Grower Notes and Pest News
Lygus bug (Lygus spp.) is a major strawberry pest in California affecting the yield and quality of berries. Although strawberry is not a preferred host for lygus bug compared to other hosts like alfalfa, as the strawberry crop is available almost throughout the year, it seems to serve as a permanent habitat for this pest. Flowering weed hosts like wild radish and wild mustard also harbor lygus bug populations.
Regular monitoring to make treatment decisions, managing weeds or alternative hosts to limit the spread of lygus bug to strawberry fields, using flowering hosts as trap crops, conserving natural enemies, vacuuming, and chemical control are common techniques used for lygus bug management. Calculating degree-days is an approach that can help estimate when immature or adult stages of lygus bug will be seen in the fields so that appropriate treatment decision can be made.
Lygus degree-day model for strawberries (Photo illustration by Surendra Dara)
Degree-day is the unit of measurement for physiological time for insect maturity. In other words, it is the time an insect takes from one point to another point in its life cycle depending on the temperature. The amount of heat accumulated in 24 hours when the temperature is one degree above the lower developmental threshold is a degree-day (oD). The lower developmental threshold for lygus bug is 54 oF and it requires 252 oD (in Fahrenheit) for egg stage and 371 oD for nymphal stages to complete. Adults require 176 oD before they start laying eggs. So, it takes a total of 799 oD from egg to next generation egg. By monitoring the presence of lygus bug and daily temperatures, degree-days can be calculated to make treatment decisions against nymphal stages following UC IPM guidelines.
For example, if lygus nymphs are first seen in March on weed hosts or adults are first seen in strawberries in April, degree-day model predicts that first generation nymphs from adults can be seen in strawberries around May-June after accumulating 252 oD. Second generation nymphs, from the nymphs on weeds, can be seen in July in strawberries after accumulating 799 oD. The third generation nymphs from first generation can also be seen after 799 oD in August.
California Strawberry Commission, in collaboration with growers in the Watsonville and the Santa Maria areas, is working on monitoring lygus populations and calculating degree days. Data loggers that record temperatures and calculate degree-days were recently set up in about 35 strawberry fields in these two areas. Growers periodically monitor the fields and once the adult lygus bug is found, calculation will begin until 252 oD are accumulated. This indicates the time when the first generation nymphs emerge so that appropriate management decisions can be made. Although lygus bugs can move into the strawberry fields and deposit eggs several times during the growing season, resulting in the emergence of nymphs at different times, degree-day model can still help tackling the first generation nymphs.
Andrew Kramer from California Strawberry Commission explaining the functioning of the unit to Dave Peck, Manzanita Farms, Santa Maria
One degree-day (01 at the extreme right) was accumulated one day after installing the data logger in one of the Santa Maria fields
Visit UC IPM web resources to find out more information about degree-days (www.ipm.ucdavis.edu/PHENOLOGY/ma-lygus_bug.html) or to calculate degree-days by uploading your own temperature data (http://www.ipm.ucdavis.edu/WEATHER/ddretrieve.html).
Identity is an important issue whether it is for an individual, a company or even a disease causing organism. In this case, it is the plant pathogenic fungus that causes powdery mildew on strawberries and several other crops. I recently attended some talks about strawberry diseases and found out that powdery mildew pathogen, previously known as Sphaerotheca macularis, is now referred to as Podosphaera aphanis. Literature search indicated that this name has been used for a few years. I have contacted Dr. David Gadoury, a senior research associate and powdery mildew specialist from Cornell Univeristy, whose talk I attended, to elaborate on the name change. Below is what he says:
“Although the causal agent of strawberry powdery mildew has long been known by the name Sphaerotheca macularis, it has more recently been reclassified as Podosphaera aphanis. Classification of all powdery mildews before 1980 was largely based upon features of the overwintering structures or fruiting bodies called cleistothecia. In particular, genera of powdery mildews were grouped and named based upon the numbers of spore containing sacs known as asci (singular ascus) in the cleistothecium and the morphology of the appendages of the cleistothecia, in particular the appendage tips. The foregoing system has been largely supplanted by the phylogeny (history of evolutionary relationships) of powdery mildew fungi inferred from internal transcribed spacer (ITS) of ribosomal DNA sequences, which correlates with conidial ontogeny (developmental changes) and morphology (structure) (Braunet al., 2002). Although such details may fascinate taxonomists, the bottom line for those concerned with the practical aspects of disease management is this: the fungus has a new name, but it's the same pathogen, not a new one that has recently attacked strawberries.
Going forward, in particular when searching for information in electronic resources, it will become increasingly important to remember that the name was recently changed. The more recent literature is most likely to be found using the new name: Podosphaera aphanis.”
Conidial chains borne atop conidiophores. It is these stalk-like conidiophores that give mildew colonies their powdery appearance. Spores form at the bottom of the chain, so the oldest spores are at the tip of the chain. They break off in wind currents and can travel short distances, generally less than 100 meters. (Photo and description by David Gadoury, Cornell University)
Cleistothecia are the overwintering structures of powdery mildews (dark, round structures in picture). They are firmly embedded in the threads of the fungal growth (mycelium) on the leaf surface. (Photo and description by David Gadoury, Cornell University)
Cleistothecium with ascus containing ascospores. The cleistothecium swells when coated with a film of water and fractures. The ascus is an elastic sac that continues to absorb water and swell, eventually bursting and ejecting the ascospores into the air. Appendages of the genus Podosphaera are simple with unbranched tips, and the cleistothecia contain only a single ascus. (Photo and description by David Gadoury, Cornell University)
Although it is the same pathogen, it is important to know the new name as it will eventually be updated in all publications. It is even more important when we look for recent updates as it is very likely to have the new name.
Brief description about the disease and symptoms: Powdery mildew is an important disease causing damage to leaves, flowers, and fruit and affecting the fruit yield and quality. Typical symptoms include white, powdery fungal growth on the lower surface of the leaves, upward curling of the leaf edges, and dry, purplish patches on the upper leaf surface as the disease advances. Dry leaf surfaces, cool to warm temperatures and high humidity favor the infection. Fungal spores are disseminated by wind and cause further infection. Recent studies indicate that cleistothecia serve as functional source of primary inoculum (Gadoury et al., 2010). Resistance of leaves and berries to the infection significantly increases as they mature (Gadoury et al., 2007, Asalf et al., 2009, Carisse and Bouchard, 2010).
Infection symptoms: Upward curling of the leaf edges and powdery growth
(Photo by Jack Kelly Clark)
Management: Clean nursery stock is very important to prevent the introduction to the production fields and can reduce the need for fungicidal applications. Fungicidal treatment prior to the onset of symptoms is critical for effective and sustainable suppression of the disease. Gadoury said that the choice of materials is generally secondary to proper timing and thorough coverage of the young, susceptible leaves, flowers, and fruit. “Keep in mind that the mildew colonies that you see result from infections that occurred up to four weeks before they became visible to the naked eye,” said Gaoudy. “Waiting until disease is apparent will result in poor control and hasten development of resistance in many of the remaining effective fungicides particularly those in the DMI and strolbilurin classes.”
You can refer to the UC pest management guidelines for additional information.
Asalf, B., A. Stensvand, D. M. Gadoury, R. C. Seem, A. Dobson and A. M. Tronsmo. 2009. Ontogenic resistance to powdery mildew in strawberry fruits. Proc. 10th International Epidemiology Workshop. (eds. Gadoury, D.M., R. C. Seem, M. Moyer and W. E. Fry). Cornell University, New York. 177 pp.
Braun, U., R.T.A. Cook, A. J. Inman. and H. D. Shin. 2002. The taxonomy of the powdery mildew fungi. In The powder mildews: a comprehensive treatise (eds., Bélanger, R. R., W. R. Bushnell, A. J. Dik and T.L.W. Carver), pp. 13-55.
Carisse, O. and J. Bouchard. 2010. Age-related susceptibility of strawberry leaves and berries to infection by Podosphaera aphanis. Crop Protection 9: 969-978.
Gadoury, D.M., A. Stensvand, R. C. Seem, and C. Heidenreich. 2007. Ontogenic resistance of leaves, leaf folding and the distribution of mildew colonies in strawberry powdery mildew (Podosphaera macularis). Phytopathology 97:S38
Gadoury, D. M., B. Asalf, M. C. Heidenreich, M. L. Herrero, M. J. Wlser, R.C. Seem, A. M. Tronsmo and A. Stensvand. 2010. Initiation, development, and survival of cleistothecia of Podosphaera aphanis and their role in the epidemiology of strawberry powdery mildew. Phytopathology 100: 246-251.
Several specimens of grass bugs have been brought to our office in the recent weeks. These are of varying sizes (about 7-12 mm), but identified by the CDFA systematist, Rosser Garrison as Arhyssus sp. They belong to the family Rhopalidae (Order Hemiptera), members of which are commonly known as scentless plant bugs. They mostly feed on xeric (require less water or adapted to dry habitats) and other weed plants. Sometimes they enter homes in search of protected places.
Grass bugs are similar to coreids or leaf-footed bugs except they do not have well-developed scent glands and smaller than coreids. These can be confused with false chinch bug, Nysius raphanus (family Lygaeidae) which are also found in weedy or grassy areas and migrate to homes.
Damage: They usually do not cause any serious damage in the home gardens. However, they can be a nuisance when entering the homes in large numbers.
Management: Sealing the windows, keeping the doors closed or using the screen doors, and vacuuming are the best practices to keep them out or clean them up.
In light of spotting a couple of Asian citrus psyllids (ACP) in Ventura County about a month ago, it is important to be aware of this exotic and invasive pest and its damage potential.
Asian citrus psyllid, Diaphorina citri Kuwayama in its characteristic posture
(Photo by Michael Rogers, UC)
ACP, Diaphorina citri Kuwayama (Homoptera: Psyllidae) looks like a miniature cicada. Psyllids are similar to aphids except that they have longer antennae and strong jumping legs hence the name jumping plantlice.
Origin and distribution: Although native to Asia, ACP has worldwide distribution in tropical and subtropical regions. After its first discovery in Florida in 1998, it has now spread to Texas and California as well as neighboring Bahamas, Cuba, Jamaica, Mexico, Puerto Rico and other areas. With its recent detection in La Conchita and Santa Paula areas, the entire Ventura County along with southern Santa Barbara and western Riverside Counties are now considered as quarantined areas.
Damage: ACP is a phloem feeding insect that consumes copious amounts of phloem sap and secrets large quantities of honey dew resulting in sooty mold growth. Nymphs feed exclusively on young leaves and shoots. Feeding damage includes cessation of terminal growth and malformation of developing parts. Mature plants can withstand feeding damage to some extent, but it is severe in nursery stock and developing young trees. Additionally, ACP transmits an endocellular, phloem-limited bacterium, Liberobacter asiaticum that causes citrus greening or huanglongbing (HLB) or yellow dragon disease. It is also called citrus Likubin or dieback or leaf mottle in different Asian countries. Native to China, citrus greening was first detected in Florida in 2005 and is a more serious threat than the feeding damage. This disease is transmitted when ACP feeds on a healthy plant after feeding on a diseased plant. Disease stays latent for sometime before symptoms appear in an infected plant. Typical symptoms include mottling and yellowing of leaves. As the disease progresses, small and narrow leaves, short stems, stunted growth, poor flowering and dieback can also be seen. Infected fruit is small, hard, lopsided with dark seeds and bitter juice.
Burnt tip of feather flush due to ACP feeding damage (Photo by Michael Rogers, UC)
Yellowing of leaves due to citrus greening disease (Photo by Susan Halbert, FDACS/DPI)
Host range:Citrus and closely related species like orange jasmine are susceptible both to ACP and citrus greening. Citropsis, Citrus and Murraya are preferred genera in the Rutaceae family.
Biology:Adults are 3-4 mm long insects with mottled brown wings and have a characteristic angular posture. Eggs are almond-shaped, bright yellow to orange and are deposited on developing shoots or feather flush. Adults lay up to several hundred eggs that hatch in about 4 to 10 days. Nymphs are 0.25 to 1.7 mm long and are yellowish orange. There are five nymphal instars which take about 13 to 39 days to mature into adults. Nymphs produce waxy tubules from their posterior end that help divert the honeydew away from their body. Eggs develop into adults in 14 to 49 days depending on temperature. Average longevity of adult female varies between 29 and 88 days at different temperatures. Optimum temperature for development is 25-28oC (77-82oF) and life cycle can be completed in 29 days at 28oC (82oF).
Management: Due to its extensive distribution in Florida it is not expected to be eradicated in that state, but eradication is possible in its new home, California. Chemical control is possible with neonicotinoids like imidacloprid and dinotefuran, pyrethroids like bifenthrin, deltamethrin and fenpropathrin, the organophosphate, chlorpyrifos, the carbamate, carbaryl, and the ketoenol, spirotetramat. Biological control is possible with several generalist predators like spiders, lacewings, syrphids, minute pirate bugs, coccinellids (lady beetles) and several parasitoids. Harmonia axyridis andOlla v-nigrum are abundant and effective coccinellid predators and imported hymenopteran, Tamarixia radiate is a well established parasitoid in Florida. Microbial control is possible with entomopathogenic fungi likeBeuveria bassiana, Metarhizium anisopliae, Hirsutella thompsonii and Isariaspp. which were found pathogenic to ACP. Several formulations of these fungi are commercially available.
Additional information: More information about ACP, HLB, tracking, control and quarantine areas can be found athttp://www.aphis.usda.gov/plant_health/plant_pest_info/citrus_greening/
index.shtml and http://www.cdfa.ca.gov/phpps/acp/.
What to do:Southern Santa Barbara Co is currently under quarantine. If you have citrus or related host plants, periodically examine the newly developing leaves for ACP feeding damage, waxy substance, honey dew and sooty mold. You can also look for leaf mottling, a symptom of citrus greening. If you find ACP in a new area, please secure the specimen, note the location, and send or bring in the specimen. You can reach me at 805-788-2321 or firstname.lastname@example.org.
Grafton-Cardwell, E. E., K. E. Godfrey, M. E. Rogers. C.C. Childers and P. A. Stansly. 2006. Asian citrus psyllid. UC ANR publication, 8205.
Halbert, S. E. and C. A. Nunez. 2004. Distribution of the Asian citrus psyllid, Diaphorina citri Kuwayama (Rhynchota: Psyllidae) in the Caribbean basin. Florida Entomol. 87: 401-402.
Liu, Y. H. and J. H. Tsai. 2000. Effects of temperature on biology and life table parameters of the Asian citrus psyllid, Diaphorina citri Kuwayama (Homoptera: Psyllidae). Annals of Appl. Biol. 137: 201-216.
Michaud, J. P. 2004. Natural mortality of Asian citrus psyllid (Homoptera: Psyllidae) in central Florida. Biological Control 29: 260-269.
Padulla, L.F.L. and S. B. Alves. 2009. Suscetibilidade de ninfas de Diaphorina citri a fungos entomoatogênicos. Arquivos do Instituto Biológico (São Paulo) 76: 297-302.
I have moved the Asian citrus psyllid article to http://ucanr.org/blogs/pestnews/. I have created this blog to address issues that are not related to strawberries and vegetables.