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.

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.

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References

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. 10^th 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.

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).

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Lewis mite, Eotetranychus lewisi (Photo courtesy: Daniel Gilrein, Cornell University)

Lewis spider mite or Lewis mite, Eotetranychus lewisi (McGregor) (Phylum Arachnida, sub-class Acarina, family Tetranychidae) is a pest of many host plants.  In the US, it has been reported on citrus and greenhouse poinsettias.  Lewis mites have been seen on strawberries and raspberries in the Ventura area for some time, but growers appear to be noticing increased infestations in the recent years.  Some growers have also seen them in Santa Maria in the recent years, but they have so far not been reported from the Watsonville area.  Considering the recent trend, growers might keep them in mind while scouting for pests.  Environment, natural enemies, cropping patterns, pesticide usage and other agronomic practices are among the factors that influence the status of pests.  It is not yet clear why infestations are increasing, but here is some information on Lewis mite.

Host range and distribution: According to an earlier report (Bolland et al, 1998), Lewis mite is distributed mostly in the western hemisphere and some parts of Africa infesting 65 species of plants including food and multipurpose plants like Erythrinia edulis, Citrus spp., Prunus sp., papaya, fig, ornamentals like Cleome sp., Bauhinia spp., Rosa sp., Euphorbia spp. (poinsettia, snow-on-the-mountain, fire-on-the-mountain), weeds like silverleaf nightshade, and various other hosts.  It has later spread to parts of Europe and East Asia.  Lewis mite infestation on strawberries was reported from Philippines nearly a decade ago (Raros, 2001).

Biology: Males are about 0.25 mm and females are about 0.36 mm long.  Species identification is tricky and requires both sexes to be examined microscopically.  They can be confused with twospotted spider mite (TSSM), Tetranychus urticae in their general appearance.  But, comparing adult females, Lewis mites are smaller than TSSM and have several small spots on their body while TSSM have a single dark spot on either side of the body.  Lewis mite has five life stages – egg, larva, protonymph, deutonymph and adult.  Eggs are round, whitish to light orange.  Females lay 60-90 eggs over a period of about a month.  It takes about 12-14 days from egg to adult stage at 21^oC (70^oF).

Twospotted spider mite (right, UC IPM)

Damage: Symptoms of damage vary according to the host plant.  On poinsettia, Lewis mite feeds on the underside of the leaves and causes very fine stippling or flecking on foliage, which can be initially mistaken for nutrient deficiency.  Infestations can build to high levels in poinsettia before mites and their webbing are noticed.  Since their infestation in strawberries currently coincides with TSSM infestations, exclusive Lewis mite damage symptoms in strawberries are yet to be determined.  In some hosts like papaya, damage resembles symptoms of viral infection and may cause distortion and yellowing of young leaves.  According to Daniel Gilrein, an extension entomologist from Cornell University, fine flecking from Lewis mite feeding almost resembles nitrogen deficiency and can delay detection until populations build up.  Leaves turn brown and webbing can be seen as the damage advances.

Management: In research on greenhouse poinsettias (D. Gilrein, pers. comm.), abamectin (Avid for ornamentals, Agri-Mek and generics for food crops), acequinocyl (Kanemite and Shuttle-O), bifenazate (Acramite and Floramite), fenpyroximate (Akari and Fujimite), spiromesifen (Judo and Oberon) were found effective in controlling Lewis mite.  These are also among the miticides recommended by UC IPM for spider mites on strawberries.   However, there have been some suspected resistance issues in some populations of Lewis mite in strawberry growing areas of California, but some of these materials act primarily as contact miticides and issues of coverage cannot be ruled out in all cases.

What to do: Look for Lewis mites while scouting for TSSM especially in fields close to infested ornamentals.  Keep in mind that there could be resistance or efficacy problems with some miticides in certain areas with TSSM or Lewis mites.  It is always a good practice to rotate miticides with different modes of action for better management and reducing the risk of resistance development.   The mode of action is indicated on most lables (e.g. Agri-Mek is a Group 6 Insecticide) If you suspect resistance in TSSM or Lewispopulations in your area, perform a simple test to evaluate the effectiveness of the intended miticides.  Dip several mite-infested leaves in a cup of miticide solution (concentration similar to the field application rate), let them dry and then cover (to keep foliage from drying out completely) and place in a cool area away from direct sun.  Check to see if any mites have survived 24 and 48 hours after dipping.  The effectiveness of the miticide will be indicated by the number of mites alive or dead.  For comparison, dip several similar mite-infested leaves in normal water and treat similarly, observing the survival of mites on those leaves as well.  This can be a useful comparison for new, untested miticides as well as for older products where miticide resistance is suspected.

UC researchers are working on this issue and will have some more information in the near future.  If you suspect or find Lewis mites in your fields, please contact me at skdara@ucdavis.edu or 805-788-2321.

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References

Bolland H. R., J. Gutierrez and C.H.W. Fletchtmann.  1998.  World Catalogue of the Spider Mite Family (Acari: Tetranychidae.) Brill, Leiden (NL).

European and Mediterranean Plant Protection Organization.  2006.  Eotetranychus lewisi.  OEPP/EPPO Bulletin 36: 161-163.

Gilrein, D.  2011.  Cornell Cooperative Extension of Suffolk County, LIHREC, Riverhead, NY.

Raros, L.A.C.  2001.  New mite pests and new host records of phytophagous mites (Acari) from the Philippines.  Philippine Agricultural Scientist 84: 341-351.