- Author: Shimat Villanassery Joseph
- Author: Richard Smith
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.
- Author: Shimat V. Joseph
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).
Experienced growers, pest control advisors, and other field professionals involved with broccoli already know that the winter period can signal increased problems due to head rot (also known as pin rot). Favored by cool temperatures and prolonged periods of moisture from rain, dew, and fog, broccoli head rot continues to be a damaging and yield-reducing factor because preventative measures have yet to be consistently effective. Spray applications have not proven to be consistently reliable tools to prevent head rot. While some cultivars (especially those having rounded, dome-shaped heads) may be less susceptible to head rot, true resistance has not been demonstrated for cultivars grown in California. Trying to avoid the use of overhead sprinkler irrigation is apparently the only cultural practice that helps reduce disease; however, the winter and early spring weather will enhance head rot even if growers use drip or furrow irrigation.
Field personnel should remember that two types of head rot affect the crop in California. For bacterial head rot, initial symptoms on the immature broccoli heads consist of a water-soaked or greasy discoloration of the surfaces of small groups of the unopened flowers. Later, the affected portions of the head turn brown to black and the infection spreads and affects larger parts of the head. The tissue becomes soft and gives off a very bad odor. For bacterial head rot there will not be any fungal growth unless secondary molds colonize and cause further decay.
The second type of head rot is Alternaria head rot. For this fungal problem, early symptoms consist of a water-soaked discoloration that later turns dark brown to black. Tissues infected with Alternaria are usually not as soft and smelly as heads infected with the bacterial pathogens. Alternaria readily produces dark green spores on the diseased head tissue. Secondary molds and bacteria cause further decay.
Photo 1: Bacterial head rot of broccoli
Photo 2: Bacterial head rot of broccoli
Photo 3: Alternaria head rot of broccoli
Photo 4: Alternaria head rot of broccoli
Downy mildew of lettuce, caused by Bremia lactucae, is the very common foliar disease that results in the familiar yellow to brown leaf lesions and accompanying white sporulation on the lesions. However, the systemic phase of lettuce downy mildew may be less familiar to growers and pest control advisors. In the spring of 2009, systemic downy mildew was very common in coastal California. Currently in 2010, systemic downy mildew is not as serious but is still being observed in some coastal plantings.
Symptoms of systemic downy mildew may be seen on both lettuce leaves and the central, internal core of the lettuce plant. For leaf symptoms, examine the plant for large, elongated regions of the leaf that are discolored and turning dark green to brown. Such regions often develop along the midrib of the leaf and extend into the flat, outer leaf panels (photos 1, 2). White sporulation is often not present on these infected areas until late in disease development. Note that for many systemically infected lettuce plants, these leaf symptoms are absent and the only evident symptoms are in the internal core.
To check for systemic infections in the plant core, cut open and examine the central part of the plant; these tissues will show a dark brown to black streaking and discoloration (photos 3, 4). In some cases, systemically infected plants may be slightly stunted or late in maturing. Exercise caution, however, before concluding that internal core discoloration is due only to systemic downy mildew. Other important lettuce problems (Verticillium wilt, Fusarium wilt, ammonium toxicity) can cause similar internal discolorations.
Confirmation of systemic downy mildew requires laboratory testing. Affected tissues can treated with biological stains and then examined using a microscope. Such procedures can show the presence of the characteristically thick mycelium that lacks cell cross walls (photo 5). In addition, incubating pieces of affected lettuce tissue can result in sporulation of the pathogen (photo 6, showing systemic downy mildew of cauliflower), again enabling confirmation of systemic downy mildew.
Systemic downy mildew of lettuce has not been studied extensively, so researchers do not know exactly what triggers this less common phase of the disease. Some suggest that early infection of young plants may allow the pathogen to infect the inner foliage of lettuce, resulting in pathogen access to the plant growing point. Field personnel also report that some lettuce cultivars are more severely affected than others.
|Photo 1: Brown discoloration due to systemic downy mildew infection in a lettuce leaf|
|Photo 2: Brown discoloration due to systemic downy mildew infection in a lettuce leaf.|
|Photo 3: Internal discoloration of lettuce core due to systemic downy mildew infection|
|Photo 4: Internal discoloration of lettuce core due to systemic downy mildew infection.|
|Photo 5: Blue-stained mycelium of downy mildew that has systemically infected lettuce tissues.|
|Photo 6: Sporulating downy mildew from a systemically infected piece of cauliflower stem.|