Introduction
One of the viruses we encounter in our apple blocks that increases in symptom expression when we have cold spring weather is the Apple Mosaic Virus (ApMV). While this virus is usually insignificant economically, it can be alarming when symptoms are observed in both greater incidence and severity during cooler springs.
Apple Mosaic Virus is one of the oldest identified viral diseases, having first been described in 1928, with different strains isolated at the East Malling Research Station in 1951. A species of the Ilarvirus genus, it is closely related to a number of other viruses including Prunus Necrotic Ringspot Virus (PNRV).
It is often found concomitantly with PNRV and Prune Dwarf Virus in stone fruit; and with Apple Chlorotic Ringspot Virus, Apple Stem Pitting Virus, and Apple Stem Grooving Virus in apple.
ApMV has a broad host range that includes both deciduous trees and > 65 species of herbaceous plants. Apple is the most common symptomatic tree fruit host, but ApMV has also been isolated in pear, apricot, sweet cherry, peach, plum, and almond. Some other horticultural hosts in our area are red currant, blackberry, raspberry, strawberry, hazelnut, and hops.
Other woody species that host ApMV include rose, birch, horse chestnut, mountain ash, buckeye and hawthorn.
Symptoms
Expression of symptoms varies depending on cultivar sensitivity, strain virulence, and the growing season, especially early spring temperatures. Infected leaves in apple may be symptomless, but most will develop mosaic patterns of yellow or cream-colored spots and flecks, expanding later to blotches or bands. These are sometimes associated with leaf veins in the early spring and expand into irregular patterns as the season progresses.
The affected areas of the leaves may become necrotic later in the season and sometimes drop in the summer heat.
Not all leaves on an infected tree express symptoms. Symptomatic leaves are often interspersed with symptomless leaves on the same branch.
No fruit symptoms have been observed on any of the varieties we grow commercially here.
The literature varies regarding effect on yield and tree growth, depending on the area and variety. Some reports indicate yield reductions of up to 50% and significant reductions in tree vigor. However, in the PNW, I’ve never seen this virus have any effect on yield, fruit quality, or tree vigor. It appears to be relatively benign with no economic impact that I’ve seen.
Transmission
ApMV is graft-transmissible. The first occurrence of it that I recall seeing was on some Fuji grafts back in the early 1980’s where the scion wood – obviously infected – was smuggled in from Japan in someone’s luggage (apparently Customs either missed or didn’t care about a few apple sticks). As those grafts grew, additional scion wood was harvested from those first grafts and the infection spread with subsequent propagation.
There was some thought years ago that the virus was present in the Malling series of rootstocks, and that served as an initial source of transmission to the field. Today, however, the Malling (M), Malling-Merton (MM), and East Malling / Ashton Long (EMLA) rootstocks are all certified virus-free.
The most common known method of virus transmission in the orchard is root grafting. Transmission through pruning is possible, but difficult even under controlled conditions.
The virus is not transmitted in pollen or in seeds, and there are no known insect vectors. However, some related ilavirus species are vectored by thrips and certain aphids, so it is possible that there is an as yet unidentified insect vector of ApMV.
Control
The only recommended control measure for ApMV in the literature is to use certified virus-free planting material. There are a few different heat treatments that nurseries can use to eliminate the virus from infected tissues, but these aren’t practical for field use:
Heat treatment at 98.6° F for 4 weeks, followed by 104° F for 2 weeks destroys the virus in potted trees. While we did have some extreme temperatures last season, we didn’t have the controlled environment needed to maintain a constant 98.6° F 24/7 for a month, followed by a constant 104° F 24/7 for another 2 weeks. It just felt that way at the time.
Infected budwood can be treated with a hot water bath of either 116.6° F for 30 minutes, or 127° F for 15 minutes. Again, a good treatment for budwood, but not a practical field application.
While there is no field control currently available, the good news is that even though the virus can appear severe I’ve never seen an impact on production, fruit quality, or tree vigor with our varieties and under our growing conditions.
