445-4528 | WSU Puyallup Research & Extension Center,2606 West Pioneer, Puyallup, WA, 98371-4998 USA