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.
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.
An exotic bug is found invading parts of southern California and Arizona. It is referred to as painted bug in some literature and is similar to the harlequin bug, Murgantia histrionic in appearance. But it is called Bagrada bug, Bagrada hilaris (Burmeister) and belongs to the stink bug family Pentatomidae of the order Hemiptera.
Origin and distribution: It is native to Africa and is reported to infest and/or cause crop damage in parts of Asia and Europe. It is an exotic pest in the US. It was first reported in Los Angeles in June, 2008 and started causing damage to broccoli, cabbage, cauliflower, kale, radish, rutabaga, collards and other crops by the next year. It is now seen in Orange, Imeperial and Riverside Counties of California and all over Yuma Co in Arizona.
Host range: Feeds mainly on crucifers like cole crops, but can infest a variety of other hosts including solanaceous plants like potato, malvaceous plants like okra and cotton, leguminoseous plants like legumes, cucurbits like cantaloupes and watermelons, and graminaceous plants like wheat, corn and millets.
Biology: Adults are 5-7 mm long and 3-4 mm wide. They are black with orange and white markings. Females are larger than males and lay an average of 95 barrel shaped whitish eggs in clusters on foliage or in the soil. Eggs turn orange as they mature in 3-6 days. Nymphs resemble ladybugs due to their dark head and thorax and reddish or orange abdomen with white or black markings. They go through five instars before adults emerge in 5-8 weeks depending on the temperature. They have multiple generations in a year.
Damage: Bagrada bugs have piercing and sucking mouthparts and feed on the plant juices. Depending on the crop and plant part they infest, damage can vary from stippling with necrotic spots, stunted growth, loss of apical dominance and formation of multiple heads to death.
Stippling of young cauliflower leaves (Courtesy: Eric Natwick, UCCE)
Multiple heads in broccoli (Courtesy: John Palumbo, Univ. of Arizona)
Management: Reports indicate that Bagrada bug can be controlled with pyrethroids, organophosphates like chlorpyrifos and malathion and neonicotinoids like imidacloprid in conventional fields using different treatment methods. Neem products have also showed some effectiveness in a study conducted abroad. Biological and microbial control options are being evaluated by some researchers.
What to do: It has not been reported in vegetable growing California Central Coast or areas other than those mentioned above. Since they are already found on several cole crops in the neighboring counties, those in Ventura, Santa Barbara, San Luis Obispo Counties or surrounding areas should keep an eye out for this bug. It is important to be aware of this new pest and report its occurrence. If you find it in a new area, please call (805-788-2321) or email (firstname.lastname@example.org) me.
Ahuja, B., R. K. Kalyan., U. R. Ahuja, S. K. Singh, M. M. Sundria and A. Dhandapani. 2008. Integrated management strategy for painted bug, Bagrada hilaris (Burm.) inflicting injury at seedling stage of mustard (Brassica juncea) in arid western Rajasthan. Pesticide Res. J. 20: 48-51.
Halbert, S. E. and J. E. Eger. 2010. Bagrada bug (Bagrada hilaris) (Hemiptera: Pentatomidae) an exotic pest of cruciferae established in the Western USA. Florida Dept of Agriculture and Consumer Services, DACS-P-01750./span>
I have gathered some more information after my first blog and here is an update on the maggot issue we are seeing in Santa Maria and parts of Oxnard.
Santa Barbara Co entomologist, Brian Cabrera and UC Davis entomologist, Frank Zalom both recovered larvae of the dark-winged fungus gnat (Family: Sciaridae) from the strawberry plants they received. These larvae primarily feed on fungus and decaying plant material, but they can also feed on plant roots. They are also known to mechanically vectoring plant pathogenic fungi.
Dark-winged fungus gnat larva from strawberry plants (Courtesy: Brian Cabrera)
A PCA had reported finding seedcorn maggot-like larvae in the strawberry fields near Oxnard. A few growers had also mentioned similar findings in Santa Maria. I too found similar maggots in fields I checked later on. So, we have two species infesting the strawberry plants at this time.
Seedcorn maggot flies are attracted to decaying plant material, but their maggots feed on living plant material. Dark-winged fungus gnats do not appear to be new in the strawberry fields. Brian Cabrera found them early this year in dead strawberry plant specimens. The probable reason for seeing more of them now is that there is more dead plant material from the other maggot damage. Is it possible that we have a primary invader, seedcorn maggot-like one and a secondary invader, the dark-winged fungus gnat larva? This is just a thought. High soil moisture from recent rains can also worsen their problem. Damage from both maggots can also increase vulnerability of plants to pathogens.
Organophosphates will control these maggots, but Frank Zalom mentioned that growth regulators like azadirachtin can also be effective. Neonicotinoids like imidacloprid are not known to have any effectiveness.
We need to continue close monitoring of this problem and share the information.
Whitish maggots with a tapering body (above) and about 1/4 inch long (below)
Several growers in the Santa Maria area reported maggot infestation in their young strawberry fields. I have visited some fields and collected some specimens. These maggots are whitish with a tapering body and approximately ¼ inch long. They are legless and seemed to have a thick and opaque skin. These look like seedcorn maggots [Delia platura (Family: Anthomyiidae)], but accurate identification is pending. UC researchers, CDFA and Ag Commissioner's office has the specimens and we are working on identifying the pest.
Maggots were found feeding in the root area at the base of the crown. Seedcorn maggot adults are usually attracted to decaying crop residue and moist soil. It is a good idea to set up yellow sticky traps in the fields for catching adults to help with the identification. It appeared that some growers have treated the fields with imidacloprid, diazinon and chlorpyrifos.
Someone at the field mentioned that these are thought to be fungus gnats. But these are not. Fungus gnat larvae have a whitish to translucent and elongated body. They have wormlike appearance and have a conspicuous shiny black head capsule.