THE LINE‐PATTERN VIRUS DISEASE OF PLUMS

The disease, now usually called line‐pattern of plum, has been described under many names in most countries where plums are grown extensively. Naturally infected trees show widely differing Symptoms; this has two causes: (1) different varieties react differently to the same isolate of the virus, and (2) different isolates cause different symptoms in the same variety. Because the virus occurs in strains with different pathogenicities, the choice of indicator varieties is important when selecting virus‐free material by transmission tests. Peach seedlings and the mazzard clone, F 12/1, were the most sensitive types found. The line‐pattern virus does not become fully systemic in some varieties of plum. In this and other respects, it resembles the viruses that cause apple mosaic; three isolates obtained from plum and two from apple produced similar Symptoms in peach and apple. It is therefore suggested that plum line‐pattern and apple mosaic viruses are caused by strains of one virus.