Cotton leafroll dwarf disease: An enigmatic viral disease in cotton

Abstract

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 S_20w 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.

Keywords: Gossypium hirsutum; Polerovirus; cotton blue disease; cotton leafroll dwarf disease; cotton leafroll dwarf virus.

FIGURE 1

(a) Worldwide distribution of cotton blue disease and cotton leafroll dwarf disease. (b) Counties where cotton leafroll dwarf virus (CLRDV) was detected on cotton fields in Georgia, USA, are highlighted in blue. Pointers in (a) indicate where CLRDV was discovered, and the year of the first report in the respective country. For the African continent, virus discovery dates are not known; hence, only disease presence is shown.

FIGURE 2

Symptoms observed on cotton plants infected with cotton leafroll dwarf virus (CLRDV) grown in commercial cotton fields in Georgia, USA. (a) Leaf deformation with bluish‐green discolouration. (b) Vein clearing, leaf curling, rolling, and deformation. (c) Reddening of leaf, petioles, and stem. (d) Puckering and crinkling. (e) Wilting and downward leaf drooping (V‐shaped lamina folding). (f) Deformation of the leaf lamina. (g) Leaf turgidity with a leathery texture. (h) Crinkling. (i) Square/leaf dropping. (j) Wilting. (k,l) Vegetative growth in the upper branches of the stem and stacked internodes and ceased reproductive growth. (m) Bushy phenotype. (n) Terminal whips or accentuated verticality. (o) Parrot beak fruits. (p) Plant infected in the early stage. (q,r) Symptoms diminish in early infected plants but plants remain dwarf without any harvestable bolls. (s,t) Volunteer cotton stalks.

FIGURE 3

(a) Transmission electron micrograph of virion particles of cotton leafroll dwarf virus (CLRDV) from bark tissues of an infected cotton plant with symptoms. The presence of CLRDV in these bark tissues was further confirmed by reverse transcription‐PCR. (b) Schematic representation of the genome of CLRDV and open reading frames (ORFs). (c) Functional overview of proteins associated with CLRDV, including a schematic representation and their functions. An artificially coloured cryo‐electron microscopy image of rice yellow mottle virus (family Solemoviridae) is shown in the centre (ICTV; https://ictv.global/report/chapter/solemoviridae/solemoviridae and Opalka et al., 2000). The functional role played by each viral protein (shown in green, blue, and lavender‐coloured circles) encoded by CLRDV and other members of the genus Polerovirus are depicted at the periphery corresponding to their functional characterization in that virus species.

FIGURE 4

(a) The evolutionary history was inferred by using the maximum‐likelihood method and a JTT matrix‐based model with 1000 bootstrap replicates. The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. This analysis involved 76 nucleotide sequences. All positions with less than 95% site coverage were eliminated. The blue colour indicates cotton leafroll dwarf virus P0 (CLRDV‐P0) sequences obtained from cotton plants in Alabama, Florida, Georgia, Oklahoma, and Texas, in 2018–2022. The light pink colour indicates CLRDV‐P0 sequences from weed species in Georgia. The CLRDV‐P0 sequences obtained from cotton in Georgia/Alabama in 2018 are represented in teal. The typical CLRDV‐P0 sequences from Brazil are shown in mustard yellow, whereas atypical CLRDV‐P0 sequences from cotton are shown in green. Four sequences from Texas forming a subclade are shown in purple. Two sequences (accessions ON954058 and ON954059) shown in golden brown were isolated from cacao (Theobroma cacao) trees in Brazil. Three CLRDVs (accessions UID85580, QLJ58259, and ARU09826), isolated from Hibiscus sp., Malvaviscus sp., and aphid species, are shown in red. The phylogenetic tree was visualized using iTOL v. 4 (Letunic & Bork, 2021). (b) Graph representing the evolution of CLRDV. The virus epidemic may have started around 1945. Analysis was conducted in BEAST v. 1.10.4 and the results were further visualized using Tracer v. 1.7.1.