- Author: Shimat Villanassery Joseph
A series of laboratory and field studies were conducted to determine if the insecticides coated lettuce seeds are an option to control key lettuce pests in the Salinas Valley: springtail (Protaphorura fimata; Fig. 1A), leafminers (Liriomyza spp.; Fig. 1B) and western flower thrips (Frankliniella occidentalis; Fig. 1C). In addition, a laboratory test was conducted to determine if “primed” lettuce seeds reduced springtail feeding damage.
Springtails. Springtail (P.fimata) is soil dwelling primitive arthropod primarily attacks germinating lettuce seeds, reducing the plant vigor or death, which cause patchy or area-wide stand loss. Most springtails possess a forked organ (furcula) in the rear-end, which is extended forward and backward to jump; hence, the common name, springtail. However, the springtail species, sampled from lettuce fields causing the stand loss, does not have furcula. This means it cannot jump.
The head lettuce seed ‘Regency' was coated with clothianidin, thiamethoxam, and spinosad (Table 1). The seeds were coated by Dr. Alan Taylor at Cornell University and coating technique mirrored commercial seed coating procedure. Laboratory studies were conducted in containers with springtail (P. fimata) infested soil. The data show that all three insecticides spinosad, clothianidin and thiamethoxam treated seeds significantly reduced the incidence of springtail feeding injury when compared with untreated seeds. Among insecticides, superior performance in efficacy was noted in the following order: clothianidin > thiamethoxam > spinosad (Fig. 2). Two field trials were conducted against springtails using the same seed treatments, however, the springtail pressure was so low that conclusive data were not obtained. Clothianidin (NipSit) in particular, is now registered on head lettuce and could be used for springtail control. This is an important information in that springtails attack the germinating seeds of lettuce especially in the spring time. During spring, we get some rain showers and the wet conditions in the field after planting makes insecticide application along seed line almost impossible. If the insecticide coated seeds are planted, the grower or PCA could avoid at-plant insecticide application which is typically targeted toward springtails. Application of insecticides such as neonicotinoids and pyrethroids along the seed line will protect the germinating seeds from springtail feeding. More field studies will be conducted in the following years to validate these results in the field.
Studies were also conducted to determine if there are any varietal effects exists (Table 2). The much needed attribute for springtail control is faster seed germination so that the springtail would not get sufficient time to feed and cause seed mortality. “Primed” lettuce seeds are used for uniform and a quick germination (cut short 2 to 3 days than “unprimed” seeds). “Primed” and “unprimed” seeds were evaluated to determine if the quick seed development would reduce springtail damage. Data show that germinating “primed” seeds were impacted with springtail feeding affecting their germination and were not different from the “unprimed” seeds when the springtail pressure was moderate to high (Fig. 3). The seeds used for this experiment were from same seed lot (“primed” and “unprimed”) for a lettuce variety. Also, there was no clear variety difference in springtail feeding damage.
Leafminers and western flower thrips. The leafminer eggs are laid within the surface layer of the leaf. The eggs hatch within couple of days and the maggots mine through the surface layer of the leaves. The egg laying and maggot mining creates stippling and mining injuries which make the leaves unmarketable. Although UC recommends few insecticides such as Agri-mek (Abamectin), Trigard (Cyromazine), Aza-direct (Azadirachtin) and Entrust (Spinosad), the management of leafminers are primarily relied upon on Agri-mek applications.
Thrips is another of the major pest of lettuce, and combination of direct feeding injury and viral disease [thrips-transmitted tospoviruses [Impatiens Necrotic Spot Virus (INSV)] can cause significant losses in lettuce production. In addition, because most of the export markets have set higher standards on prevalence of live and dead thrips in the produce, the lettuce industry is constantly battling ways to significantly reduce thrips in the produce targeted for export.
A replicated-field trial was conducted to determine the efficacy of seed coated insecticides (Table 1) on leafminers and western flower thrips incidence and their infestation. The results show that insecticide seed coating may not be an effective option for leafminers and thrips control in head lettuce (Table 3 and 4) under the conditions this experiment was conducted. There was no reduction of leafminer or thrips feeding with insecticide coated seeds compared with untreated control. Further evaluations under varying conditions might be necessary to validate the consistency of these results.
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.
Impatiens necrotic spot on lettuce
Impatiens necrotic spot on lettuce
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.
Photos: Impatiens necrotic spot virus (INSV) on lettuce.
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.
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|
|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.