Denomination of Pfs: 15, a new race of downy mildew in spinach

Recently, a widespread outbreak of foxglove aphid (Aulacorthum solani) (Figures 1, 2 and 3) has been reported in the Salinas Valley. Reports were primarily from Salinas to Soledad at this point. Growers have been losing several acres of lettuce to foxglove aphid since last one and half months.

Foxglove aphid nymphs and wingless adults have a light to dark green patch at the base of the cornicle (Figures 4 and 5). The lettuce aphids (Nasonovia ribis-nigri) do not have any patch at the base of the cornicle. Cornicle is a tube-like organ at the back of the aphid as shown in the figures 4 and 5. The foxglove aphids observed were yellowish to green in color than pink or red. At the base of the antennae, foxglove aphid doesn't have any prominent converging tubercles (a projecting structure), which is typical for green peach aphid (Myzus persicae). The joins of legs and antennae of foxglove aphid are darker (dusky) than other regions. Winged adults of foxglove aphid and lettuce aphid are practically indistinguishable from each other. 

Generally, aphids are capable of reproducing parthenogenetically meaning female aphids lay eggs without mating and all the eggs turn into females. Moreover, in warmer conditions like in summer, they pretty much give birth to young ones as the eggs hatch within the reproductive canal of the female. Typically, a single aphid give birth to about 50 to 100 nymphs in two weeks or about 10 nymphs every day, which could vary with environmental conditions such as temperature and humidity. Foxglove aphids can complete a generation in less than 2 weeks in the summer. Unlike lettuce aphid, foxglove aphid has a broad host range meaning it could survive on several host plants.

Detection of foxglove aphid early in the crop stage is critical. They initially infest the cap or outer leaves of head lettuce. Eventually, foxglove aphids move into the deeper layers of leaves then form colonies. Green peach aphids on the other hand, form colonies on the wrapper leaves from the onset. Based on anecdotal observations, foxglove aphid colonies have been observed in lettuce about 20 days to harvest. The lettuce crops during thinning stage often appeared clean (without foxglove aphids).

Management of foxglove aphid with insecticides has been monitored more closely than ever before. It is important to note that spirotetramat (Movento), which is widely used against aphids has to metabolize from applied form (spirotetramat) and convert into more toxic derivatives within the plant in order to be toxic or effective against aphids. Normally, it will take at least 10 days for this metabolic process and movement of derivatives into the growing tissues of the lettuce. Thus, two applications of spirotetramat (5 fl oz/acre each) should be timed at the onset of first foxglove infestation without delay. Late application of spirotetramat (e.g. 10 days before harvest) may not offer any control. Also, it is important to keep in mind that there is little use if the applications are made when high populations of foxglove aphid have been detected. Make sure that the insecticides such as acetamiprid (Assail), imidacloprid, thiamethoxam (Actara) are used in rotation. Back-to-back use of insecticides in same class or IRAC group (http://www.irac-online.org/teams/mode-of-action/) will increase the risk of development of aphid resistance to a particular class of insecticides. Once a super aphid has been created, there is no value in using insecticides from that class to manage aphids. The industry is currently seeking emergency registration of sulfoxaflor (Closer or Sequoia) for use to tackle foxglove aphid problem this season. Sulfoxaflor is systemic (moves within the plant), trans-laminar (move through leaves) and acts in hours once applied and is from a new insecticide class or IRAC group. Please email (Shimat Joseph: svjoseph@ucanr.edu) or call (831) 229 8589 if you have further questions.

Figure 1. Foxglove aphid infested head lettuce

Figure 2. Foxglove aphid infested head lettuce

Figure 3. Foxglove aphids

Figure 4. Diagnostic character of foxglove aphid

Figure 5. Diagnostic character of foxglove aphid

Bagrada bug (Bagrada hilaris) (Figure 1), an invasive stink bug species native to old world countries of southern Africa, Middle East and Asia, was first detected in North America from Los Angeles Co, California in 2008, and is now established in brassica crop production regions of central coast of California.

Bagrada bug prefers cruciferous hosts (Family: Brassicaceae) including broccoli, cauliflower, cabbage, kale, arugula, or collards. Also, bagrada bug could survive on cruciferous weeds such wild radish, London rocket, short pod mustard, and shepherd's purse, as well as the insectary plant, sweet alyssum. Mustard weed species are very common in ditches, roadsides, and along the edges of agricultural fields in the central coast of California. Mustard cover crops such as white mustard and Indian mustard in particular, could harbor bagrada bug populations, which are not often monitored for insect pests.

Damage to brassica crops varies but can be severe. Severe economic loss has been reported when injury occurred during early developmental stages of the crop such as cotyledon or < 4 leaves stages. Injury on leaves appears initially as small puncture marks, which turn into white patches as leaves expand. In broccoli and cauliflower, the economic injury occurs when the bug feeding kills the apical meristematic tissue of young seedling, which later results in “multiple heads” or sometimes “blind head” (without a head). Severe feeding also depletes the nutrient reserves of the plant leading to desiccation or wilting. Other leafy brassica crops such as mizuna mustard or arugula are especially at risk from direct feeding damage of bagrada bug.

At this time, no baseline information on bagrada bug abundance, seasonal activity, or generations has been established in the coastal brassica crop production regions in California. Limited information exists to determine threshold based pest management decisions for bagrada bug. Monitoringfor bagrada bug during mid-day hours might increase the probability of finding them as the bugs typically hide and stay in the cracks and crevices or on the underside of leaves when the temperature is cooler. Strong attraction to specific color could be utilized when developing selective traps for monitoring bagrada bug in the field. Yellow traps are most commonly used for monitoring true bugs such as false chinch bugs, plant bugs, psyllids or stinkbugs. Yellow pyramid traps were used to monitor brown and dusky stink bugs.

This study was conducted to determine the influence of trap color on catches of bagrada bug. In fall 2013, an organically-managed broccoli field in San Ardo, CA was chosen for the study. Planting sweet alyssum is a common practice among organic growers in the Salinas Valley to harbor beneficial insects such as syrphid flies (hover flies) and lady beetles within the cole crop field. Strips of sweet alyssum were planted within the broccoli field. Cross-vane traps were constructed using corrugated plastic sheets and were painted with white, yellow, red, purple, or black paint (Figure 2). All the trap-base containers used in this study were light green colored. Traps were placed within the sweet alyssum plants (Figure 3).Bug captures were recorded for up to 5 weeks.

In total, 10,654 bagrada bug were captured and were predominantly adults (95%). Adult bagrada bug captures were influenced by trap color (Figure 4a). Black traps captured almost twice the number of adult bagrada bug than purple traps; however, adult catches were similar among purple, white or red traps. Catches of adults were higher in purple than in yellow traps. Similarly, the numbers of bagrada bug nymphs collected among various colored-traps were higher in black traps than other colored traps (Figure 4b). Future research will investigate the potential use of color preference with plant host odor to develop traps for field monitoring of bagrada bug

For those of you, who are interested to read more on it, please find the published article after clicking the link (below) and feel free to contact me (Shimat Joseph) at svjoseph@ucanr.edu or 831 759 7359.

http://cemonterey.ucanr.edu/files/195965.pdf

Figure1: Bagrada bug adults and nymphs

Figure 2: Various colors of trap used in the experiment

Figure 3: A trap deployed in the broccoli field

Figure 4: Influence of trap color on bagrada bug captures

Cabbage maggot (Delia radicum) is one of the most destructive pests of cruciferous crops in the Salinas Valley. Cabbage maggot flies lay eggs in the soil around the base of the plant. A single female can lay about 300 eggs under laboratory conditions. Legless,8-mm long white-maggots feed on the taproot and affect normal plant development. After about 3 weeks of feeding, the maggot pupates in the surrounding soil and remains at this stage for 2-4 weeks before emerging into an adult fly. The most common above-ground feeding symptoms of cabbage maggot are yellowing, stunting and slow growth.

Because the winter weather in the Salinas Valley is mild and rarely goes below freezing point, not all cabbage maggot pupae go into a resting stage, often called as diapause. This means our unique environment enables cabbage maggot flies to remain active even in winter months, producing multiple overlapping generations throughout the year. In this post organophosphate era with stringent restrictions for chlorpyrifos and diazinon use and less persistent insecticides being available for cabbage maggot management, knowledge of field-level incidence of cabbage maggot infestation is critical to determine precise timing for insecticide applications in brassicas. We studied the temporal incidence of cabbage maggot relative to seeded broccoli and turnip in the Salinas Valley.

Cage studies showed that severe injury from cabbage maggot did not appear during the first 14 days after plant emergence but was greater during 15-28 days after plant emergence. Similarly, survey in broccoli fields indicates that cabbage maggot flies did not lay a high number of eggs at the base of the plant until three weeks after plant emergence, despite presence of adult cabbage maggot in the field during the early stages of plant development (Figures below). On turnip, notable injury from cabbage maggot did not appear until five weeks after plant emergence. This is important information because typically insecticides targeting cabbage maggot were applied mostly at planting. Researchers showed that cabbage maggot infestation could be suppressed by using organophosphate insecticides, particularly chlorpyrifos, for more than a month after planting because product residues persisted for an extended period. However, most of us are not using these insecticides and a consistent cabbage maggot control using organophosphate insecticides was never attained in the Salinas Valley. I'm working on insecticides to determine their effectiveness against cabbage maggot and will share that information as soon as it is available.

It is unclear why increased cabbage maggot oviposition did not occur during the early stages of plant development. It is possible that the invading cabbage maggot flies cannot distinguish the young seedlings at a certain size relative to the surrounding area of bare soil. Cabbage maggot populations and crop injury from this pest tend to be more abundant in the border than the interior zone of the field; this invasion pattern continues throughout the growing period. In conclusion, our data suggest that the important season periods in the central coast vegetable production area to consider targeting cabbage maggot control are three to four weeks after planting the seeds. I will continue to monitor if other maggot species (seedcorn maggot or onion maggot) which could attack when brassica plants are at younger stages especially during spring or early summer.

For those of you, who are interested to read more on it, please find the published article after clicking the link (below) and feel free to contact me (Shimat Joseph) at svjoseph@ucanr.edu or 831 759 7359.

 http://cemonterey.ucanr.edu/files/190245.pdf