ISHS Acta Horticulturae 193: XIII International Symposium on Fruit Tree Virus Diseases
OBSERVATIONS ON THE ETIOLOGY OF SOME VIRUS DISEASES OF APPLE AND PEAR |
| Author: | F.A. van der Meer |
| Abstract:
During the last two years the presence of a sap transmissible virus in apple and pear has been found to be correlated with apple stem pitting (Posnette and Cropley, 1963), pear vein yellows (Posnette, 1963) and pear stony pit (Kristensen, 1963). The virus has a very narrow range of herbaceous hosts. Out of 93 Nicotiana accessions (kindly supplied by Dr. V.A. Sisson, Tobacco Research Laboratory, Oxford, N.C., USA) only N. occidentalis 37B reacted with clear symptoms, i.e. systemic vein yellowing and vein mosaic, sometimes followed by leaf epinasty and growth reduction. Out of many plants commonly used in plant virology, Chenopodium murale was the only other host that developed symptoms, viz. pin point local lesions in inoculated leaves. The virus is difficult to detect in trees that are also infected with apple chlorotic leaf spot virus (CLSV) (Lister, 1979) because many strains of CLSV induce severe systemic mosaic in N. occidentalis 37B. Recognizing the symptoms of the new virus becomes impossible then. Isolates from stem pitting-infected apples and isolates from vein yellows and stony pit-infected pears cause the same symptoms in N. occidentalis 37B, suggesting that apple stem pitting, pear vein yellows and pear stony pit are caused by identical or by similar viruses. However, further characterization, back transmission from herbaceous hosts to apple and pear and further tests concerning the correlation between the three diseases and the newly isolated virus are necessary to confirm this. CLSV isolates obtained from five different russet ring-affected apple trees were graft transmitted from Chenopodium amaranticolor to seven out of 42 grafted apple seedlings. These seven seedlings were indexed on the usual range of indicators. Two of the seedlings were found to induce russet ring in Belle de Boskoop apple and mild but definite scaly bark symptoms in Malus platycarpa. All seven seedlings induced the usual CLSV symptoms in M. platycarpa, Spy 227 and R 12740–7A apple, however, there was much difference in severity of symptoms. Of two seedlings, infected with CLSV from the same tree, one induced russet ring and scaly bark (Luckwill and Campbell, 1963), whereas the other did not. Of two other seedlings, infected with CLSV from another tree, neither one induced russet ring or scaly bark. Within both pairs of seedlings clear differences in severity of symptoms were noticed on the three CLSV indicators, showing that clearly different CLSV strains can be obtained from the same tree. By grafting a Belle de Boskoop containing all latents (Mink et al., 1971) and showing severe russet ring, to seedlings of Cotoneaster species, several CLSV sources were obtained that were free of the other apple latents. Some of these sources induced russet ring and leaf pucker/leaf flecking (Welsh and May, 1967) on Belle de Boskoop, Golden Delicious and Lord Lambourne, although the parent Belle de Boskoop tree and its offspring never showed leaf pucker/leaf flecking. Other sources induced only moderate russet ring on Belle de Boskoop but not on Golden Delicious, while some sources did not induce symptoms on either of these varieties. | |
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