The following UF/IFAS Pest Alert was provided by Zhang, S. and K. Ling on November 6, 2014.
Shouan Zhang Kai-Shu Ling
Tropical Research and Education Center U.S. Vegetable Laboratory
Department of Plant Pathology USDA-Agricultural Research Service
University of Florida, IFAS Charleston, SC 29414
Homestead, FL 33031 (843) 402-5313
(305) 246-7001 x 213 kai.ling@ars.usda.gov
In April of 2013, some plants of zucchini squash ‘Senator’ in an 80-acre field in Homestead, Florida showed an unusual mosaic disorder on leaves. Symptoms occurred primarily on top young leaves including mosaic, leaf curling, yellowing, and stunting of plant tissues (Fig. 1 & 2). Young leaves were usually small and distorted showing narrow and pointed “fern leaf” symptoms (Fig. 1 & 2). On average, approximately 5% of squash plants were infected, and the most severe problem occurred at one corner of the field where nearly 100% plants demonstrated the symptoms (Fig. 3). It was noticed that an abandoned pepper field full of weeds was right across a road to this area of the field (Fig. 4 & 5). A series of serological tests turned out to be negative against 13 commonly known viruses in cucurbits (Agdia Inc., Elkhart, IN). Using specific molecular technologies, i.e. deep sequencing of small RNAs and assembly, we were able to identify a novel genotype of Tobacco streak virus (TSV) infecting these squash plants (Padmanabhan et al., 2014). To the best of our knowledge, this is the first report of TSV infecting squash in Florida and the U.S. albeit similar symptoms being observed for years in the Homestead area.
Tobacco streak virus (TSV), a member of the genus Ilarvirus, is reported from over 26 countries in the world, and has wide host range infecting more than 200 plant species in 30 plant families (EPPO, 2005). Cucurbits are among the hosts of TSV, including cucumber, bottle guard, and pumpkin, with watermelon (Vemana and Jain, 2010) and squash (Fulton, 1984) being reported as experimental hosts. In the United States, TSV was first reported in Nicotiana tabacum from Wisconsin by Johnson (1936). Other natural hosts of TSV reported in the USA include asparagus, beans, clover, cowpea, and soybeans.
TSV is efficiently transmitted in fields by thrips (Frankliniella occidentalis, Thrips tabaci) and through mechanical means, and could be seed transmitted in some plant species. The existence of diverse genotypes of TSV with distinct serological reactivity and variable symptom expression makes it difficult to accurately diagnose the disease.
The identification of this emerging TSV isolate with diverse genotypes to the currently known TSV in the U.S. is important to understand the nature of its broad host range. The wide distribution of vector thrips species in the U.S. also possesses another serious threat to the American agriculture. How this novel genotype of TSV was introduced to the U.S. was unknown. Seed transmission of TSV should be considered a potential for long distance dispersal of the seed-borne pathogen through international seed trade. It is recommended to use seed that has been tested free of TSV in commercial vegetable production. Due to the high sequence diversity, the current commercial ELISA kit for TSV may not be so reliable in detecting this new genotype. Therefore, development of a sensitive and reliable molecular technique that is capable of detecting all TSV genotypes would be necessary and imperative.
Control of TSV is difficult. It is always wise to use virus-free seed and to control weed hosts. Maintaining insect control programs is advised even though controlling the vectors alone will not provide sufficient control of TSV.
References:
EPPO 2005. PQR database (version 4.4). Paris, France: European and Mediterranean Plant Protection Organization.
Fulton, R. W. 1984. Plant Viruses Online – Tobacco streak ilarvirus. http://pvo.bio-mirror.cn/descr811.htm.
Johnson, J. 1936. Tobacco streak, a virus disease. Phytopathology 26: 285.
Padmanabhan, C., Gao, S., Li, R., Zhang, S., Fei, Z. and Ling, K.-S. 2014. Complete genome sequence of an emerging genotype of Tobacco streak virus in the U.S. Genome Announc. 2(6): e01138-14. doi:10.1128/genomeA.01138-14.
Vemana, K. and Jain, R. K. 2010. New experimental hosts of Tobacco streak virus and absence of true seed transmission in leguminous hosts. Indian J. Virol. 21(2):117–127
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