Posts Tagged: broccoli
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 email@example.com or 831 759 7359.
Bulb mites (Rhizoglyphus spp. or Tyrophagus spp.) (Photo 1) are known to attack spinach, onion and garlic but recently I have observed them feeding on germinating seeds of broccoli (Photo 2). Cool and wet soil conditions especially during early spring, trigger rapid multiplication of these mites in the soil and they infest germinating seeds. These mites often feed on the internal content of the seeds leaving behind only the seed coat. Severe infestation will result in inconsistent crop stand (Photo 3, and 4). Those affected seedlings would have distorted shoot and/or delayed germination (Photo 5). Cotyledon leaves of those affected seedlings would have damaged margins (Photo 6). Unlike the damage from garden symphylan which appears in hot spots in a field, bulb mites affect the entire crop stand (Photo 3).
Bulb mites seem to be composed of several species of soil mites including crown mites. Bulb mites are shiny, spherical and clear or off-white in color. These mites often have distinctive long hairs on their abdomen. These mites survive on the decaying plant material in the soil.
Bagrada bug, Bagrada hilaris is now well established in the southern region of the Salinas Valley. This invasive stink bug, if left unmanaged, could become a serious pest of brassica crops. We studied their occurrence on cover crop, a mustard cover crop blend, broccoli and surrounding weed species, and would like to report few observations.
1. Cover crop, mustard
We observed significant populations of bagrada bug on 4-5 week-old mustard cover crop blend (Brassica juncea and Sinapis alba) in San Ardo. Interestingly, the adjacent field was also cover cropped with mustard but was disced few weeks ago. It is likely that the previous adjacent mustard crop might have had a bagrada bugs infestation and they moved to new planting when it was disced. When we walked into mustard stand from the disced field, we noticed that number of bagrada bug numbers declined from the field edge to the interior of the field. To understand better, a border zone (edge of the field), interior zone (90 feet into the field), and intermediate zone (between border and interior zones) were designated. Within each zone, six spots (~ 5.6 feet) were randomly selected and number of bagrada bugs was quantified after spending two minutes per spot. Similarly, two weed species and one native shrub surrounding the mustard field were randomly selected and number of bagrada bugs on them was counted after spending about one minute per plant.
Bagrada bugs tend to be more abundant on the edge of field than interior zones of the field (Fig. 1). It seems that bugs settled on the border plants of the field rather aggressively moving into the field. All life stages were detected. Most of the adults were in the mating position (connected by the rear ends) but very mobile. The adults tend to hide into the soil or under the leaves when we approached the infested plants.
Among the weed species investigated, bagrada bugs were only found on short pod mustard (Fig. 2 and 3). Other weed species investigated were shortpod mustard (Hirschfeldia incana), common purslane (Portulaca oleracea), lambsquarter (Chenopodium album), and the native shrub coyote bush (Baccharis pilularis). It is interesting to note that shortpod mustard plants were senescing, yet we found bagrada bugs on them.
The broccoli field was located in San Ardo adjacent to the Salinas River. The riparian plant community along the river contained stands of perennial pepperweed (Lepidium latifolium) as well as other species. Bagrada bug infestation was severe on pepperweed (Figs. 4a, and b).
Bagrada bug feeding injury symptom on broccoli plants was clearly visible on plants along the edge of the field. Feeding injury symptoms on broccoli include leaf distortion, chlorotic patches along the leaf margin and stunting (Fig. 5). Again, feeding symptoms drastically declined and/or plant vigor improved as we walked few steps (~ 10 feet) into the broccoli field from river side. Both nymphs and adults of bagrada bug were active along the edge of the field.
These preliminary observations indicate that some plants in the mustard family are highly attractive to bagrada bug. Shortpod mustard is a common summer-growing species that is commonly found on roadsides, in vineyards and in rangeland. Perennial pepperweed is an invasive plant that is commonly found in the riparian strip along the Salinas River. Both plants provide sufficient food resources for bagrada bug to successfully breed. After the onset of the winter rains, other mustard family weed species such as field mustard (Brassica rapa), black mustard (B. nigra), London rocket (Sisymbrium irio) and wild radish (Raphanus sativus) will begin their growth cycle. These plants are very common along roadsides and in ditches and may also provide over wintering habitat for bagrada bug.
ATTN: Recently, bagrada bug adults were found on Chinese or napa cabbage in Santa Cruz County.
Although this bug feeds on a wide range of hosts, we are more concerned because the bug prefers cruciferous hosts (Family: Brassicaceae) including broccoli and cauliflower, which are grown as rotation crops in the Salinas Valley. It is believed that other major crops especially lettuce and spinach are NOT a suitable host for bagrada bug. At the same time, bagrada bug can survive on cruciferous weeds such as mustard species (Brassica sp), wild radish, London rocket, short pod mustard and shepherd’s purse, as well as the insectary crop sweet asylum. Mustard weeds species are very common in the Salinas Valley along ditches, roadsides and even along the edges of agricultural fields. Other species of mustards such as white mustard (Sinapsis alba) and Indian mustard (Brassica juncea) are grown as cover crops. It is clear that given the abundance of mustard family weeds and crops, there is a readily available source of habitat for this insect in the Salinas Valley.
Bagrada bug adult is often confused with harlequin bug. Adult of harlequin bug is orange with black and white marks, whereas bagrada bug adult is black with orange and white marks; and adult harlequin bug is about 3 times larger than bagrada bug (Fig. 2). Eggs of harlequin bug are white with horizontal, black strips, whereas bagrada bug has no strips but has a “dirty” white appearance.
It is believed that bagrada bug overwinters as adult in the cracks and crevices in soil or on plants. Generally, female bug is larger in size than male. Eggs are laid on the underside of leaves, cracks and crevices in soil or on hairy stems. There are five nymphal stages for bagrada bug. Typically, bagrada bug is found in aggregation with various nymphal stages and adult rather than individuals (Fig. 3). Because Salinas Valley has relatively mild temperature through year, it is expected that the development of bagrada bug would be prolonged compared with its populations in the warmer regions where it has been established. This also indicates that, if the bug is established, the number of generations of bagrada bug would be fewer in the Valley than in the warmer locations such as southern California or in the desert regions. Normally, its population size is small during early spring to mid-summer but eventually increases in size during later summer or fall.
At this point, preventing the dispersal of bagrada bug to the Salinas Valley is the key strategy. Growers often move plant materials including transplants to the Valley for production from the regions where the bug has been established. Special care should be given to inspect the plant materials while moving them. Monitoring for 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 on the cooler side. Cruciferous weeds in the drains, river bottoms, edges of the field or near residential area increase the risk of establishment. Based on the insecticide efficacy studies conducted in University of Arizona, pyrethroids and neonicotinoids are effective in reducing bagrada bug infestation and injury. For organic growers, none of the products are efficacious but pyrethrin and azidirachtin are suggested.
If you detect bagrada bug in Monterey, Santa Cruz and San Benito Counties, please do not hesitate to contact me at firstname.lastname@example.org or (831) 759-7359.
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White mold disease, caused by the fungus Sclerotinia sclerotiorum, is causing damage to a number of vegetable crops in California and Arizona during the late 2010 and early 2011 months. On the coast of California, white mold is being found on crucifer crops such as broccoli and cauliflower. In the desert regions white mold is causing damage on broccoli, cauliflower, celery, lettuce, and other vegetables (for lettuce this disease is commonly called lettuce drop). White mold incidence on these crops appears to be greater than normally observed. See photos 1 through 6 below.
The first symptoms on most vegetable crop hosts are small, irregularly shaped, water-soaked areas on stems, leaves, pods, or flower heads. These infections quickly develop into soft, watery, pale brown to gray rots. Rotted areas can expand rapidly and affect a large portion of the plant. Diseased tissues eventually are covered with white mycelium, white mycelial mounds that are immature sclerotia, and finally mature, hard, black sclerotia. Mature sclerotia usually form after tissues are rotting and breaking down. Plants with infections on the main stems can completely collapse and fall over.
The black sclerotium is the survival stage of the fungus and can measure from ¼ to ½ inch long. Sclerotia are found in the soil and can directly infect plants if stems are in close proximity. However, these winter cases of white mold are due to ascospore infections. If sufficient soil moisture is present, shallowly buried sclerotia germinate and form small, tan mushroom-like structures called apothecia (photos 7 and 8). Ascospores (photos 8 and 9) are released from apothecia and carried by winds to the host plant. These ascospores are responsible for these winter infections and result in disease of the above-ground parts of plants. The relatively cool, moist weather found in most regions has allowed for the production of apothecia production and ascospore releases.
For ascospores to start colonizing plant tissues, nutrients and plant fluids from damaged tissues are usually needed. This is why white mold is very severe if ascospores land on compromised tissues such as lettuce leaves with tip burn, leaves and heads damaged by frost or other factors, stems with open wounds or exposed leaf traces (vascular tissue in the stem that is left exposed when a lower leaf falls off), and senescent leaves and stems.
Controlling white mold under these winter weather conditions is difficult. Protective fungicides provide some assistance and can be used effectively in lettuce. However, such fungicides need to be applied prior to ascospore flights and usually will require multiple sprays. Fungicides may not be warranted for crucifer crops.
Steve Koike thanks Jeff Rollins and Karen Chamusco for assistance with photographs for this article.
Photo 1: White mold (lettuce drop) on romaine lettuce.
Photo 2: White mold (lettuce drop) on romaine lettuce, showing white mycelium and two black sclerotia.
Photo 3: White mold on broccoli stems.
Photo 4: White mold on broccoli stem, showing white mycelium and one black sclerotium (center).
Photo 5: White mold on cauliflower head, showing white mycelium.
Photo 6:White mold on celery, showing numerous black sclerotia.
Photo 7: One sclerotium and several apothecia (spore producing structures) of Sclerotinia sclerotiorum.
Photo 8: Microscopic view of the spore-producing apothecium of Sclerotinia sclerotiorum. Note the lined-up ascospores (red) ready to be released. Photo used by permission (K. Chamusco).
Photo 9: Microscopic view of ascospores lined-up in a tube (called an ascus) and ready to be released. Photo used by permission (J. Rollins).