Cotton leafroll dwarf disease: An enigmatic viral disease in cotton.

TAXONOMY: Cotton leafroll dwarf virus (CLRDV) is a member of the genus Polerovirus, family Solemoviridae. Geographical Distribution: CLRDV is present in most cotton-producing regions worldwide, prominently in North and South America. PHYSICAL PROPERTIES: The virion is a nonenveloped icosahedron with T = 3 icosahedral lattice symmetry that has a diameter of 26-34 nm and comprises 180 molecules of the capsid protein. The CsCl buoyant density of the virion is 1.39-1.42 g/cm3 and S20w is 115-127S. Genome: CLRDV shares genomic features with other poleroviruses; its genome consists of monopartite, single-stranded, positive-sense RNA, is approximately 5.7-5.8 kb in length, and is composed of seven open reading frames (ORFs) with an intergenic region between ORF2 and ORF3a. TRANSMISSION: CLRDV is transmitted efficiently by the cotton aphid (Aphis gossypii Glover) in a circulative and nonpropagative manner. Host: CLRDV has a limited host range. Cotton is the primary host, and it has also been detected in different weeds in and around commercial cotton fields in Georgia, USA. SYMPTOMS: Cotton plants infected early in the growth stage exhibit reddening or bronzing of foliage, maroon stems and petioles, and drooping. Plants infected in later growth stages exhibit intense green foliage with leaf rugosity, moderate to severe stunting, shortened internodes, and increased boll shedding/abortion, resulting in poor boll retention. These symptoms are variable and are probably influenced by the time of infection, plant growth stage, varieties, soil health, and geographical location. CLRDV is also often detected in symptomless plants. CONTROL: Vector management with the application of chemical insecticides is ineffective. Some host plant varieties grown in South America are resistant, but all varieties grown in the United States are susceptible. Integrated disease management strategies, including weed management and removal of volunteer stalks, could reduce the abundance of virus inoculum in the field.

Effect of Cotton Leafroll Dwarf Virus on Physiological Processes and Yield of Individual Cotton Plants.

Cotton leafroll dwarf disease (CLRDD) caused by cotton leafroll dwarf virus (CLRDV) is an emerging threat to cotton production in the United States. The disease was first reported in Alabama in 2017 and subsequently has been reported in 10 other cotton producing states in the United States, including Georgia. A field study was conducted at field sites near Tifton, Georgia in 2019 and 2020 to evaluate leaf gas exchange, chlorophyll fluorescence, and leaf temperature responses for a symptomatic cultivar (diseased plants observed at regular frequency) at multiple stages of disease progression and for asymptomatic cultivars (0% disease incidence observed). Disease-induced reductions in net photosynthetic rate (A n, decreased by 63-101%), stomatal conductance (g s, decreased by 65-99%), and efficiency of the thylakoid reactions (32-92% decline in primary photochemistry) were observed, whereas leaf temperature significantly increased by 0.5-3.8°C at advanced stages of the disease. Net photosynthesis was substantially more sensitive to disease-induced declines in g s than the thylakoid reactions. Symptomatic plants with more advanced disease stages remained stunted throughout the growing season, and yield was reduced by 99% by CLRDD due to reductions in boll number per plant and declines in boll mass resulting from fewer seeds per boll. Asymptomatic cultivars exhibited more conservative gas exchange responses than apparently healthy plants of the symptomatic cultivar but were less productive. Overall, it is concluded that CLRDV limits stomatal conductance and photosynthetic activity of individual leaves, causing substantial declines in productivity for individual plants. Future studies should evaluate the physiological contributors to genotypic variation in disease tolerance under controlled conditions.

Genome analysis of cotton leafroll dwarf virus reveals variability in the silencing suppressor protein, genotypes and genomic recombinants in the USA.

Cotton leafroll dwarf virus (CLRDV) is an emerging virus in cotton production in Georgia and several other Southeastern states in the USA. To better understand the genetic diversity of the virus population, the near complete genome sequences of six isolates from Georgia and one from Alabama were determined. The isolates sequenced were 5,866 nucleotides with seven open reading frames (ORFs). The isolates from Georgia were >94% identical with other isolates from the USA and South America. In the silencing suppressor protein (P0), at amino acid position 72, the isolates from Georgia and Alabama had a valine (V), similar to resistant-breaking 'atypical' genotypes in South America, while the Texas isolate had isoleucine (I), similar to the more aggressive 'typical' genotypes of CLRDV. At position 120, arginine (R) is unique to Georgia and China isolates, but absent in Alabama, Texas and South American isolates. Ten potential recombinant events were detected in the isolates sequenced. An increased understanding of CLRDV population structure and genetic diversity will help develop management strategies for CLRDV in the USA cotton belt.