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DIFFERENCES BETWEEN TWISTED LEAF AND TOMATO BUSHY STUNT VIRUS IN SWEET CHERRY
It is graft- and bud-transmissible, and can be distinguished from most cherry diseases by the leaf twist symptom which it induces.
The causal agent of TL has never been transmitted to herbaceous hosts.
Abnormal fruit shapes reported from some affected trees are not a regular symptom of TL (Lott and Reeves, 1951).
Tomato Bushy Stunt virus (TBSV) occurs sporadically in Europe and South America on ornamentals, grapes and vegetables (Bercks, 1967; Lovisolo et al., 1965; Pontis et al., 1968; Smith, 1957). It has been detected only once in a rosaceous host, sweet cherry, and this outbreak in Ontario constitutes the only report of TBSV occurrence in North America (Allen and Davidson, 1967).
During studies of cherry fruit disease symptoms in British Columbia, Canada, in 1971 and 1972, an unusual virus was consistently isolated from green and mature ‘Lambert’ fruits affected by a target-like symptom, usually consisting of a black necrotic center 1–2 mm in diameter surrounded by one or two roughly concentric sharp lines (Fig. 1A). Isolates sap-transmitted from affected ‘Lambert’ fruits in four locations, and from misshapen ‘Van’ and ‘Sam’ fruits in two other locations strongly resembled each other on a range of herbaceous hosts.
A type isolate reacted positively with 1:200 diluted antiserum to an English isolate of TBSV, and the other five cherry isolates reacted similarly with an antiserum produced against the type isolate.
All are therefore considered to be closely related or identical to TBSV.
Further orchard studies showed that all trees with fruit symptoms also displayed foliar necrotic lesions.
Where these lesions occurred on the petiole or the main vein, a sharp leaf kink resulted (Fig. 1B). About 35% of the affected trees also displayed a general decline.
TBSV was consistently isolated from spring until mid-June from all affected fruits and leaves, but not from symptomless parts of the same trees, nor from neighbouring healthy-looking trees or from common orchard weeds.
Transmission of TBSV by insertion of buds from infected field trees into healty 2-year-old sweet cherry trees induced the typical leaf symptoms; however, the virus moved only a few cm per year and never invaded these trees completely.
In similar tests with TL-affected trees, no causal agent could be isolated on herbaceous hosts, but the disease was consistently reproduced by insertion of buds from the diseased trees into healthy ones.
Although Twisted Leaf and Tomato Bushy Stunt induce similar and variable symptoms in cherry, and have been confused in the past, the syndromes can generally be distinguished under orchard conditions: TL