Diseases of lettuce (
Reported virus diseases
Several viruses have historically been severe problems of lettuce in Arizona although their incidence and severity fluctuate. These are listed with their host range, symptoms, vector and available control strategies:
Lettuce infectious yellows virus (LIYV) infects a very wide variety of crops including: cucurbits, spinach, carrots and cilantro; and of weeds including: cheeseweed, dock, wild lettuce, sowthistle, sowbane, sunflower and Physalis; symptoms vary with time of infection, planting date, and variety, but in general plants are stunted with overall yellowing and leaf margins of older leaves may turn brown (photo 1); it is transmitted by Bemisia tabaci, the sweet potato whitefly; disease was very severe throughout Arizona in the 1980's but has not been a problem recently since the race of Bemesia that vectors the virus has all but disappeared in recent years.
Lettuce mosaic virus (LMV) infects a very wide variety of hosts, including many common weeds and all types of lettuce; the virus causes mosaic and mottle symptoms in all types of lettuce but vary according to virus strain, time of infection and temperature; early infected plants are often stunted; it is transmitted by seed and aphids; infection can be prevented by planting only mosaic free seed, destroying weeds around the field, and planting new lettuce fields as far as possible from old fields.
Cucumber mosaic virus (CMV) infects cucurbits, tomatoes, peppers, spinach and many other hosts including many weeds; the symptoms are very similar to LMV but is not seed borne like LMV so distribution within the field is usually along margins; the virus is transmitted by several species of aphids including green peach and melon aphids in a non-persistent manner; disease can be prevented by planting in fields at least one mile from vector sources.
Beet western yellows virus (BWYV) infects many hosts including beets, spinach, broccoli, cauliflower, radish and lettuce; the virus causes stunting and yellowing of foilage and is difficult to distinguish from other problems such as nutrients, poor drainage or other viruses; it is transmitted persistently in aphids, especially the green peach aphid, so it can be carried for some distance; disease has not been a problem in Arizona.
Alfalfa mosaic virus (AMV) has many different strains infecting alfalfa, peppers, potatoes and many other hosts; it causes brilliant yellow blotches of color in lettuce leaves; the virus is transmitted in a non-persistent manner by many different species of aphids including the pea aphid, the alfalfa aphid and spotted alfalfa aphid; it can be controlled by planting lettuce at some distance from alfalfa or other hosts.
Big vein is caused by a virus-like agent that causes a pronounced clearing of the chlorophyll next to major veins which is very prominent when leaves are held up to bright light (photo 1) and (photo 2); the symptoms appear in cool weather about one month after seeding but are prevented when daily temperatures are constantly above 70 F ; big vein is transmitted by motile spores, zoospores, of the soil borne fungus, Olpidium brassicae; disease can be controlled by planting tolerant varieties and avoiding fields with known history of disease.
February 20, 2013