- Author: Steven T. Koike
Coastal California growers, pest control advisors, and other field professionals might be on the alert for early outbreaks of lettuce problems caused by viruses. In particular, early confirmations have been made of the Tomato Spotted Wilt Virus (TSWV) on lettuce in this region. Affected leaves have tan, brown, or blackish spots and dead areas; this necrotic tissue can resemble damage caused by pesticide or fertilizer applications (see photos below). Some leaf yellowing can also be observed. If disease is advanced, symptoms can also be found on the newer leaves near the center of the plant axis. If plants are affected early in their development, growth can be stunted. Symptoms caused by TSWV in lettuce are indistinguishable from symptoms caused by the closely related Impatiens necrotic spot virus (INSV) which in some seasons can be commonly found on lettuce in the Salinas Valley. All lettuce types (iceberg, romaine, greenleaf, redleaf, butter) are susceptible to both TSWV and INSV. Both viruses are spread by thrips, are found in hundreds of crop and weed species, and are not seedborne. In our coastal region, TSWV and INSV are spread only by the western flower thrips (Frankliniella occidentalis).
Our laboratory has confirmed TSWV on lettuce in late March, which is rather early for such developments. PCAs also report early buildups of thrips populations in lettuce and other fields. The relatively dry spring is resulting in early drying up and senescing of weeds and hillside vegetation; this early decline of surrounding vegetation very likely is driving thrips populations into fields earlier than normal. Lettuce growers should be aware of possible early problems due to thrips vectoring tospoviruses into their fields.
Lettuce with tospovirus-like symptoms can be sent for analysis to the UC Cooperative Extension diagnostic laboratory in Salinas.
Tomato spotted wilt on lettuce.
Tomato spotted wilt on lettuce.
Impatiens necrotic spot on lettuce
Impatiens necrotic spot on lettuce
- Author: Steven T. Koike
Beginning in early April, 2012, the UC Cooperative Extension diagnostic lab in Salinas began to receive lettuce samples exhibiting obvious symptoms of a virus problem. Samples continued to be submitted throughout the month. All samples tested positive for the thrips-vectored Impatiens necrotic spot virus (INSV). This virus first began to cause damage to California lettuce in 2006. Since that time, INSV has occurred to a greater or lesser degree every season. However, significant INSV outbreaks usually are found beginning in late May or early June. Seeing INSV on lettuce in April is therefore unusual and growers and pest control advisors should carefully monitor these situations.
INSV-infected plants have leaves with brown to dark brown spots and dead (necrotic) areas; this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications (see photos below). Affected leaves can be distorted and twisted. Extensive necrosis can cause much of the leaf to become brown, dry, and dead. Some leaf yellowing can also be observed. Yellowing and the brown spotting tend to be observed on the newer leaves near the center of the plant’s growing point. If plants are affected with INSV early in their development, growth may be stunted. All lettuce types are susceptible, and INSV has been confirmed on iceberg, butterhead, romaine, and leaf lettuces. INSV can infect many other crops and weed species; the virus is vectored by the Western flower thrips (Frankliniella occidentalis).
Growers, PCAs, and other field personnel should exercise caution in diagnosing INSV disease because the lettuce dieback viruses cause very similar symptoms (see table below). Lettuce dieback is caused by two pathogens: Lettuce necrotic stunt virus (LNSV) and Tomato bushy stunt virus (TBSV). Infected lettuce can be severely stunted, especially if infected early in plant development. The oldest, outer leaves can be severely yellowed. Brown, necrotic spots and lesions later develop in these outer leaves. The younger, inner leaves remain dark green in color, but can be rough and leathery. LNSV/TBSV infects only romaine, butterhead, and leaf lettuces; modern cultivars of iceberg lettuce are immune. The LNSV/TBSV virus complex is a soilborne problem and no vector (insect, nematode, fungus) is known to spread these viruses.
For help in diagnosing these and other plant problems, submit samples to the UC Cooperative Extension diagnostic lab in Salinas.
Photos: Impatiens necrotic spot virus (INSV) on lettuce.
- Author: Steven T. Koike
As the coastal California lettuce crop heads into the ending fall season and as the inland county region initiates its fall lettuce cycle, both crops are being affected by Tomato spotted wilt virus (TSWV). Diseased plants have leaves with irregularly shaped, brown to dark brown lesions and dead (necrotic) areas (Photo 1); this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications. Chlorosis (yellowing) can also be observed. Depending on the age of the plant when first infected, these necrotic and chlorotic symptoms can occur on both the older, outer foliage as well as the younger, inner leaves. If plants are affected with TSWV early in their development, growth may be severely stunted. The virus is vectored by thrips and in California is primarily spread by the western flower thrips (Frankliniella occidentalis) (Photo 2).
All lettuce types are susceptible, and TSWV has been identified in iceberg, butterhead, romaine, and leaf lettuces. TSWV has an extremely wide host range that includes over 500 crop and weed species. Vegetable crop hosts include basil, bean, celery, cucumber, eggplant, endive, escarole, fava bean, lettuce, pea, pepper, potato, radicchio, spinach, and tomato. This host range may explain, in part, why TSWV has been observed in a number of lettuce fields in the San Joaquin Valley. The relatively cool summer temperatures have resulted in delayed tomato harvests, causing an overlap of the summer tomato and fall lettuce crops. Thrips vectoring TSWV are therefore able to readily move from the late tomato plantings and into the lettuce fields. (For related information see research conducted by the Gilbertson team (UC Davis) and sponsored by the California Processing Tomato group.)
Growers and pest control advisors should exercise caution if attempting to identify TSWV in the field and without testing. In the coastal region, symptoms caused by the very closely related, thrips-vectored Impatiens necrotic spot virus (INSV) are identical to those caused by TSWV. Romaine and leaf lettuces are susceptible to the lettuce dieback virus complex (Lettuce necrotic stunt virus [LNSV] and Tomato bushy stunt virus [TBSV]) which is also common on the coast.
Photo 1: Tomato spotted wilt virus (TSWV) on lettuce.
Photo 2: Western flower thrips, vector of TSWV.
- Author: Steven T. Koike
- Author: Robert L Gilbertson
In spring and summer of 2010, plants in some tomato and pepper fields in coastal California have developed virus-like symptoms. Affected plants were found in both Monterey and San Benito counties, though disease incidence was low. Samples submitted to the Gilbertson lab at UC Davis tested positive for the curly top virus, indicating that these symptoms were due to curly top disease.
Curly top can be a severe disease for both tomato and pepper. Symptoms of the disease vary depending on the host plant infected and the stage of growth when infections occur. In general, affected plants show some degree of stunted and distorted growth and leaf curling and light green to yellow discoloration. In tomato, curly top symptoms begin with leaves showing light green coloration, up-curling, and vein purpling on the underside of leaves. Plants become stunted with twisted, distorted, and yellow leaves. Plants infected at an early stage of development stop growing and die, often standing out among nearby healthy plants. Plants infected at later stages of development show distorted growth and light green-yellow leaves with vein purpling; these symptoms will develop on the upper part of the plant or on entire shoots, but the plants generally do not die. Any fruits produced will be stunted and ripen prematurely.
In peppers, plants are stunted with shortened internodes. Leaves are light green or yellow in color, show strong up-curling, and are thick and brittle. Plants infected at a young stage of growth may die, whereas older plants are stunted and leaves again curl upwards and are light green to yellow; any fruits that develop are small, wrinkled, and unmarketable. As for all diseases caused by viruses, confirmation of curly top disease requires laboratory testing because other tomato and pepper viruses can cause symptoms that are similar to those caused by the curly top viruses (e.g., symptoms caused by Tomato spotted wilt virus).
In California curly top disease affects many plants including tomato, pepper, bean, pumpkin, squash, spinach, sugar beet, other crops, and many weeds. Researchers have discovered that curly top disease in the western USA is actually caused by several distinct virus species. Therefore, curly top of tomato or pepper could be caused by one or more of the following viruses: Beet curly top virus (BCTV), Beet mild curly top virus (BMCTV), Beet severe curly top virus (BSCTV). For example, samples in 2008 from Monterey County tested positive for either BMCTV or BSCTV.
The curly top viruses are vectored by the beet leafhopper (Circulifer tenellus). The virus is not carried in seeds nor is it mechanically transmitted (i.e., by touch or physical contact). Disease incidence and distribution in tomato and pepper fields in a given year are dependent on the populations of the beet leafhopper and the migratory feeding patterns of this insect. Most infections tend to occur early in the growing season, but late season infections may also occur, as in 2010. In general, for coastal California, disease incidence is usually low and symptomatic plants occur randomly in a field, indicating where the fast moving leafhopper has stopped to feed. The random, scattered distribution of diseased plants reflects the fact that tomato and pepper are not preferred hosts of the beet leafhopper and that the insects move on in search of preferred hosts, such as plants in the beet family (Chenopodiaceae).
Tomato infected with curly top viruses: If infected early, plants can be severely stunted.
Tomato infected with curly top viruses: Leaves can be thickened, deformed, yellowed, and rolled upwards. Symptomatic tomato leaves often develop purple veins.
Pepper infected with curly top viruses: If infected early, plants can be severely stunted.
Pepper infected with curly top viruses: Leaves can be thickened, deformed, and rolled upwards.
- Author: Steven T. Koike
Two important virus pathogens have been affecting coastal lettuce crops for a number of years. As expected, both problems have shown up again this summer of 2010. These virus diseases are familiar to experienced growers and pest control advisors. However, one should exercise caution in diagnosing these problems because their respective symptoms can resemble each other.
Impatiens necrotic spot virus (INSV): INSV-infected plants have leaves with brown to dark brown spots and dead (necrotic) areas; this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications. Extensive necrosis can cause much of the leaf to become brown, dry, and dead. Some leaf yellowing can also be observed. Yellowing and the brown spotting tend to be observed on the newer leaves near the center of the plant’s growing point. If plants are affected with INSV early in their development, growth may be stunted. All lettuce types are susceptible, and INSV has been confirmed on iceberg, butterhead, romaine, and leaf lettuces. INSV can infect many other crops and weed species; the virus is vectored by thrips.
Lettuce necrotic stunt virus (LNSV) and Tomato bushy stunt virus (TBSV): Diseased lettuce can be severely stunted, especially if infected early in plant development. The oldest, outer leaves can be severely yellowed. Brown, necrotic spots and lesions later develop in these outer leaves. The younger, inner leaves remain dark green in color, but can be rough and leathery. LNSV/TBSV infects only romaine, butterhead, and leaf lettuces; modern cultivars of iceberg lettuce are immune. The LNSV/TBSV virus complex is a soilborne problem. No vector (insect, nematode, fungus) is known to spread these viruses.
See table below for comparison of these virus disease symptoms. For help in diagnosing these and other plant problems, submit samples to the UC Cooperative Extension diagnostic lab in Salinas.
Symptom comparisons for INSV and LNSV/TBSV pathogens of lettuce | |||
Symptom | INSV | LNSV/TBSV | |
Presence of yellowing, chlorosis | yes | yes | |
Yellowing mostly on older leaves | no | yes | |
Brown necrotic spots, lesions | yes | yes | |
Stunting if infected early | yes | yes | |
Central part of plant remains green | no | yes | |
Affects romaine and leaf lettuce | yes | yes | |
Affects iceberg head lettuce | yes | no |
Impatiens necrotic spot virus (INSV) on lettuce
Lettuce necrotic stunt virus (LNSV) on lettuce.