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Potato virus Y (PVY) is a serious plant pathogen, causing severe yield losses worldwide on members of the Solanaceae, including potato, pepper, tomato and tobacco. During the last two decades new virus strains have been detected, including those representing recombinants between N-and O-strains, now designated PVY NTN and PVY N -Wilga, respectively. The question of whether recombination is easily induced in nature by mixed infections of potato might be answered by an investigation of strains appearing under isolated conditions such as those in New Zealand. More than 30 PVY isolates collected during the last 20 years were characterized biologically, serologically and using molecular biological approaches. The New Zealand population of PVY isolates was mainly composed of PVY N and PVY O . To date no recombinant strains have been found among the isolates tested. Similarly, experiments performed with these isolates on potatoes under greenhouse conditions with mixed infection PVY N ⁄PVY O did not result in signs of recombination. This raises the question as to the driving force for the appearance of recombinant strains. It also demonstrates the efficacy of plant quarantine measures in New Zealand over the past 20 years.

The plasmodiophorid Polymyxa graminis transmits plant viruses to cereal crops such as wheat, rye, barley and triticale. Soil samples from different locations and cereal host plants were analyzed for the presence of P. graminis ribotypes I and II, and tested for the occurrence of soil-borne viruses. P. graminis sequences mainly from fields in Germany used for virus resistance trials, but also from a site each in Poland and Denmark were obtained and deposited in the European Nucleotide Archive. The interactions between the components of the pathogen complex -vector ribotype and virus -and the host are discussed.

The complete nucleotide sequence of RNA1 of an Aschersleben isolate of barley mild mosaic virus (BaMMV) was determined. It consists of 7263 nucleotides (nt) excluding the 3′ poly (A) tail. The 5′ and 3′ nontranslated regions (NTR) are 148 and 338 nt in length, respectively, and flank a single large open reading frame coding for a precursor polypeptide with a calculated molecular mass of 256 kDa. Sequence comparison revealed a 96% amino acid (aa) identity to RNA1 translation products of Japanese and French BaMMV isolates. Conserved nucleotide motifs in the 3′ sense and 5′ complementary sense NTR of the two genomic RNAs were identified that may represent the polymerase recognition sites. A range of constructs containing various parts of the coding region of the P3 nonstructural protein was prepared for expression in Escherichia coli. A short stretch of 35 aa in the C‐proximal region of P3 appeared to be highly toxic to the bacterium.

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