1975) SALAZAR AND JONES: MOP-TOP VIRUS IN PERU 143 SOME STUDIES ON THE DISTRIBUTION AND INCIDENCE OF POTATO MOP-TOP VIRUS IN PERU L. F. Salazar and R. A. C. Jones* Abstract Symptoms of PMTV are distributed widely in potatoes in the highlands (Andes) of Peru, but have not been found in coastal plantings; they were observed in 9 out of 22 Peruvian Departments. Incidences of over 10 % of affected plants in one or more crops were found in 14 different localities. Thirteen cultivars with symptoms were observed, 9 were Solanum tuberosurn subsp, andigena Juz. & Buk., 3 were hybrids ofS. tuberosum subsp, andigena x S. tuberosum subsp, tuberosurn L orS. Curtilobum Juz. & Buk. and 1 was S. juzepczukii Buk. Symptoms were also seen in 29 clones of the potato germplasm collection of the International Potato Center. Distribution and prevalence of PMTV in Peru appear to be related to those rainfall and temperature conditions that favour its fungus vector Spongospora subterranea (Wallr.) Lagerh. Resumen Los sintomas de PMTV estfin distribu/dos amp!iamente en los cul- tivos de papa en la sierra (Andes) del Peril, pero no se han encontrado en plantaciones de la costa; estos fueron observados en 9 de los 22 Departa- mentos del Peril. En 14 localidades diferentes se han encontrado inciden- cias mayores a 10% de plantas afectadas en uno o mils cultivos. Se observa- ron 13 cultivares con sintomas, de los cuales 9 fueron Solanum tuberosum subsp, andigena Juz. & Buk., 3 tu~bridos de S. tuberosum subsp, andigena • S. tuberosum subsp, tuberosum L. 6 S. curtiloburn Juz. & Buk. y uno rue S.juzepczukii Buk. Tambi~n se han observado sintomas en 29 clones de la colecci6n del germoplasma de papa del Centro Internacional de la Papa. Parece que la distribuci6n y prevalencia del PMTV en el Peril estfi re- lacionada alas condiciones de Iluvia y temperatura que favorecen a su hongo vector, Spongospora subterranea (Wallr.) Lagerh. Introduction Potato mop-top virus (PMTV) has been found in several countries in Europe (3, 13, 18) and in Peru (9, 12, 17). It is one of a number of viruses which cause yellow symptoms in leaves of Peruvian potatoes but it can be *Associate and post doctorate virologists respectively, Pathology Dept., The International Potato Center, Apartado 5969, Lima, Peril Received for publication May 22, 1974