Greenhouse Comparison of Two Detection Methods for Potato virus YN-Wi at Four Potato Growth Stages

Potato virus Y (PVY) causes significant crop and monetary losses. Owing to the prevalence of newly emerging strains of PVY such as PVYN-Wi, which often cause asymptomatic to mild reactions on certain potato cultivars, accurate tools are required to detect the virus in potato production. This study compared the sensitivity of a rapid field detection method (immunostrips) with a common laboratory detection method (triple antibody sandwich ELISA) on cultivar Chieftain, grown under isolated conditions in a greenhouse and mechanically inoculated with PVYN-Wi, at four potato growth stages (emergence, preflower, postflower, and senescence). Plants inoculated at emergence displayed severe symptoms of mosaic, veinal necrosis, and leaf drop. Plants inoculated at preflower, postflower, and senescence had veinal necrosis but low or no incidence of mosaic and leaf drop. Overall, few or no tuber symptoms were observed, but a trend of lower tuber yield occurred for emergence-inoculated plants. Low variability in PVYN-Wi detection occurred in both tests for emergence-inoculated plants, whereas those inoculated at preflower and postflower had more variability. Because symptom expression may differ depending on the growth stage when a plant becomes infected, these variations should be heeded with either detection method when collecting samples for PVY testing.

Veinal Necrosis Induced by Turnip mosaic virus Infection in Arabidopsis Is a Form of Defense Response Accompanying HR-Like Cell Death

In the pathosystems of Turnip mosaic virus (TuMV) with Brassicaceae crops, various symptoms, including mosaic and necrosis, are observed. We previously reported a necrosis-inducing factor TuNI in Arabidopsis thaliana, a model species. In this study, we show that the necrotic symptom induced by TuNI, observed along the veins, was actually a form of defense response accompanying a hypersensitive reaction (HR)-like cell death in the veinal area. The virus is often localized in the necrotic region. The necrotic response is associated with the production of H2O2, accumulation of salicylic acid (SA), emission of ethylene, and subsequent expression of defense-related genes. Additionally, this HR-like cell death is eased or erased by a shading treatment. These features are similar to the HR-associated resistance reaction to pathogens. However, unlike HR, two phytohormones—SA and ethylene—are involved in the necrosis induction, and both SA- and ethylene-dependent pathogenesis-related genes are activated. We concluded that the veinal necrosis induced by TuMV is regulated by a complex and unique network of at least two signaling pathways, which differs from the signal transduction for the known HR-associated resistance.

Identification and Characterization of a Carlavirus Causing Veinal Necrosis of Coleus

A filamentous virus identified in coleus (Coleus × hybrida) in Minnesota and New York was found to cause veinal necrosis in coleus, although this symptom was observed only under certain conditions. The virus was transmitted readily by mechanical inoculation to coleus and Nicotiana spp. and was not transmitted by Myzus persicae. The particles of the coleus virus had a modal length of 640 nm and a single capsid protein with an estimated molecular mass of 34 kDa. The amino acid sequence of the coat protein region of the coleus virus genome had significant similarities only to the corresponding domain of carlaviruses. Based on virion morphology, capsid protein size, genome size and organization, amino acid sequence, and phylogenetic analyses, the coleus virus, which was named provisionally Coleus vein necrosis virus (CVNV), was concluded to be a new definitive member of the genus Carlavirus. A 2-kb fragment of the 3′ terminus of the CVNV genome sequence is accessible under accession number DQ915963 in GenBank.

First Report of Tobacco streak virus Infecting Safflower (Carthamus tinctorius) in Maharashtra, India

Safflower, Carthamus tinctorius L. (Asteraceae), is extensively cultivated in India, China, and other parts of Asia for edible oil, dyeing agent, and its medicinal value. In 2003, safflower entry (NARI-6) in the All-India Coordinated Research Project on Oilseeds (Safflower) grown in the experimental fields of M/s Syngenta India Ltd., Aurangabad (Maharashtra State, India) exhibited symptoms of veinal and leaf necrosis, necrotic streaks on the stem, necrosis of the terminal bud, and ultimately plant death. The disease was attributed to Tobacco streak virus (TSV) because sunflower growing adjacent to safflower showed similar symptoms caused by TSV (1). Mechanical inoculations of sap from symptomatic safflower leaves caused typical symptoms of TSV (local, irregular, necrotic rings, veinal necrosis, and systemic veinal necrosis) on Vigna unguiculata (L.) Walp. cv. C-152 and Phaseolus vulgaris (L.) cv. Topcrop, and symptoms of local, necrotic lesions, veinal necrosis, and systemic necrosis of leaf and growing bud on Arachis hypogaea L. cv. JL-24. Sap-inoculated safflower cv. Manjeera showed chlorotic and necrotic local lesions followed by systemic leaf necrosis, leading to necrosis and death of the terminal bud. Safflower cvs. A-1, BIP-2, Co-1, and Bheema (10 plants of each cultivar) inoculated with sap from safflower plants showing typical TSV symptoms did not show any visible symptoms except stunting, but six to nine plants of each cultivar tested positive for TSV using enzyme-linked immunosorbent assay (ELISA) tests. In direct antigen coating-ELISA, the virus reacted positively with antiserum produced to an isolate of TSV from peanut (2) and to antiserum to TSV (ATCC-PVAS 276 for Datura stramonium), but did not react to peanut bud necrosis tospovirus antiserum. Examination of leaf extracts using leaf-dips and immunosorbent electron microscopy with the antiserum of TSV-peanut isolate showed isometric particles resembling those in the genus Ilarvirus. To our knowledge, this is the first report of an isolate of Tobacco streak virus infecting safflower. References: (1) R. D. V. J. Prasada Rao et al. J. Oilseeds Res. 17:400, 2000. (2) A. S. Reddy et al. Plant Dis. 86:173, 2002.

First Report of the Necrotic Strain of Potato virus Y (PVYN) on Potatoes in the Northwestern United States

More than 50 isolates of Potato virus Y (PVY) with characteristics of strains that cause tobacco veinal necrosis (PVYN) were obtained from potatoes (Solanum tuberosum L.) grown in the northwestern United States. These isolates are being characterized at the biological and molecular levels. Isolate RR1 was obtained from leaves of potato cv. Ranger Russet showing distinct mottling and leaf deformity, which is in contrast to the leaf-drop and necrosis usually observed with ordinary strains of PVY (PVYO) in this variety. Isolate AL1 was obtained from tubers of potato cv. Alturas showing distinct internal light brown rings and blotches. When RR1 and AL1 were transmitted to tobacco (Nicotiana tabacum L. cvs. Samsun NN and 423), they caused systemic veinal necrosis, including stem and petiole lesions typical of PVYN strains (2). Symptoms induced by RR1 and AL1 on tobacco appeared 9 to 11 days after inoculation, whereas some other isolates caused delayed veinal necrosis. All isolates that produced veinal necrosis on tobacco were detectable with PVY polyclonal antisera. Potato virus X was not detected by enzyme-linked immunosorbent assay in tobacco plants showing veinal necrosis. Some isolates, including AL1, failed to react in serological tests using PVYN-specific monoclonal antibodies obtained from three commercial sources. Other isolates, including RR1, were detectable with these monoclonal antibodies. Reverse transcription-polymerase chain reaction (RT-PCR) products obtained with primers specific for the coat protein (CP) open reading frame (ORF) were cloned and sequenced. AL1 possesses a CP more closely related to PVYO type isolates, which would account for its failure to react with PVYN monoclonal antibodies. In this regard, AL1 is similar to the PVYN-Wilga isolate (1). Other isolates that are detectable with the PVYN monoclonal antibodies possess a CP more consistent with N strains of the virus. Results of RT-PCR tests using primers derived from the P1 ORF sequence (3), and the restriction enzyme analysis and sequencing of the RT-PCR products, all suggest that AL1 and RR1 are related to European-type members of PVY tuber necrotic (NTN) or N strains. However, other isolates under investigation appear to be more closely related to previously reported North American NTN types (3). The symptomatology of these viruses on tobacco and potato, and the serological and molecular data clearly show that at least two distinct variants of PVYN have been found for the first time in a major potato production area of the United States, and pose a potential threat to the potato industry. References: (1) B. Blanco-Urgoiti et al. Eur. J. Plant Pathol. 104:811, 1998. (2) J. A. de Bokx and H. Huttinga. Potato virus Y. Descriptions of Plant Viruses. No. 242, CMI/AAB, Surrey, England, 1981. (3) R. P. Singh et al. Can J. Plant Pathol. 20:227, 1998.