Written by Achour Amiri, Associate Professor, Department of Plant Pathology, Washington State University, Tree Fruit Research and Extension Center, Wenatchee, WA and Lederson Gañán-Betancur. February 2019, updated July 2024.
Powdery Mildew Spec Sheet
Table 1. Apple powdery mildew.
| Key element | Description |
|---|---|
| common name | Podosphaera leucotricha |
| pathogen type | fungal pathogen |
| reproductive mode | asexual and sexual |
| inoculum sources | dormant buds, sexual bodies (Cleistothecia) |
| occurrence in the pacific northwest | widespread |
| active season | spring, early summer, fall |
| critical environmental factors | temperature optimal between 55 and 80 F |
| known host resistance | yes, varies between cultivars |
| best management practices | sanitation, host resistance, chemical, biological |
Description and Identification
Powdery mildew, caused by the fungus Podosphaera leucotricha, is a fungal disease that affects apple and pear trees. Unlike some other pathogens, this fungus can only survive on its specific hosts, making it an obligate pathogen. Powdery mildew primarily affects the vegetative tissues of the trees. Under high disease pressure, it can even cause the blossoms to girdle and abort (Figure 1B) and the fruit to russet (Figure 1D), especially on highly susceptible cultivars. If left unmanaged, severe and repeated infections can significantly reduce fruit set and yield, affect the vigor of the trees, and ultimately impact the longevity and profitability of an orchard. It is important to note that the powdery mildew fungus that infects pome fruit is specific to its hosts. However, recent studies have reported its presence on related and on unrelated crops (Amiri and Ganan, 2019).
Infection and Life Cycle of Organism
The fungus (P. leucotricha) causes spring and early-summer epidemics, starting from the green tip stage in the early season until the end of new shoot growth (Figure 2). In apples, the fungus overwinters in the form of mycelia in dormant buds (Hickey and Yoder, 2014), whereas its survival mode remains unknown in pear trees. Because infected buds open later than healthy ones (Hickey and Yoder, 2014), there is a higher risk of infection when conidia released from the infected buds infect new leaves that have emerged earlier from healthy buds. Additionally, the fungus may also survive as cleistothecia (sexual bodies) on or stems leaves (Ganan-Betancur et al., 2021) (Figure 1C), although the exact role of these structures in early infections is not yet fully understood (Gadoury and Pearson, 1988; Grove and Boal, 1991, Ganan et al. 2021).
Temperature is the primary environmental factor that has the most significant impact on the development of mildew. The optimal temperature range for spore release is between 59 and 68 °F, while germination is optimal at 68 °F for P. leucotricha (Grove and Boal, 1991; Grove, 1998; Hickey and Yoder, 2014; Ogawa et al., 1995; Xu and Butt, 1998). The incubation period, prior to the emergence of the first symptoms, can vary from 3 to 12 days depending on the temperature, with an average of 8 days under favorable conditions (Xu, 1996). Temperatures below 50 °F and above 86 °F significantly inhibit growth and infections.
The germination and growth of P. leucotricha are not significantly influenced by ambient humidity. Germination on apple leaves can occur within a relative humidity range of 40% to 100% (Butt, 1978; Molnar, 1971; Roosje, 1961; Xu and Butt, 1998). However, the dispersal of P. leucotricha conidia is negatively affected by higher relative humidity (Sutton and Jones, 1979), and germination is completely inhibited in the presence of free water (Butt, 1978). Rainfall can limit dispersal. In apple orchards, the dispersal of P. leucotricha is primarily diurnal, with concentrations of conidia peaking around midday (Grove, 1998; Sutton and Jones, 1979). Therefore, it is recommended to perform early morning sprays.
Management of apple powdery mildew
The incorporation of robust host resistance (Amiri and Betancour, 2019), represents the first step towards achieving a more sustainable approach to managing powdery mildews in tree fruit. Nevertheless, due to the lack of absolute immunity and the potential emergence of physiological strains capable of circumventing specific host resistance, implementation of Integrated Pest Management (IPM) strategies is advised to effectively control the disease.
Cultural Controls
Host resistance
Commercial apple cultivars display variable levels of susceptibility ranging from high susceptibility to high levels of resistance to P. leucotricha (Figure 3A), whereas no cultivar is known to be immune. Different levels of susceptibility to P. clandestina have also been reported in cherry cultivars (Figure 3B). On pearP. leucotricha can be a concern on d’ Anjou which makes more than 90% of the total acreage in central Washington. The pear cv. ‘Louis Bonne’ has also been reported to be susceptible to P. leucotricha (Hickey and Yoder, 2014).
Sanitation and removal of hosts
Although it can be labor-intensive, pruning infected shoots showing the first symptoms of powdery mildew in early spring or during the vegetative growth period will help to reduce the inoculum load. Research has shown that pruning can significantly decrease apple powdery mildew incidence on both shoots and fruit, especially when combined with fungicide applications (Hickey and Yoder, 2014; Holb and Kuntz, 2016). Another recommended practice to lower inoculum loads is the removal of other potential susceptible hosts, such as rose bushes near commercial tree fruit orchards.
Chemical Controls
Mildew management begins at the tight cluster stage when buds are breaking and continues every 10 to 14 days until the end of vegetative growth. In wet apple growing regions, the management of apple powdery mildew is often combined with scab management, as most registered fungicides are effective against both pathogens. However, in dry regions like central Washington where scab is not a major concern, reduced fungicide regimes use can increase the risk of apple powdery mildew infection. In addition to multi-sites carbonates and sulfur, fungicides from three main chemical groups – demethylation inhibitors (DMI), quinone inhibitors (QoI), and succinate dehydrogenase inhibitors (SDHI) – are registered to control mildews in tree fruit and other crops (Table 1). Apart from the fungicides listed in Table 1, cyprodinil (FRAC group 9) is mixed with difenoconazole (DMI, FRAC 3) and used in apple orchards for mildew control.
Table 2 Fungicides registered to control of powdery mildews in conventional orchards.
| Trade name | Active ingredient | MOA | Rate per acre | PHI | Efficacy |
|---|---|---|---|---|---|
| Procure 480SC, Trionic 4SC | triflumizole | 3 | 8-16 fl oz | 14 | 4 |
| Rally 40WSP | myclobutanil | 3 | 5-19 oz | 14 | 4 |
| Topguard | fluritriafol | 3 | 8-12 fl oz | 14 | 4 |
| Inspire Super | cyprodinil + difeneconazole | 3 + 9 | 12 fl oz | 14 | 3 |
| Fontelis | penthiopyrad | 7 | 14-20 fl oz | 28 | 3 |
| Aprovia | benzovindiflupyr | 7 | 5.5-7 fl oz | 30 | 3 |
| Luna Privelege | fluopyram | 7 | 2.4-6.8 fl oz | 7 | 3 |
| Miravis | pydiflumetofen | 7 | 3 fl oz | 30 | 3 |
| Excalia | indiflin | 7 | 3-4 fl oz | PF | 3 |
| Flint Extra | trifloxystrobin | 11 | 2.5-3 fl oz | 14 | 4 |
| Luna Sensation | fluopyram + trifoxystrobin | 7+11 | 5-5.8 fl oz | 14 | 4 |
| Luna Tranquility | fluopyram + pyrimethanil | 7+9 | 11-16 fl oz | 3 | 4 |
| Oso SC, Ph-D | polyoxin D zinc salt | 19 | 3.7-13 fl oz | 0 | 3 |
| Gatten | flutianil | U13 | 6-8 fl oz | 14 | 3 |
| Lime sulfur | calcium polysulfide | M02 | 3-6 gal | 0 | 2 |
MOA = mode of action. PHI = pre harvest interval in days. Efficacy based on recent trials conducted in Washington state. Results may vary by season and location.
In addition to evaluating the effectiveness of a specific spray program, management strategies must also consider two crucial factors: i) managing fungicide resistance and ii) impact on non-target organisms. It is highly recommended to implement rotation and tank-mixing of fungicides with different modes of action to reduce the risk of selecting for resistant fungal populations. In central Washington, it is advised to utilize DMI fungicides instead of SDHI fungicides early in the season to limit the development of resistant populations in other fungal pathogens such as gray mold (Botrytis cinerea) and other preharvest and storage rots against which Group 7 fungicides have a better activity. The combinations of boscalid + pyraclostrobin (Pristine) and fluxapyroxad + pyraclostrobin (Merivon) are commonly employed in the weeks leading up to harvest to control storage rot pathogens. Thus, their usage during spring and mid-summer should be avoided to prevent resistance development in storage pathogens. Polyoxin D (OSO, PhD), which has shown efficacy against APM in apple (Amiri, 2018), is another fungicide with relatively low risk and potential for rotation with Group 3 fungicides. Ultimately, it is important to consider that the nature and effectiveness of a given spray program may vary depending on the region and environmental conditions.
In organic orchards, the management of powdery mildew can be quite challenging, particularly on susceptible cultivars. In central Washington, for instance, the susceptible apple cultivars ‘Granny Smith’, ‘Cripps Pink’, and ‘Honeycrisp’ make up nearly 50% of the total organic acreage. Therefore, it is crucial to implement enhanced disease management programs. Alongside appropriate sanitation practices, preventive applications, and rotations of various bio-pesticides from tight cluster to the end of new shoot growth, in early to mid-summer, should aid in keeping powdery mildew below the economic threshold. While sulfur is widely used for controlling apple powdery mildew in organic orchards, it is worth noting that sulfur may not be effective in high disease pressure conditions and can potentially cause phytotoxicity, particularly at temperatures above 28 °C. Table 2 provides a list of other bio-pesticides registered for powdery mildew control in organic orchards in the U.S. Pacific Northwest region. However, their efficacy in this region and under different environmental conditions in various regions requires further assessment.
Table 3 Products registered to manage powdery mildew of pome in organic orchards in the U.S. Pacific Northwest
| Trade name | Active ingredient | MOA | Rate per acre | Efficacy |
|---|---|---|---|---|
| Lime sulfur | calcium polysulfide (29%) | M02 | 3-6 gal | 2 |
| OSO SC (5%) | polyoxin D zinc salt | 19 | 3.7-13 fl oz | 3 |
| Kaligreen | potassium bicarbonate | o. chemical | 2.5-3 lbs | 2 to 3 |
| Stylet oil | paraffin oil | o. chemical | 1-2 gal | 3 |
| Rango (1.25%) | neem oil cold pressed | o. chemical | 6 qt | 3 |
| Actinovate AG | Streptomyces lydicus WYEC 108 | biological | 3-12 fl oz | 2 |
| Double Nickel LC | Bacillus amyloliquefaciens strain D747 | biological | 64 fl oz | 2 |
| Sonata | Bacillus pumilis strain QST 2808 | biological | 2-4 qt | 2 |
| Aviv | Bacillus subtilis strain IAB/BS03 | biological | 10-30 fl oz | 2 |
| Stargus | Bacillus amyloliquefaciens strain F727 | biological | 1-4 qt | 2 |
| Pathocurb | thyme oil | plant extract | 64-128 fl oz | 2 |
| Timorex ACT | tea tree oil | plant extract | 20 fl oz | 3 |
| Seican | cinnamaldehyde | plant extract | 32 fl oz | 3 |
| ProBlad Verde | blanda de lupinus albus doce | plant extract | 45 fl oz | 2 |
| Regalia | extract Reynoutria sachalinensis | plant extract | 48 fl oz | 2 |
MOA = mode of action. PHI = pre harvest interval in days.
Materials available for apple and pear
Excerpt from the WSU Crop Protection Guide. For timings at which each pesticide can be used refer to the Crop Protection Guide.
Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.
Contact
Dr. Achour Amiri,
Postharvest Pathology
Tree Fruit Research and Extension Center
Wenatchee, WA
a.amiri@wsu.edu
509-293-8752
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