In Japan a general decline of apple trees (referred to as Taka tsugi byo) occurs frequently after old trees are top-grafted with any of several new apple varieties.
This disease occurs most often in trees growing on Malus prunifolia var. ringo (Maruba kaido) rootstocks.
However, it also occurs in some trees growing on M. sieboldii (Mitsuba kaido) and M. sieboldii var. arborescens (Kobano zumi) rootstocks.
Indexing tests demonstrated that apple chlorotic leafspot virus (CLSV) was present in all declined trees and in most apparently healthy trees tested.
Stem pitting virus (SPV) and stem grooving virus (SGV) were detected in some declined trees as well as some apparently healthy trees.
Inoculations with single latent virus isolates from America demonstrated that type isolates of CLSV induced characteristic decline symptoms in Maruba kaido cuttings but remained symptomless in seedlings of Mitsuba kaido and Kobano zumi.
In contrast, SPV isolates induced decline in Mitsuba kaido and Kobano zumi, while remaining symptomless in Maruba kaido.
These results indicate that CLSV-type strains cause topworking disease in trees propagated on Maruba kaido rootstocks whereas SPV causes the disease in trees propagated on either Mitsuba kaido or Kobano zumi rootstocks.
A strain of CLSV (CLSV-Maruba) was isolated from symptomless Maruba kaido cuttings that differed from CLSV-type strain in reactions produced in woody indicator plants.
This strain appeared similar to the CLSV-Hopa strain in that neither produced symptoms on Hopa crab or Maruba kaido.
Two CLSV-isolates, one of which was sap-transmitted to Chenopodium quinoa from a topworking diseased 'Golden Delicious' tree (P-1) and another from an apparently healthy Maruba kaido rootstock (MO-17), were purified and characterized.
The two isolates could be distinguished by symptoms on C. quinoa and by spur reactions in agar gel diffusion tests, but were indistinguishable in other herbaceous plants, in sedimentation, particle morphology, or general properties.
From the results obtained here, it is clear that use of scions free from both CLSV-type strain and SPV is the best method to control top-working disease of apples in Japan.