Further research on the etiology of cherry chlorotic rusty spot disease
As reported in EPPO RS 97/053, a new disease called ‘cherry chlorotic rusty spot’* was observed in Italy, in a cherry orchard in the province of Avellino (Campania). The disease was first described in May 1992 in a young orchard (3 years old) of 0.5 ha, comprising 120 sweet cherry trees (Prunus avium, cultivars belonging to the Bigarreau group and a local one ‘La Signora’) grafted on sour cherry rootstocks (P. cerasus). Leaf symptoms appeared in spring and were characterized by chlorotic spots which later developed a rusty appearance with small reddish spots. Defoliation was observed later in the season. Fruits were small and deformed with colour alterations (reddish line patterns). In the affected orchard, the first symptoms had in fact already been observed in spring 1989 on 2 trees, and within 3 years 30% of the trees were affected by the disease. Since then, the etiology of cherry chlorotic rusty spot has been investigated. The first hypothesis was that a viral agent was involved but ELISA tests failed to detect any known viruses of stone-fruit trees. In addition, the disease could not be transmitted by grafting to cherry seedlings or peach indicator GF305. Bacteria could not be isolated either. Light and electronic microscopy revealed the presence of mycelium-like structures in naturally infected cherry leaf and fruit tissues. However, it was not possible to isolate any fungus on several nutritive substrates. Molecular studies were undertaken and showed that several dsRNAs (10 to 12) and 2 small circular RNAs were consistently associated with the disease. Further characterization and phylogenetic analysis of 6 of these dsRNAs showed that they are most probably the genomic components of 2 new species of mycoviruses belonging to the genera Chrysovirus and Partitivirus. Therefore, it is hypothetized that ‘cherry chlorotic rusty spot’ is a disease of complex etiology which may involve a fungus, itself infected by at least 2 mycoviruses. Finally, it is recalled that two other cherry diseases showing very similar symptoms had also been described in Emilia-Romagna in 1962 and in the Amasya region in Turkey in 1970 (the disease was there called Amasya cherry disease). It is suggested that these disorders might be identical and caused by the same pathogen(s).
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* EPPO note: Cherry chlorotic rusty spot ‘virus’ was added in 1998 to the EPPO Alert List as a new disease of cherry, but deleted in 2001 in the absence of data on its etiology, distribution and severity.
Sources
Alioto D, Covelli L, Zaccaria F, Di Serio F, Vitale T, Ragozzino A (2005) Cherry chlorotic rusty spot: a disease with complex aetiology in cherry orchards of Campania region. Informatore Fitopatologico no. 3, 45-50.