Studies on the resistance of potato varieties to virus X

I. On the virus causing top necrosis in May Queen potatoes

Abstract

Ten to eighty percent of the potato plants of variety May Queen were found to show top necrosis when tuber-grafted with virus-X-infected potatoes of eight varieties, but show no reaction when grafted with virus-X-free varieties. Hence the virus which induces top necrosis on May Queen was considered to be closely related to or associated with virus X. Some May Queen plants did not show top necrosis, although the donors were undoubtedly infected with virus X. To confirm this, cores from the same virus-X-infected tuber were grafted to different potatoes of May Queen. The reaction of the test plant varied with individuals.
Fortunately, a May Queen plant showing mosaic symptoms, infected with virus X, was found in 1959 at Tsumagoi Potato Foundation Stock Farm, Gunma Prefecture. By using this material, cross protection tests were made. The results showed that the top-necrosis-causing virus was completely antagonistic to X-virus, which originated in May Queen (Tab. 4) Cockerham (1943) identified the virus causing top necrosis on May Queen as virus B, but other workers have considered this virus B to be a strain of X-virus. Bawden (1944) demonstrated that his top-necrosis had incomplete cross-protective reaction with X-virus. It seems that the virus causing top necrosis on May Queen in Japan is a strain of virus X, which may be related to B, but it differs from B in the cross protective activity with virus X.