Podosphaera clandestina (Powdery Mildew) risk


Powdery Mildew of Stone Fruits

Powdery Mildew is the most common disease of tart cherry and peach trees . Mildew infections result in a white powdery mass of fungal growth on susceptible tissue. The fungi that cause this disease are Podosphaera clandestina on cherry. These fungi frequently infect new vegetative growth, causing reduced vigor, leaf malformation, and reduced viability of buds. In addition to Sphaerotheca pannosa, peach can also be infected by the apple powdery mildew fungus, Podosphaera leucotricha, which causes rusty spot disease on peach fruit.

Mildew is more serious on tart than on sweet cherries, but both can be seriously affected under ideal conditions. Mildew causes uneven ripening of tart cherries and makes mechanical harvesting more difficult. In some years, mildew may infect sweet cherry fruit or petioles, causing distortion and poor quality. Under prevailing  climatic conditions, peach powdery mildew affects leaves and shoots and less commonly fruit. Mildew may reduce the vigor of all fruit trees and reduce return bloom. Young trees and vigorously growing shoots are the most susceptible.

Conditions for the infection

This fungus overwinters as cleistothecia. The cleistothecia drop to the orchard floor and tree crotches or become trapped in bark crevices.  Spores released from the cleistothecia in the spring are spread by rain or irrigation to young leaves. The earliest infections are found on leaves of suckers or succulent terminal growth near the crotches. These infections produce conidia in repeated cycles during the summer, resulting in the powdery appearance of infected leaves. Late in the summer, the fungus produces the cleistothecia.

Powdery mildew is most common when the relative humidity exceeds 90 percent and daytime temperatures are between 50 - 78F (10°C-26°C)  although some infections can occur when humidity is quite low. Long periods of rain are not necessary for infections since the spores will not germinate in free water.

In Fieldclimate.com the risk of a Powdery Mildew infection is determined by temperature and leaf wetness period. Risk will be increased by temperatures above 15°C and leaf wetness and reduced by temperatures lower than 15°C and dry conditions. On the beginning of May temperatures have been above 15°C and a leaf wetness period on the 7th of May supported the development of the disease. A risk of 100% could be determined under this favourable conditions.


Control options

Where powdery mildew has been a problem in the past, fungicides may be needed. Fungicides function as protectants, eradicants, or both.

A protectant fungicide can only prevent a new infection from occurring, but an eradicant will kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears (for example in FC a risk of 80% and favourable conditions in the next hours (forecast). Eradicants should be used at the earliest appearance of the disease (at 100% risk). Once mildew growth is extensive, control with fungicides becomes more difficult.

Several least-toxic fungicides are available for backyard trees and vines, including horticultural oils, neem oil, jojoba oil, sulfur, and the biological fungicide Serenade. With the exception of the oils, these materials are primarily preventive. Oils work best as eradicants but also work as good protectants.

Oils. To eradicate powdery mildew infections, use a horticultural oil such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil or one of the plant-based oils such as neem oil (such as Green Light Neem Concentrate) or jojoba oil (such as E-rase). Be careful, however, never to apply an oil spray within 2 weeks of a sulfur spray or plants may be injured. Some plants may be more sensitive than others, however, and the interval required between sulfur and oil sprays may be even longer; always consult the fungicide label for any special precautions. Also, oils should never be applied when temperatures are above 90°F or to drought-stressed plants. Horticultural oils and neem and jojoba oils are registered on a wide variety of crops.

Sulfur. Sulfur products have been used to manage powdery mildew for centuries but are only effective when applied before disease symptoms appear. The best sulfur products to use for powdery mildew control in gardens are wettable sulfurs that are specially formulated with surfactants similar to those in dishwashing detergent (such as Safer Garden Fungicide). To avoid injury to the plant or tree, sulfurs should not be applied within 2 weeks of an oil spray, used on any plant when the temperature is near or over 90°F (80°F for caneberries and strawberry), and never applied at any temperature to apricot trees.

Biological Fungicides. Biological fungicides (Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on the plant, inhibit or destroy fungal pathogens. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew from infecting the plant. While this product functions to kill the powdery mildew organism and is nontoxic to people, pets, and beneficial insects, it has not proven to be as effective as the oils or sulfur in controlling this disease.

Apply protectant fungicides to susceptible plants before disease develops. Once mildew growth is mild to moderate, it is generally too late for protective fungicides to effectively control powdery mildew except for on new plant growth. The protectant fungicides are only effective on contact, so applications must provide thorough coverage of all susceptible plant parts. As plants grow and produce new tissue, additional applications may be necessary at 7- to 10-day intervals as long as conditions are conducive to disease growth. On highly susceptible plants, sulfur can be applied early in the season when temperatures are below 90°F and then to switch to other materials as the season progresses. However, applying oil, which is both a protectant and an eradicant, for the early sprays provides the best control.

If mild to moderate powdery mildew symptoms are present, the horticultural oils and plant-based oils such as neem oil and jojoba oil can be used.

Source:http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7494.html (University of Davis,


Created by system. Last Modification: Wednesday 02 of September, 2015 08:12:07 GMT by cpilz.