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Iris yellow spot virus (IYSV), a new tospovirus associated with a disease in onion (Allium cepa) that is known to growers in Israel as “straw bleaching,” was identified and further characterized by host range, serology, electron microscopy, and molecular analysis of the nucleocapsid gene. The transmissibility of IYSV by Thrips tabaci and Frankliniella occidentalis was studied. IYSV was efficiently transmitted by T. tabaci from infected to healthy onion seedlings and leaf pieces. Two biotypes of F. occidentalis, collected from two different locations in Israel, failed to transmit the virus. Surveys to relate the incidence of thrips populations to that of IYSV were conducted in onion fields. They revealed that the onion thrips T. tabaci was the predominant thrips species, and that its incidence was strongly related to that of IYSV. Forty-five percent of the thrips population collected from IYSV-infected onion and garlic fields in Israel transmitted the virus. IYSV was not transmitted to onion seedlings from infected mother plants through the seed, and was not located in bulbs of infected plants.

Most plant viruses depend on insect vectors for their survival, transmission and spread. They transmit plant viruses by two principal modes, circulative (circulating through the insect's haemocoel, CV) and non‐circulative (carried on the cuticle lining of mouthparts or foregut, NC). Transmissibility and specificity between NC viruses and their vectors depends on the coat protein (CP) of the virus in addition to virus‐encoded helper proteins. Circulative viruses cross the gut, circulate in the haemocoel and cross the salivary glands to render the insect infective. Circulative luteoviruses depend on small CP and the read‐through protein (RTD) for transmission. Electrical penetration graphs have provided evidence on insect feeding behaviour and virus transmission. Recently, studies have shown that viruses can modify vector behaviour in a way that transmission is enhanced. Cultural, physical and novel biotechnological tools can provide virus control by interfering with vector landing and the retention of viruses in their vectors. Key Concepts Most plant viruses rely on insect vectors for survival, transmission and spread. Aphids and whiteflies are by far the most important insects transmitting plant viruses. Some plant viruses are associated to the cuticle of the insect mouthparts (non‐circulative), whereas others are retained in the salivary glands after they circulate through the insect's body (circulative). Some non‐circulative viruses are retained in the common duct of the aphid's maxillary stylets and are inoculated by salivation during brief superficial intracellular probes. Circulative viruses are inoculated during salivation stylet activities in phloem sieve elements. Recent findings in the nature of insect proteins involved in the retention of virus or virus‐encoded helper proteins will help to develop new molecules to interfere with the transmission process.

The nucleotide sequence of the helper component protease (HC-Pro) genes of three zucchini yellow mosaic virus (ZYMV) strains has been compared with that of a helper-deficient strain of ZYMV-HC. The comparisons revealed three unique deduced amino acid differences. Two of these mutations were located in regions which are conserved in other potyviruses. The role of these mutations in aphid transmissibility was examined by exchanging DNA fragments of part of the deficient HCPro gene with the respective section within the gene of the infectious full-length clone of the aphid-transmissible ZYMV. The first exchange included two of the three mutations, the first coding for a change from Asp to Gly (in a non-conserved region) and the second coding for a change from Arg to Ile [within the Phe-Arg-Asp-Lys (FRNK) conserved box]. This exchange resulted in a reduced transmission (20-6% for the mutated virus compared with 57-4% in the normal ZYMV when acquired from plants and 37.2 % compared with 83"1%, respectively, when acquired from membranes). The second exchange incorporated a single mutation [conferring a change from Thr to Ala within the Pro-Thr-Lys (PTK) conserved box]. This single mutation resulted in almost total loss of HC activity in aphid transmission both from plants and from membranes. The Lys residue in the conserved Lys-Ile-Thr-Cys (KITC) box, which is related to loss of HC activity in potato virus Y, tobacco vein mottling virus and in the Michigan strain of ZYMV, is unchanged in the helper-deficient ZYMV. It is therefore proposed that more than one site in HC-Pro may be functionally related to aphid transmissibility. The possible reasons for the role of these mutations in helper activity in aphid transmission of ZYMV are discussed.

An aphid-transmissible (AT) and two non-aphidtransmissible (NAT) isolates of zucchini yellow mosaic virus (ZYMV) were studied. The predicted amino acid sequences of the coat protein (CP) of the three virus isolates were analysed and compared. The NAT isolates differed from the AT isolate in having a Thr instead of an Ala residue at position 10 in the conserved Asp-Ala-Gly triplet in the N-terminal region of CP.Aphid transmissibility was restored in a progeny virus derived from an infectious clone of the ZYMV-NAT isolate in which Thr was changed back to Ala by sitedirected mutagenesis. However this mutation did not have any effect on the multiplication rate in squash, which was significantly higher than that of the AT isolate. The involvement of this mutation in aphid transmission and virus multiplication is discussed.

The titer of chrysanthemum yellows phytoplasma (CYP, "Candidatus Phytoplasma asteris") in the three vector species Euscelis incisus Kirschbaum, Euscelidius variegatus Kirschbaum, and Macrosteles quadripunctulatus Kirschbaum (Homoptera: Cicadellidae) was measured after controlled acquisition from infected Chrysanthemum carinatum (Schousboe) (daisy) plants. Phytoplasma DNA was quantified in relation to insect DNA (genome units [GU] of phytoplasma DNA per ng of insect DNA) by using a quantitative real-time polymerase chain reaction (PCR) procedure. The increase in phytoplasma titer recorded in hoppers after they were transferred to plants that were nonhosts for CYP provides definitive evidence for phytoplasma multiplication in leafhoppers. CYP multiplication over time in M. quadripunctulatus was much faster than in E. incisus and E. variegatus. CYP titer was also highest in M. quadripunctulatus, and this was reflected in the latent period in the insect. The mean latent period of CYP in M. quadripunctulatus was 18 d versus 30 d in E. variegatus. M. quadripunctulatus was the most efficient vector, giving 100% transmission for single insects compared with 75-82% for E. incisus or E. variegatus, respectively. By sequential transmission, we analyzed the time course of transmission: E. variegatus were persistently infective for life or until shortly before death. Occasionally, leafhoppers failed to maintain continuity of infectivity even after completion of the latent period. PCR analysis of transmitter and nontransmitter E. variegatus adults showed that some nontransmitters were CYP positive, whereas others were CYP negative. These findings suggest that both midgut and salivary gland barriers play a role in transmission efficiency.

Potyviruses, as typical non-persistently transmitted viruses, are carried within the stylets of aphids. Cuticle proteins (CuPs), which are a major component of the insect cuticle, were examined for in vitro binding to the potyviral helper component-proteinase (HC-Pro). Proteins in 8 M urea extracts from Myzus persicae were separated by SDS-PAGE, electroblotted onto membranes and identified as CuPs by using specific antibodies to M. persicae CuP. Blotted M. persicae protein extracts were overlaid with two HC-Pros, differing by the presence of K or E in the KLSC domain. The HC-Pro with KLSC, known to assist transmission, was found to bind M. persicae proteins, whereas the HC-Pro with ELSC, being deficient in assisting transmission, did not. To identify CuPs that react with HC-Pro, protein extracts were separated by two-dimensional gel electrophoresis. Nine proteins reacting with HC-Pro were sequenced by mass spectrometry. Sequences of peptides in four proteins, of molecular masses between 22 and 31 kDa, were identified as CuPs according to comparison with sequences in GenBank. The putative CuPs from M. persicae that bind HC-Pro are potentially of interest in locating receptors for virions bound to HC-Pro in aphids' stylets.