According to a previous ANR blog post, the 2013 processing tomato crop in California is experiencing worse-than-usual curly top problems, which is caused by a complex of Beet Curly Top Viruses (BCTVs) (Processing tomatoes face critical threat: curly top). Infections are generally lethal to young transplants. Older, more developmentally mature plants will become chlorotic and stunted with cupped leaves and curled petioles. Leaf veins will often turn purple. Fruit set can be significantly impacted (Heflebower et al. 2008).
BCTVs are members of the Geminiviridae family of viruses (specifically, the Curtovirus genera). Viruses in this family have circular, single-stranded DNA genomes that are encapsulated in a 'twinned' (hence the name 'Geminiviridae') set of icosahedral protein shells. According to Chen et al. (2010), three distinct viruses are responsible for curly top disease in the western United States: Beet Curly Top Virus (BCTV, formerly the Cal/Logan strain), Beet Mild Curly Top Virus (BMCTV, formerly the Worland strain) and Beet Severe Curly Top Virus (BCSTV, formerly the CFH strain). BMCTV and BSCTV were the predominant pathogens associated with curly top in processing and fresh market tomatoes in California, as determined by surveys conducted between 2002 and 2008 (Chen et al. 2010).
The viruses that cause curly top have an extremely wide host range (>300 species), including crops (i.e. beets, tomatoes, peppers, cucumbers, watermelon, spinach), weeds (i.e. filaree, perennial pepperweed, Russian thistle and mustard species), and native shrubs (i.e. buck brush [Ceanthus cuneatus]) (Chen et al. 2010; Creamer et al. 1996; Davis et al. 1998). Symptomology can vary dramatically among species; crops tend to exhibit obvious signs of infection, whereas weeds and perennial shrubs are generally asymptomatic (Creamer et al. 1996; Davis et al. 1998). Differences in disease pressure (and viral concentration) are likely due to the varying degrees of virulence among the virus strains, pathogen/host interations that affect viral replication and leafhopper feeding preferences.
Similar to the virus, its vector, the beet leafhopper (Circulifer tenellus), also has a wide host range (Chen et al. 2010; Creamer et al. 1996; Davis et al. 1998; Heflebower et al. 2008). Adult leafhoppers migrate into the coastal range foothills in late fall to overwinter on perennial weeds (i.e. Russian thistle). Their eggs are oviposited on annual species (i.e. buckhorn plantain) that germinate in the following spring. The leafhopper nymphs feed on infected plants, thus acquiring the virus. Mature beet leafhoppers carry the virus to the Central Valley during their spring flights. Successive generations of leafhoppers will feed on both weeds and crops (transmitting the virus while doing so) before the remaining adults migrate back to the foothills for winter. Warm and windy conditions in the springtime increase virus problems, as this facilitates the 'dry-down' of the foothills reservoir species and brings about the start of the migratory cycle (Chen et al. 2010; Creamer et al. 1996; Davis et al. 1998; Heflebower et al. 2008). Additionally, both Chen et al. (2010) and Creamer et al. (1996) suggested that the viruses could survive, year round, in host plants located within the San Joaquin Valley, thereby precluding the need for yearly re-introductions.
Curly top can be confused with tomato spotted wilt, which is caused by the Tomato Spotted Wilt Virus (TSWV, genus Tospovirus, family Bunyviridae). Tomato Spotted Wilt Virus is extremely complex, relative to the BCTVs. The TSWV virion consists of three negative, single-standed, ribonucleoprotein-encapsulated RNAs (the Large [L], Middle [M] and Small [S] stands), which are enclosed, along with an RNA-dependent polymerase, in a host-derived membrane bilayer (Sherwood et al. 2003). TSWV can infect more than 1000 species across 85 families, including: tomato, pepper, potato, eggplant, radicchio, begonia, geranium, sowthistle, cheeseweed, purslane and prickly lettuce. Symptoms of TSWV vary with respect to host plant identity, plant age and environmental conditions, although the most common symptoms on tomatoes include: chlorosis, wilting, bronzing and the development of necrotic spots. Concentric rings may develop on infected fruit. The tospoviruses are vectored exclusively by thrips (Thysanoptera: Thripidae). Frankliniella occidentatlis, the western flower thrips, is considered to be the most important TSWV vector species because of its global distribution (Sherwood et al. 2003).
The management of viral diseases can be quite difficult. With respect to curly top, control strategies could include: altering the timing of tomato plantings to avoid exposing young transplants to the early migration of leafhoppers, increasing planting densities to compensate for losses and discourage visitation by leafhoppers, and eliminating weeds and tomato volunteers following harvest. The California Department of Food and Agriculture manages a statewide program (Curly Top Virus Program) designed to control the beet leafhopper. For tomato spotted wilt, growers are advised to ensure that their transplants are both virus and thrips free. Fields should be monitored for both thrips and viral symptoms. Thrips management should occur early in crop development; if reasonable, symptomatic plants should be removed. As with curly top, weeds and volunteers should be controlled. The use of resistant varieties is encouraged (the resistances of some tomato varieties to TSWV are noted here).
References:
Chen et al. 2010. Characterization of curtoviruses associated with curly top disease of tomato in California and monitoring these viruses in beet leafhoppers. Plant Disease 94:99-108
Creamer et al. 1996. Epidemiology and incidence of Beet Curly Top Geminivirus in naturally infected weed hosts. Plant Disease 80:533-535.
Davis, et al. 1998. Curly top virus found in perennial shrubs in foothills. California Agriculture. 52:38-40.
Heflebower et al. 2008. Curly top of tomato. Utah Pests Fact Sheet PLP-007. http://extension.usu.edu/files/publications/factsheet/curly-top-tomato08.pdf
Sherwood et al. 2003. Tomato spotted wilt. The Plant Health Instructor. http://www.apsnet.org/edcenter/intropp/lessons/viruses/Pages/TomatoSpottedWilt.aspx