- Author: Shimat Villanassery Joseph
Among several species of thrips that invade vegetable crops, western flower thrips [Frankliniella occidentalis (Fig. 1)] is the most destructive species of thrips in the Salinas Valley. They can cause severe feeding injury to all stages of plant development. Early feeding can cause severe stunting or reduce plant development whereas; late feeding can cause visible feeding patches - affecting marketable yield in both the instances. Severe feeding injury is usually associated with very high populations of thrips on the crop. It is likely that recent early surge in thrips populations in the Salinas Valley is related to warmer (high day temperatures) and dry winter.
Thrips is a tiny insect (less than 3 millimeters) and prefers to stay and feed within tight protected areas of the plant such as, between veins or near mid-rib or within the layers or stacks of lettuce leaves or celery stems. Thus, thrips injury was detected in those tight areas of the plant. It is likely that colonizing at those tight areas provide protection from insecticide sprays.
Thrips has a “piercing-sucking” or “punch and suck” mouthpart. Mouthpart of thrips is referred as mouthcone (Fig. 2). Thrips typically feeds using two structures of its mouthcone: (1) a mandible and (2) stylets. As illustrated in the Figure 2, thrips uses the mandible to pierce or punch the plant cell wall and stylets (or needles), which often form a single tube, sucks the liquid food from the plant cell. This feeding apparatus allows thrips to feed on liquid food on a surface or within a plant cell.
As indicated, thrips can cause significant crop loss once its population increases to very high levels. Thrips injury on lettuce may appear as brown streaks, or scarring on the leaves (Fig. 3). If examined closely using magnifying glass, it appears like punctured plant cells and the content removed (Figs. 4 and 5). On celery, the thrips feeding injury is similar but the injured cells appear as raised ridges (Fig. 6). When attacked at the younger stages of the plant development, for e.g., on the growing tips of the cotyledons (Fig. 7), the feeding could deform the true leaves that develop later (Fig. 8).
In addition to feeding injury, western flower thrips are able of transmiting plants viruses (tospoviruses) such as Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV). In the family: Thripidae, there are 1710 species of thrips but only 14 thrips species are currently reported to transmit tospoviruses. Both larval and adult stages of thrips vectors actively feed on the host plants but only early larval instars can acquire tospoviruses and later instar larvae and adults can transmit tospoviruses after a latent period. Adult thrips can acquire tospoviruses, but they do not transmit them because virus could not multiply to sufficient numbers. Also, tospoviruses are not transmitted when the thrips lay eggs into the plant. Thus, each new generation of thrips vectors must acquire the virus as larvae. The weed plants outside the field can be the reservoir for tospoviruses and when the larvae feed on them, they acquire the virus. In the field, larvae feeding on the tospoviruses-infected lettuce plants may also aid virus acquisition. The thrips carrying the virus just need to feed for 10-15 minutes to transmit the virus to uninfected plants.
Typically, bean-shaped eggs are inserted by female western flower thrips into the leaf. Within 5 days, eggs hatch to first instars. If the temperature stays at 86ºF, first instars molt into second instars. This can happen in couple of days in the Salinas Valley. Second instars develop into prepupae within 4-5 days. Most of the prepupae drop to the soil and emerge into adults within 3 days at 86ºF. Pupal stage is the only non-destructive stage of the thrips. Clearly, thrips development is associated to temperature. Adult females lay about 50 eggs and can live up to 4-5 weeks at 86ºF. So, in Salinas Valley due to milder temperature range, western flower thrips may live longer than 5 weeks. Western flower thrips requires a minimum 194 degree days (DD) (min. temp. 49.5oF) to complete a generation, but has been estimated to be as high as 254 DD with a minimum temperature of 43.7oF. Western flower thrips can lay eggs with and without mating. The mated female thrips (fertilized) tend to produce more female offsprings than males whereas, unmated female thrips tend to produce more male offsprings than females.
Thrips are weak flyers but they have fringed wings which help them to get airborne and glide short and long distances. Thrips can stay airborne for about 24 hours in the cooler conditions and can remain without feeding and drinking. They get quickly desiccated if they stay longer in the air. The dispersal of thrips is largely depending on temperature, light, and wind.
To prevent direct feeding injury and viral transmission, it is important that we manage thrips on the crops using the tools such as targeted insecticide sprays. Recent insecticide efficacy studies against western flower thripssuggested that insecticides such as Radiant, Entrust, Lannate, Exirel and Beleaf have decent activity against western flower thrips. Other products, Gladiator and Torac are effective but are not registered for use. Please read the Monterey County crop note (May edition) for details on insecticide efficacy trials. It is important that the growers restrain from repeated use of insecticides within same IRAC class (http://www.irac-online.org/documents/moa-classification/?ext=pdf) in a given season instead rotate insecticides with distinctly different modes of action to reduce development of resistance.
- Author: Steven T. Koike
Typically, spinach crops in the Salinas Valley and other coastal regions rarely are infected with viruses. However, that changed this spring 2015 due to a number of virus disease outbreaks on coastal spinach. Two viruses, Impatiens necrotic spot virus and Cucumber mosaic virus,have been confirmed in a number of spinach plantings. Symptoms caused by the two viruses are similar and cannot readily be differentiated in the field. All such reports have occurred on the larger, older plantings destined for processing (freezer spinach). To date no virus diseases have been reported yet on baby leaf or other fresh market spinach. This development on spinach is consistent with virus outbreaks on lettuce and other leafy vegetables grown on the coast (see Blog entry for April 9).
1. Impatiens necrotic spot virus (INSV): Diseased spinach plants are severely stunted and short. Leaves are at first yellow and later can become tan to whitish in color. Leaf texture is leathery and the foliage appears crinkled and deformed (Photos 1 and 2 below). Symptoms can be seen on both the older, outer leaves as well as on the newer leaves near the center of the plant growing point. The central growing point of the plant is severely affected. INSV is vectored by thrips.
Photo 1. INSV on spinach
Photo 2. INSV on spinach
2. Cucumber mosaic virus (CMV): On spinach, CMV also causes significant plant stunting, poor growth, and yellow to tan foliage. Leaves are leathery in texture and are crinkled and deformed (Photos 3 and 4 below). Symptoms occur on both the older, outer leaves as well as on the newer leaves near the center of the plant growing point. The central growing point of the plant likewise is severely damaged. CMV is vectored by aphids.
Photo 3. CMV on spinach
Photo 4. CMV on spinach (healthy leaf on right)
The dry spring is contributing to early drying up and senescing of weeds and hillside vegetation; this early decline of surrounding vegetation could be driving thrips and aphid populations into fields earlier than normal. In addition, growers and PCAs reported moderate to high thrips and aphid pressure in the early spring. This abundance of vectors certainly accounts for these early outbreaks of virus disease.
Plants suspected of being infected with viruses can be sent for analysis to the UC Cooperative Extension diagnostic laboratory in Salinas.
- Author: Larry J Bettiga
Pinot leaf curl (PLC) is a physiological disorder that affects the Pinot cultivars. The symptoms are general seen in the coastal production areas during cool spring temperatures. During the cool spring temperatures of 2010 symptoms of this disorder were very common to Pinot vineyards. This week I have had several reports of this disorder showing in Pinot vineyards in the central coast.
Symptomatic leaves on elongating shoots curl downward across the middle of the leaf blade, perpendicular to the mid-vein. The angle of the downward bend is acute; the marginal tip of the center lobe may come into contact with the petiole. Mild symptoms involve only the blades; continued expansion of which results in a misshaped and reduced leaf size. A necrotic region is present on the mid-vein preventing further elongation. If the necrosis expands to include the petiole, the leaf will abscise from the shoot. Severe symptoms occur when necrosis involves the node at which a leaf abscised, killing the shoot distal to that point.
Symptoms appear somewhat similar to Botrytis shoot blight caused by Botrytis cinerea on foliage under cool, wet spring conditions. In the past samples of PLC affected tissue have been tested and no known foliar pathogens have been associated with the samples. Fungicide applications are unwarranted and will have no effect on subsequent PLC symptom development.
Research conducted by Rhonda Smith, UCCE Sonoma County and Doug Adams, Department of Viticulture and Enology UC Davis, have shown the level of putrescine were found to be elevated in symptomatic versus asymptomatic samples. Relative amino acid levels were less clearly aligned with symptoms; symptomatic blades were associated with elevated gluatamine levels in addition to elevated putrescine. Glutamine is the predominate amino acid in vine sap and the main amino transport form of reduced nitrogen in the vine. With the current information, pinot leaf curl is a physiological disorder most likely associated with elevated nitrogen levels. Elevated putrescine levels are toxic to plant tissue and are thought to be involved in symptom development in false potassium deficiency (“spring fever”) and early bunch stem necrosis. Pinot leaf curl may be an additional disorder associated with putrescine.
Source: Adapted from UCCE Sonoma County (http://cesonoma.ucanr.edu/viticulture717/Viticulture_Publications/)
- Author: Steven T. Koike
As early as March 2015, leafy green crops in the Salinas Valley exhibited symptoms indicative of virus diseases. In fact, viruses in the tospovirus group have been confirmed in a number of fields in the Valley:
1. Impatiens necrotic spot virus (INSV) on lettuce. Diseased leaves have tan, brown, or blackish spots and dead sections; this necrotic tissue can resemble damage caused by pesticide or fertilizer applications (see photos 1 and 2 below). Some leaf yellowing can also be observed. Symptoms can be found on both the older, outer leaves as well as on the newer leaves near the center of the plant growing point. If plants are affected early in their development, growth can be stunted. A number of fields have been confirmed to be positive for INSV, though in each case disease incidence was low (well below 1%).
2. Tomato spotted wilt virus (TSWV) on radicchio. Diseased leaves have yellow spots, streaks, mottles, and other symptoms (see photos 3 and 4 below). Brown necrotic leaf spots are sometimes present. If radicchio is infected when young, the plants are severely stunted, leaves are deformed and twisted, and head-forming cultivars fail to develop normal heads. Disease incidence was moderately high.
The INSV and TSWV pathogens are spread by thrips. In our coastal region, these tospoviruses are spread only by the western flower thrips (Frankliniella occidentalis). In the early part of 2015, growers and PCAs reported moderate to high thrips pressure already. This abundance of the vector certainly accounts for these early outbreaks of tospovirus disease. A number of weeds and other hosts serve as reservoirs of the viruses. Immature thrips feed on these infected plants, develop into adults, and can then spread INSV and/or TSWV to lettuce and other surrounding host crops. The dry spring is contributing to early drying up and senescing of weeds and hillside vegetation; this early decline of surrounding vegetation could be driving thrips populations into fields earlier than normal. Radicchio and all lettuce types (iceberg, romaine, greenleaf, redleaf, butter) are susceptible to both TSWV and INSV, so field personnel should be aware of this early development.
Plants suspected of being infected with tospoviruses can be sent for analysis to the UC Cooperative Extension diagnostic laboratory in Salinas.
Photo 1. Lettuce infected with Impatiens necrotic spot virus.
Photo 2. Lettuce infected with Impatiens necrotic spot virus.
Photo 3. Radicchio plant on right infected with Tomato spotted wilt virus.
Photo 4. Radicchio leaves on right infected with Tomato spotted wilt virus.
- Author: Lennis Arriaga