Little cherry virus

Prunus avium and P. cerasus are the species most sensitive to the effects of little cherry virus. However, the degree of varietal symptom severity varies greatly and is influenced by choice of rootstock. P. avium Lambert develops the most severe fruit symptoms including small, triangular fruit with high shoulders, leather textured patches on the flat faces of the fruit, poor colour development and little or no flavour. Fruit of cv. Hedelfingen is similarly affected. Varieties such as Bing and Early Rivers exhibit shock symptoms with small, poorly coloured fruit for the first 1 or 2 years after infection, but fruit appearance in subsequent years returns close to normal size. The lack of flavour persists. Varieties such as Amber, Merton Bigarreau, Bradbourne Black, Dawson, Florence, Roundel and Napolean exhibit mild fruit symptoms. Partial or complete tolerance of some European (Posnette, 1965) and New Zealand (Wood and McLaren, 1990) varieties has probably led to the virus not being of primary concern in these areas.The impact of little cherry virus on fruit size was greater when cultivars were grown on Mazzard rootstock than on Mahaleb rootstock, regardless of whether P. avium or P. cerasus scions were compared. The differential response to little cherry virus of scions on different rootstocks was also observed when several cultivars of sweet cherry were compared (Milbrath and Reynolds, 1964)In addition to fruit symptoms, sensitive varieties such as Lambert exhibit a slight upward curl of leaves and carry the leaves at an elevated angle during fruit maturation. This has the effect of exposing more fruit to view, and enhances the apparent effect of infection of fruit symptoms.In some varieties, premature leaf reddening is observed 6 to 8 weeks before normal leaf senescence. Cultivars Sam, Mazzard F12/1 and Deacon all exhibit leaf reddening. Canindex 1, a non-commercial sweet cherry, shows extreme leaf reddening and is used as a indicator host for little cherry virus testing (Hansen and Green, 1985).Little cherry virus reduces growth of cv. Lambert trees (Wilks and Milbrath, 1956). This effect is more severe on cultivars such as Canindex 1 and Sam that exhibit the leaf reddening symptom. Infected trees of sensitive varieties produce little or no annual growth.

The impact of little cherry virus(es) varies greatly from one region of the world to another. The sweet cherry industry in western Canada was very severely affected because the industry was reliant on sensitive cultivars of sweet cherry (primarily Lambert) and because the apple mealybug had just been introduced and its populations increased dramatically (Eastwell, 1997). With the introduction of biological control and chemical spray programmes for the apple mealybug, and an aggressive control programme, the sweet cherry industry has been re-established in western Canada. A large number of virus infected trees have been rogued in Northern Germany. They have often been replanted with certified virus-free plant material. The situation remains somewhat difficult until air-borne vector(s) have been identified.In Europe and New Zealand, a selection of tolerant or resistant cultivars of sweet cherry has resulted in limited impact by little cherry disease. The cultivars still experience a reduction in fruit quantity and quality.