During the 2019 growing season, cotton (Gossypium hirsutum L.) plants in North Carolina were observed to have virus-like symptoms including leaf rugosity, leaf curling, and shortened upper internodes, consistent with Cotton Leafroll Dwarf Disease (CLRDD) associated with cotton leafroll dwarf virus (CLRDV, family: Luteoviridae, genus: Polerovirus) (Avelar et al. 2020). Sentinel plots planted on June 17, 2019 at the Sandhills Research Station in Moore County, NC exhibited CLRDD symptoms and disease incidence was estimated at 75% on a 0.1-ha field. Cotton aphids (Aphis gossypii Glover), the reported vector of CLRDV (Michelotto and Busoli 2007; Heilsnis et al. 2020; McLaughlin et al. 2020), were detected on plants throughout the growing season. Samples (n = 24) were collected from sentinel plots on September 20, 2019 to test for CLRDV through RT-PCR. Each sample represented five symptomatic plants collected from a single plot. Total RNA was extracted from the petiole tissue of each sample using Qiagen RNeasy Plant Mini kit (Qiagen, Germantown, MD), following the manufacturer’s recommendations. The cDNA was synthesized using SuperScript IV first-strand synthesis system (ThermoFisher Scientific, Waltham, MA) and amplified with CLRDV-specific PCR primers CLRDV3675F/Pol3982R (Sharman et al. 2015) targeting a 310 bp genome segment of ORF3-5. Seven CLRDV positive samples were further amplified with two additional primer sets specifically designed to detect CLRDV: AL674F/AL1407R (Avelar et al. 2019), targeting a 733 bp portion of the ORF0-ORF1, and CLPOF/CLPOR (Cascardo et al. 2015), amplifying a 880 bp fragment spanning the ORF0. Nucleotide BLAST searches showed that the best matches for all sequences in this study were derived from CLRDV with a range of pairwise identity of 99.2¬¬ - 100% for all genome segments. From symptomatic samples (n = 14 ), the isolated virus was confirmed as CLRDV in several cotton varieties, including Deltapine 1646 B2XF (GenBank Accessions MN958131 (ORF3-5), MN958147 (ORF0-ORF1), MN958138 (ORF0), MN958133 (ORF3-5), MN958145 (ORF0-ORF1), MN958140 (ORF0)), Phytogen 480 W3FE (MN958134 (ORF3-5), MN958144 (ORF0-ORF1), MN958141 (ORF0), Stoneville 5471 GLTP (GenBank Accessions MN958135 (ORF3-5), MN958143 (ORF0-ORF1), MN958142 (ORF0)), and Nextgen 5711 B3XF (GenBank Accessions MN958130 (ORF3-5), MN958148 (ORF0-ORF1), MN958137 (ORF0), MN958132 (ORF3-5), MN958146 (ORF0-ORF1), MN958139 (ORF0), and MN958136 (ORF3-5). CLRDD is a newly named disease of cotton in the United States that was first reported in Alabama (Avelar et al. 2019), Georgia (Tabassum et. al. 2019), Mississippi (Aboughanem-Sabanadzovic et. al. 2019), and Texas (Alabi et al. 2019). While the virus has been reported with variable impacts, losses can be extensive in some fields that are affected (Avelar et al. 2019). North Carolina produced over one million bales of cotton in 2019 and given reported losses among fields with high disease incidence, CLRDV has the potential to significantly reduce cotton yield and quality for the state if it becomes widespread.

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A new variant of cotton leafroll dwarf virus (CLRDV) (genus: Polerovirus, family: Solemoviridae) was discovered in cotton (Gossypium hirsutum L.) fields that were reported to be infested with aphids and whiteflies in southern Alabama in 2017. Prior to the confirmation of CLRDV, speculation focused on whiteflies as a potential vector of the then-unknown virus. Although the only vector reported to transmit CLRDV to cotton is the cotton aphid, Aphis gossypii (Glover), two recombinant poleroviruses have been reported recently to be transmitted by the whitefly, Bemisia tabaci (Genn.). Due to the emergence of a new CLRDV variant in the U.S., and the recent studies on recombinant poleroviruses, conflicting messages that whiteflies and/or aphids could be transmitting CLRDV have been relayed to growers and stakeholders in the Cotton Belt. The objective of this study was to determine if A. gossypii or B. tabaci (B Mitotype) transmit CLRDV to cotton. The results demonstrated that the CLRDV-AL variant was transmissible by alate and apterous morphs of A. gossypii, but not by B. tabaci. These findings emphasize the importance of screening insect vectors for the transmission of novel plant virus variants to correctly identify the vector(s) and provide growers and stakeholders with appropriate information to make informed management decisions.

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Reniform nematode (RN, Rotylenchulus reniformis Linford & Oliveria) is a parasite that reduces and limits cotton (Gossypium hirsutum L.) yields. At present, nematicides and crop rotations are the most effective management tool for producers. Our study investigated the use of six composite breeding lines that have quantitative trait loci (QTLs) for RN resistance and the interaction effect of a nematicide under high RN pressure. The study compared these genotypes in both a non-RN field and a RN-infected field under a Velum Total (active ingredients fluopyram and imidacloprid) treatment at Tennessee Valley Research and Extension Center near Belle Mina, AL. In both years, the breeding lines outyielded commercial controls and the parental lines in RN infested field with and without Velum Total. Five out of the six breeding lines contained both chromosome 21 (Ren barb1 and Ren barb2) resistant QTLs, which reduced RN egg numbers (P < .05) and produced higher yields than the commercial checks. These lines can be considered moderately resistant to RN. The remaining line contained one QTL Ren barb1 , produced good yield, and is considered tolerant, despite the fact that it had high RN egg numbers. However, adding a nematicide treatment did increase yields in all material tested. Therefore, further work at increasing resistance from other genetic sources is needed to completely eliminate nematicide use.

Target spot is caused by Corynespora cassiicola, which heavily affects soybean production areas that are hot and humid. Resistant soybean genotypes have been identified; however, the molecular mechanisms governing resistance to infection are unknown. Comparative transcriptomic profiling using two known resistant genotypes and two susceptible genotypes was performed under infected and control conditions to understand the regulatory network operating between soybean and C. cassiicola. RNA-Seq analysis identified a total of 2571 differentially expressed genes (DEGs) which were shared by all four genotypes. These DEGs are related to secondary metabolites, immune response, defense response, phenylpropanoid, and flavonoid/isoflavonoid pathways in all four genotypes after C. cassiicola infection. In the two resistant genotypes, additional upregulated DEGs were identified affiliated with the defense network: flavonoids, jasmonic acid, salicylic acid, and brassinosteroids. Further analysis led to the identification of differentially expressed transcription factors, immune receptors, and defense genes with a leucine-rich repeat domain, dirigent proteins, and cysteine (C)-rich receptor-like kinases. These results will provide insight into molecular mechanisms of soybean resistance to C. cassiicola infection and valuable resources to potentially pyramid quantitative resistance loci for improving soybean germplasm.

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