Stolbur phytoplasma transmission to maize by Reptalus panzeri and the disease cycle of maize redness in Serbia.

Maize redness (MR), induced by stolbur phytoplasma ('Candidatus Phytoplasma solani', subgroup 16SrXII-A), is characterized by midrib, leaf, and stalk reddening and abnormal ear development. MR has been reported from Serbia, Romania, and Bulgaria for 50 years, and recent epiphytotics reduced yields by 40 to 90% in South Banat District, Serbia. Potential vectors including leafhoppers and planthoppers in the order Hemiptera, suborder Auchenorrhyncha, were surveyed in MR-affected and low-MR-incidence fields, and 33 different species were identified. Only Reptalus panzeri populations displayed characteristics of a major MR vector. More R. panzeri individuals were present in MR-affected versus low-MR fields, higher populations were observed in maize plots than in field border areas, and peak population levels preceded the appearance of MR in late July. Stolbur phytoplasma was detected in 17% of R. panzeri adults using nested polymerase chain reaction but not in any other insects tested. Higher populations of R. panzeri nymphs were found on maize, Johnsongrass (Sorghum halepense), and wheat (Triticum aestivum) roots. Stolbur phytoplasma was detected in roots of these three plant species, as well as in R. panzeri L(3) and L(5) nymphs. When stolbur phytoplasma-infected R. panzeri L(3) nymphs were introduced into insect-free mesh cages containing healthy maize and wheat plants, 89 and 7%, respectively, became infected. These results suggest that the MR disease cycle in South Banat involves mid-July transmission of stolbur phytoplasma to maize by infected adult R. panzeri. The adult R. panzeri lay eggs on infected maize roots, and nymphs living on these roots acquire the phytoplasma from infected maize. The nymphs overwinter on the roots of wheat planted into maize fields in the autumn, allowing emergence of phytoplasma-infected vectors the following July.

First Report of Iris yellow spot virus on Onion (Allium cepa) in Serbia.

Iris yellow spot virus (IYSV; genus Tospovirus, family Bunyaviridae) is established in several European countries (France, Italy, The Netherlands, Poland, Slovenia, Spain, and the UK) and its distribution in the EU region has increased since 2002 (3). In July 2007, symptoms resembling those of IYSV were observed in an onion (Allium cepa) seed crop in the Sirig locality in Serbia. Onion plants exhibited characteristic symptoms of chlorotic or necrotic spindle and diamond-shaped lesions on the leaves and scapes. Symptomatic plants were found throughout the field and disease incidence was estimated at 80%. Leaf and scape samples were tested for the presence of IYSV and two other tospoviruses, Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV), using commercial double-antibody sandwich (DAS)-ELISA diagnostic kits (Loewe Biochemica, Sauerlach, Germany). All samples tested negative for TSWV and INSV. IYSV was detected serologically in 26 of 34 onion samples. To determine an experimental host range, samples of IYSV-infected onion plants were homogenized in chilled 0.05 M phosphate buffer pH 7 containing 1 mM Na-EDTA, 5 mM Na-DIECA, and 5 mM Na-thioglycolate (2), and host plants were inoculated with the sap. Mechanical transmission of the virus occurred rarely. All inoculated test plants were assayed by DAS-ELISA and only four species tested positive for IYSV, but not in all replications. Inoculated Chenopodium quinoa developed local chlorotic lesions, Nicotiana tabacum cvs. Samsun and Prilep showed mild mosaic, while infected N. benthamiana were symptomless. For further confirmation of IYSV, conventional reverse transcription (RT)-PCR was performed on extracts made from symptomatic onion leaf material and from the ELISA-positive symptomless leaves of N. benthamiana. Total RNAs were extracted with an RNeasy Plant Mini Kit (Qiagen, Hilden, Germany) and RT-PCR was carried out with the OneStep RT PCR Kit (Qiagen) following the manufacturer's instructions. The primer pair, IYSV56U/IYSV917L, covering the entire nucleocapsid (NC) gene was used for both amplification and sequencing (1). A product of the correct predicted size (896 bp) was obtained from each of the plants assayed, and that derived from isolate 605-SRB was purified (QUIAqick PCR Purification Kit, Qiagen) and sequenced (GenBank Accession No. EU586203). BLAST analyses revealed 86 to 97% sequence identity with the NC gene from all other IYSV. The highest identity (97%) was with leek and onion isolates (GenBank Accession Nos. EF427447 and EF19888) from Spain. To our knowledge, this is the first report of IYSV infection of onion seed crop in Serbia. Thorough inspections and subsequent testing would be needed to establish the distribution and incidence of IYSV in Serbia. References: (1) I. Robène-Soustrade et al. Plant Pathol. 55:288, 2006. (2) P. Roggero et al. Plant Dis. 86:950, 2002. (3) C. Sansford and J. Woodhall. Pest Risk Analysis for Iris Yellow Spot Virus. Online publication. Central Science Laboratory, Sand Hutton, UK, 2007.