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Oats With Powdery Mildew – How To Treat Powdery Mildew On Oats

Oats With Powdery Mildew – How To Treat Powdery Mildew On Oats


By: Mary H. Dyer, Credentialed Garden Writer

Oatsare a common cereal grain, grown primarily for the seeds. Although we’refamiliar with oats for baked goods and breakfast cereal, their main purpose isas livestock feed. Like all plants, oats are sometimes affected by variousdiseases. While powdery mildew on oats isn’t the worst thing that can happen,it can markedly diminish crop quality and yield. Unfortunately, there isn’t alot that growers can do about the pesky fungal disease.

About Powdery Mildew on Oats

The severity of powderymildew outbreaks is dependent on climate, as the disease is favoredby mild, humid weather. It often shows up when temperatures are between 59 and72 F. (15-22 C.), but may disappear when the weather is dry and temperaturesexceed 77 F. (25 C.).

Powdery mildew spores can overwinter on stubble andvoluntary oats, as well as on volunteer barley and wheat. The spores spread byrain and can also travel great distances in wind.

Powdery Mildew Symptoms

Powdery mildew of oats appears as fluffy white patches onthe lower leaves and sheaths. As the disease progresses, the cottony patchesdevelop a gray or brown powder.

Eventually, the area around the patches and the underside ofleaves turn pale yellow, and leaves may die if the outbreak is severe. You mayalso notice tiny black spots on oats with powdery mildew. These are thefruiting bodies (spores).

How to Treat Powdery Mildew

There isn’t much you can do for oats with powdery mildew.The most important thing is to plant disease-resistant varieties. It also helpsto keep volunteer grains under control, and to manage stubble properly.

Fungicides may be of some help if applied early, before thedisease becomes severe. However, the limited control may not be worth theexpense. Even with fungicide, you aren’t likely to totally eradicate thedisease.

Also, keep in mind that powdery mildew is resistant to somefungicides. If you’re thinking about using fungicides, talk to the crop expertsat your localcooperative extension office.

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  • 1 Systematics
  • 2 Morphology
  • 3 Ecology
  • 4 Genetics
    • 4.1 Hosts and symptoms
    • 4.2 Disease cycle
    • 4.3 Environment
    • 4.4 Management
    • 4.5 Genetics
    • 4.6 Evolution of Blumeria gramimis f.sp. tritici
    • 4.7 Importance
  • 5 References

Previously B. graminis was included within the genus Erysiphe, but molecular studies have placed it into a clade of its own. Thus since 1975, the species graminis was moved into the new taxa Blumeria of which it is the only species. Blumeria differs from Erysiphe in its digitate haustoria and in details of the conidial wall. As well Blumeria is considered to be phylogenetically distinct from Erisiphe as it is a plant pathogen that hosts solely on the true grasses of Poaceae.

Eight special forms or formae speciales (ff.spp.) of B. graminis have been distinguished, each of which is parasitic on a particular genus of grasses. Those that infect crop plants are B. graminis f.sp. tritici, which causes powdery mildew of wheat and infects other grasses in the genera Triticum and Aegilops, f.sp. hordei on barley, f.sp. avenae on oats and f.sp. secalis on rye. Other formae speciales are pathogenic on wild grasses, including agropyri on grasses in the genera Agropyron and Elymus, bromi on Bromus spp., poae on Poa spp. and lolii on Lolium spp. (ryegrass).

The mycelium can cover the plant surface almost completely, especially the upper sides of leaves. Ascocarp is dark brown, globose with filamentous appendages, asci oblong. Ascospores hyaline, ellipsoid, 20–30 x 10–13 µm in size. Anamorph produces on hyaline conidiophores catenate conidia of oblong to cylindrical shape, not including fibrosin bodies, 32–44 x 12–15 µm in size. Haustoria are palmate.

Blumeria graminis asexually produced conidia and sexually formed ascospores.

Conidia were mainly distributed by wind, pests, or human activities. The water initiating ascospores were hypothesized to be dispersed not only by wind but also by splashing water-droplets. [1]

It is biotrophic, and does not grow on synthetic media. Relatively cool and humid conditions are favourable for its growth. Its relatively great genetic variability enables it often to infect previously resistant plant varieties.

The genome of Blumeria graminis f. sp. hordei has recently been sequenced. [1], as well as the genome of Blumeria graminis f. sp. tritici [2] Sequencing of the genome of the wheat powdery mildew Blumeria graminis f. sp. tritici, has allowed to infer important aspects of its evolution. It has been seen that it is the most repetitive fungal genome sequenced with 90% transposable elements. Additionally, 6540 genes were annotated, from which 437 encoded candidate secretor proteins and 165 for non-secreted candidate secretor proteins. These were shown to be subject to positive selection, due to their implication in the gene-for-gene relationship to defeat plant disease resistance. The ability to infect tetraploid as well as domesticated hexaploid wheat, was seen to be the result of mildew genomes being mosaics of ancient haplogroups that existed before wheat domestication. This has allowed wheat powdery mildew to maintain genetic flexibility, variability and thus a great potential for pathogen variation. It is hypothesized that this mosacisism can be maintained through clonal reproduction in population with small effective size or quasi-clonal reproduction in populations with large effective size.

Powdery mildew of wheat is relatively easy to diagnose [2] due to the characteristic little white spots of cotton like mycelia. [3] These can appear on the upper and lower epidermis of the leaves. As the disease progresses they become a light tan color. [3] Blumeria graminis f. sp. tritici is an obligate parasite which means it only grows on living tissue. Though present throughout wheat growing regions, it especially favors the eastern seaboard of the United States as well as coastal regions of the United Kingdom.

Hosts and symptoms Edit

Triticum spp. (wheat) is the only host of Blumeria graminis f. sp. tritici. [2] Signs on the foliage of wheat are white, powdery mycelium and conidia. [4] As the disease progresses, the patches turn gray and small dark black or brown cleistothecia form in the mycelium mass. [5] Symptoms progress from lower to upper leaves. Symptoms of powdery mildew are chlorotic areas surrounding the infected areas. [4] The lower leaf surface corresponding to the mycelial mat will also show chlorosis. [5] Lower leaves are commonly the most infected because of higher humidity around them. [2]

Disease cycle Edit

Blumeria graminis f. sp. tritici has a polycyclic life cycle typical of its phylum, Ascomycota. Powdery mildew of wheat overwinters as cleistothecia dormant in plant debris. Under warmer conditions, however, the fungus can overwinter as asexual conidia or mycelium on living host plants. It can persist between seasons most likely as ascospores in wheat debris left in the field. Ascospores are sexual spores produced from the cleistothecia. These spores, as well as conidia, serve as the primary inoculum and are dispersed by wind. Neither spore requires free water to germinate, only high relative humidity. [5] Wheat powdery mildew thrives in cool humid conditions and cloudy weather increases chances of disease. When conidia land on a wheat leaf’s hydrophobic surface cuticle, they release proteins which facilitate active transport of lightweight anions between leaf and fungus even before germination. This process helps Blumeria recognize that it is on the correct host and directs growth of the germ tube. [6] Both ascospores and conidia germinate directly with a germ tube. Conidia can recognize the host plant and within one minute of initial contact, the direction of germ tube growth is determined. The development of appressoria then begins infection following the growth of a germ tube. [7] After initial infection, the fungus produces haustoria inside of the wheat cells and mycelium grows on the plant’s outer surface. [5] Powdery mildew of wheat produces conidia during the growing season as often as every 7 to 10 days. [8] These conidia function as secondary inoculum as growth and reproduction repeat throughout the growing season.

Environment Edit

Powdery mildew of wheat thrives in cool, humid climates and proliferates in cloudy weather conditions. [9] The pathogen can also be an issue in drier climates if wheat fields are irrigated. [10] Ideal temperatures for growth and reproduction of the pathogen are between 60 °F (16 °C) and 70 °F (21 °C) with growth ceasing above 77 °F (25 °C). Dense, genetically similar plantings provide opportune conditions for growth of powdery mildew. [5]

Management Edit

Controlling the disease involves eliminating conducive conditions as much as possible by altering planting density and carefully timing applications and rates of nitrogen. Since nitrogen fertilizers encourage dense leafy growth, nitrogen should be applied at precise rates, less than 70 pounds per acre, to control decrease severity. Crop rotation with non-host plants is another way to keep mildew infection to a minimum, however the aerial nature of conidia and ascospore dispersal makes it of limited use. Wheat powdery mildew can also be controlled by eliminating the presence of volunteer wheat in agricultural fields as well as tilling under crop residues. [8]

Chemical control is possible with fungicides such as triadimefon and propiconazole. Another chemical treatment involves treating wheat with a silicon solution or calcium silicate slag. Silicon helps the plant cells defend against fungal attack by degrading haustoria and by producing callose and papilla. With silicon treatment, epidermal cells are less susceptible to powdery mildew of wheat. [11]

Milk has long been popular with home gardeners and small-scale organic growers as a treatment for powdery mildew. Milk is diluted with water (typically 1:10) and sprayed on susceptible plants at the first sign of infection, or as a preventative measure, with repeated weekly application often controlling or eliminating the disease. Studies have shown milk's effectiveness as comparable to some conventional fungicides, [12] and better than benomyl and fenarimol at higher concentrations. [13] Milk has proven effective in treating powdery mildew of summer squash, [13] pumpkins, [12] grapes, [14] and roses. [14] The exact mechanism of action is unknown, but one known effect is that ferroglobulin, a protein in whey, produces oxygen radicals when exposed to sunlight, and contact with these radicals is damaging to the fungus. [14]

Another way to control wheat powdery mildew is breeding in genetic resistance, using "R genes" (resistance genes) to prevent infection. There are at least 25 loci on the wheat genome that encode resistance to powdery mildew. If the particular variety of wheat has only one loci for resistance, the pathogen may be controlled only for a couple years. If, however, the variety of wheat has multiple loci for resistance, the crop may be protected for around 15 years. Because finding these loci can be difficult and time consuming, molecular markers are used to facilitate combining resistant genomes. [9] One organization working towards identifying these molecular markers is the Coordinated Agricultural Project for Wheat. With these markers established, researchers will then be able to determine the most effective combination of resistance genes. [15]

Genetics Edit

It is the most repetitive fungal genome sequenced to the moment with 90% transposable elements [16] (March 2013). 6540 genes have been annotated, a number similar to that in yeasts, but lower than for the rest of fungal genomes. The analysis of these genes has revealed a similar pattern to that found in other obligate biotrophs of lower presence of genes implied in primary and secondary metabolism.

Evolution of Blumeria gramimis f.sp. tritici Edit

Wheat powdery mildew is an obligate biotroph with a poorly understood evolutionary history. Sequencing its genome in 2013, many aspects of the evolution of its parasitism were unveiled [16] . Obligate biotrophy has appeared multiple times in evolution in both Ascomycetes like B. graminis and Basidiomycetes, thus different selective pressure must have acted in the different organisms through time. It has been seen that B. graminis f.sp. tritici's genome is a mosaic of haplogroups with different divergence times, which explains its unique pathogen adaptability. Haplogroup Hold (diverged 40-80 mya) allows for the infection of wild tetraploid wheat and Hyoung (diverged 2-10 mya) allows for the infection of both domesticated hexaploid wheat. It is hypothesized that this mosaicisms has been maintained through clonal propagation in populations with small effective size or through quasi-clonal propagation in populations with large effective size. Additionally, it has been seen that there is a positive selective pressure acting on genes that code for candidate secretor proteins and non-secreted candidate secretor proteins, indicating that these might participate in the gene-for-gene relationship of plant disease resistance.

Importance Edit

Powdery mildew can be found in all wheat growing areas of the United States but usually will be most severe in the east and southeast. [5] It is more common in areas with a humid or semi-arid environment where wheat is grown. [5] Powdery mildew has become a more important disease in some areas because of increased application of nitrogen fertilizer, which favors the development of the fungus. [4] Severe symptoms of powdery mildew can cause stunting of wheat. [4] If unmanaged, this disease can reduce yields significantly by reducing photosynthetic areas and causes non-seed producing tillers. [2] Powdery mildew causes reduced kernel size and lower yields. [8] The sooner powdery mildew begins to develop and how high on the plant it develops by flowering the larger the yield loss. [8] Yield Losses up to 45 percent have been shown in Ohio on susceptible varieties when plants are infected early and weather favors disease. [8]


What are the Symptoms?

Powdery mildews are one of the easier diseases to recognize. They differ from other fungi as most of the fungus is outside of the plant as opposed to inside the host plant. As the name suggests, the masses of fungal spores (conidia) responsible for these diseases, give the plant the appearance of being coated with flour or talcum powder (See Figure 1). Areas of white to grayish growth (mycelium) can appear on young plant tissues (leaves, stem, and fruit), and can become severe enough that the entire surface is covered. As the mycelium ages, the mildew may take on a light reddish brown to gray appearance, the result of the production of fruiting structures (cleistothecia) (See Figure 2).

In addition to the growth over plant surfaces, symptoms of powdery mildew can range from symptomless to significant distortion of leaves, flowers, fruit, and even entire shoots on broad leaf plants. Many monocots become chlorotic and eventually senescent, and stunted. In the case of cereal crops, the quality of the grain may be reduced.


Figure 2
Fruiting bodies (cleistothecia) forming on Phlox leaves


Materials and methods

Plant materials and DNA isolation

The Pm3 mapping population Kanota × Rollo comprised 79 F2:3 lines and was reported in Mohler et al. (2012). A set of 104 oat cultivars/lines (Table S1) was used to assess the genotype frequency and the predictive ability of SNP markers linked to Pm3. For the diversity panel, genomic DNA was extracted from lyophilized primary wheat leaves as described by Plaschke et al. (1995). For 53 oats of the collection, the Pm3 and other Pm phenotypes were known from previous studies (Hsam et al. 1997, 1998, 2014 Yu and Herrmann 2006 Herrmann and Mohler 2018). The pedigrees of the oat lines carrying Pm3 (Table S2) were accessed from the POOL database (Tinker and Deyl 2005 https://triticeaetoolbox.org/POOL/).

Phenotypic data

The phenotypic data for Kanota × Rollo originated from Mohler et al. (2012) and are based on seedling inoculation tests that used 12 to 16 plants for each F2:3 line. The isolate HGB2/1, known to carry avirulence for Pm3 from monosomic analysis of powdery mildew resistance in Rollo (Hsam et al. 2014), was spread in a settling tower on leaf segments at densities of 400–500 spores/cm 2 . The leaf segments were cultured in plastic dishes on 6 g/l agar and 35 mg/l benzimidazole. The conditions of incubation were under continuous lighting at 10 μE/m 2 s in a growth chamber at 17 °C and at 70% relative humidity. Ten days after inoculation, two classes of host reactions relative to the susceptible control cultivar Fuchs were distinguished: resistant (0–20%) and susceptible (> 50% infection) no intermediate (30–50%) infections were observed.

BLASTn analysis

Similarity to oat DNA sequences was searched for barley cDNA RFLP markers cMWG704 and cMWG733 by using their DNA sequences as queries against genetically mapped sequences and the hexaploid oat genome assembly lodged in the T3/oat database using default settings (https://triticeaetoolbox.org/oat/). The RFLP marker sequences were obtained from the GrainGenes database (https://wheat.pw.usda.gov). Matched sequences as well as all other oat marker sequences from the target linkage group Mrg18 used for genetic mapping were subsequently blasted against the wheat reference genome sequence (RefSeq_v1.0 International Wheat Genome Sequencing Consortium, 2018) in the Ensembl Plants database (http://plants.ensembl.org/index.html). The function of the detected high confidence protein-coding genes was retrieved from the T3/wheat database (https://triticeaetoolbox.org/wheat/) (Tables S3 and S4).

Genetic mapping

A total of 32 framework markers consisting of 6 K array SNP markers (GMI) and GBS markers (avgbs) and distributed along the linkage group Mrg18 of the oat consensus map (Chaffin et al. 2016) were used for SNP development and data collection (Table S3). The SNP assays were designed by Fluidigm Corporation (South San Francisco, USA). SNP marker genotypes were recorded on an EP1 genotyping platform using 192.24 Dynamic Array integrated fluidic circuits. All SNP genotyping analysis protocols can be found in the user guide published by the manufacturer (https://www.fluidigm.com). Genotyping with polymorphic SNP markers was done in double. The SNP data of the mapping population Kanota × Rollo were merged with previously established genotypic data, i.e., RFLP and amplified fragment length polymorphism (AFLP) markers, and the binary Pm3 phenotype (Mohler et al. 2012). To avoid complications in positioning tightly linked dominant markers from opposite linkage phases as accurately as possible (Mester et al. 2003), two separate but related linkage maps, both share the co-dominant markers, were computed with JoinMap® software version 5.0 (Kyazma BV, Wageningen, The Netherlands). The “maternal” map included dominant markers that were scored as heterozygous in the female parent (Kanota) and homozygous in the male parent (Rollo), while “paternal” markers were heterozygous in Rollo and homozygous in Kanota. Linkage of loci was claimed at a logarithm of the odds ratio (LOD) score ≥ 3.0, with a maximum recombination fraction of 0.4. Regression mapping was performed using the Haldane mapping function. Genetic linkage maps were drawn with Mapchart 2.1 software (Voorrips 2002). Chi-squared tests for goodness of fit were used to test for deviation of observed data from theoretically expected segregation ratios. Chi-squared values were corrected for continuity (http://vassarstats.net/csfit.html).


Why Choose Safer® Brand?

Sometimes with a do-it-yourself option, it can be difficult to ensure the ratios of ingredients are correct, and if they aren’t just so, the treatment may not work how you were anticipating. You may also need to use caution on which parts of the plants you apply homemade remedies. When trusting your plant’s health to the experts at Safer® Brand, you know for sure what you’re getting in each bottle and that it’s safe and effective for all parts of the plant.

Safer® Brand’s powdery mildew treatments are OMRI certified, which means it’s approved for use in organic gardening. It works by utilizing the power of sulfur compounds, which ultimately alter the plant’s pH. It’s not harmful to the plant, but powdery mildew and other fungi cannot survive.

Another benefit of using a fungicide that’s formulated for organic gardening like those Safer® Brand offers is that it doesn’t harm the soil. Millions of tiny microbes reside in the soil, providing your plant with nutrients and protecting it from pathogens. Using organic disease-control methods can help keep beneficial microorganism populations where they should be and your soil and plants healthy.

So, whether it’s powdery mildew on squash or powdery mildew on roses, Safer® Brand’s line of organic gardening treatments can help your plants, and the environment, stay disease-free and beautiful.


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