Cause Apple flat apple disease is caused by Cherry rasp leaf virus (CRLV). This virus is transmitted by dagger nematodes, Xiphinema americanum. The virus is believed to have originated in native vegetation and can move between cherry and apple plantings. Apple trees planted on sites that previously contained CRLV-infected cherry trees will become infected. Many weed species that inhabit the orchard floor are hosts to the virus. They are reservoirs of infection but do not exhibit any obvious symptoms. Since the virus is nematode transmitted, it spreads slowly in the orchard.
The virus has been found in eastern Washington and Hood River Oregon.
Symptoms Flat apple symptoms are most severe on 'Red Delicious' and related cultivars. The length of the fruit is significantly reduced and the stem cavity becomes very shallow. At first, only a few fruit will become symptomatic on lower limbs, but eventually, the entire tree will be involved.
- The use of virus-tested (and found to be free of all known viruses) planting material is important for control.
- Avoid future orchard sites that test positive for the presence of the nematode.
Chemical control Fumigation between replanting may only delay disease development.
References Parish, C.L. 1977. A relationship between flat apple disease and cherry rasp leaf disease. Phytopathology 67:199-202.
Wagnon, H.K., Traylor, J. A., Williams, H.E., and Weiner, A.C. 1968. Investigations of cherry rasp leaf disease in California. Plant Disease Reporter 52:618-622.