92 | Chapter 18
I) and two European almond (Venza) and apple (H) isolates were identified to have insertions of 6 nt to 15 nt after nucleotide position 141, in comparison with consensus apple isolates sequence. The almond isolate has a CP 220 aa long, with 2 aa additions due to the insertions of 6 nt (9 nt insertion and deletion of a consensus AAG codon). The length of the CP gene of an isolate from plum sequenced in the same study was 218 aa and was the same as that for the previously published isolate from P. mahaleb.
Economic Impact The presence of ApMV in stone fruits is recorded occasionally. The infection may result in growth reduction and yield losses in susceptible cultivars (Nemeth, 1986; Diekmann and Putter, 1996), however, no precise information on economic losses is available. ApMV infection also exerts a detrimental effect on leaf content of bittering acids in hops, decreasing alpha acid yields by 5–34%, can cause important yield reduction in hazel (Aramburu and Rovira, 1995; Aramburu and Rovira, 2000), and causes reduction of apple bud take in nurseries (Nemeth, 1986; Crowle et al., 2003). In some Mediterranean countries, ApMV is economically important on almond. In particular, old cultivars are heavily affected by the virus in mixed infections with other viruses and diseased plants decline and show yield reduction.
Symptoms The leaves of infected trees almond, plum, apricot, peach, and cherry show typical yellow line pattern, bright yellow blotches, rings, bright yellow vein clearing, and oak-leaf pattern (Figs. 18.1 and 18.2) (Posnette and Ellenberger, 1957; Canova, 1960; Ellenberger, 1962; Nemeth, 1986; Diekmann and Putter, 1996). Symptoms generally appear at the beginning of summer and, in some cases, are present only on a limited number of leaves randomly distributed on the plants. Bright chrome yellow discolorations on leaves in the form of patchy or more or less widespread mottling, ringspot and line patterns are caused by the presence of ApMV in almond (Savino et al., 1995) (see pertinent chapter in this volume). In some sensitive almond cultivars, the virus induces failure of blossom and leaf buds to grow, a symptom known as almond leaf failure (Diekmann and Putter, 1996; Desvignes et al., 1999).
The symptomatology is generally not of diagnostic significance, because similar symptoms may be produced on Prunus spp. by other Ilarviruses, e.g., PNRSV and APLPV. Moreover, some peach cultivars may fail to display any symptoms (Choueiri et al., 2001), so that laboratory tests are needed for the reliable identification of the virus.
Host Range The natural host plants of ApMV are various Prunus spp., such as P. armeniaca L., P. cerasifera L., P. domestica L., P. instititia L., P. mahaleb L., P. persica L., P. salicina L., P. serrulata L., P. amygdalus L., P. triloba L., P. cerasus L., and P. avium L. ApMV also occurs naturally in Malus spp., Rubus spp, Rosa spp., birch (Betula spp), hop (Humulus lupulus), horse chestnut (Aesculus hippocastanum) and red horse chestnut (A. carnea), hazelnut (Corylus avellana) and Chenomeles japonica (Sweet, 1980; Nemeth, 1986; Diekmann and Putter, 1996; Desvignes et al., 1999; ICTVdB, 2002). Experimentally, over 65 herbaceous plant species in 19 families are susceptible (ICTVdB, 2002). Among these, several diagnostic hosts are used as indicators in biological tests (EPPO, 2000).
Transmission No insect vector is known for ApMV and the virus is only transmitted by vegetative propagation of rootstocks and planting material from infected mother trees, and by graft-inoculation of woody plants. The virus can be sap-transmitted by mechanical inoculation, but not easily, to several herbaceous plants such as Cucumis sativus, Petunia hybrida, Chenopodium quinoa, C. amaranticolor, Cucurbita maxima, C. pepo, Nicotiana benthamiana, and N. megalosiphon. Petal extracts prepared in phosphate buffer containing antioxidants such as sodium diethyldithiocarbamate, sodium thioglycolate, and ascorbic acid represent the best inoculum for mechanical transmission. ApMV has not been detected in pollen grains from infected plum and apricot trees, either externally or internally (Digiaro et al., 1992), which suggests that it is not pollen-transmissible. Similarly, the results of Barba et al. (1986) proved that ApMV is not transmissible by seeds of infected almond trees, in agreement with the findings of Sweet (1980) who suggested that ApMV is not seed-transmissible in Aesculus species.
Geographical Distribution and Epidemiology ApMV is distributed worldwide, more commonly on Prunus spp. than on Malus spp. in Europe. It is often found in mixed infection with PNRSV and PDV, but the frequency of ApMV infection seems to be much lower than of these two other viruses, especially in plum, apricot, peach, and cherry (Petrzik and Lenz, 2002; Myrta et al., 2003).
Fig. 18.1. Diffuse yellow discolorations on leaves of an almond plant naturally infected with ApMV.
Fig. 18.2. Yellow line pattern on cherry leaves infected with ApMV.