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Powdery Mildew on Gerbera by Paul Pilon

April 19, 2011

Powdery mildew is one of the most common diseases observed on Gerbera. Although, this pathogen can be observed in the plug stages, it occurs most frequently late in the production cycle of the finished crop. The powdery mildew that most commonly attacks Gerbera is Erysiphe cichoracearum. This pathogen is particularly problematic for growers because the infections occur so frequently. Under severe infestations, the aesthetic appeal and plant quality is greatly reduced which may render the plants unmarketable.

Initially, powdery mildew appears as small whitish-grey talcum-like powdery patches (¼ to ½ inch in diameter) on the upper leaf surfaces. When the environmental conditions are favorable for disease development, these patches (also referred to as colonies) quickly coalesce together and often cover the entire plant. Many growers are surprised how quickly powdery mildew covers leaves and entire plants; seemingly overnight.

Unlike many foliar diseases, powdery mildew does not require free water on leaf surfaces for infections to occur. A fairly high relative humidity (greater than 70%) and moderate temperatures (62 to 72º F) will promote the initial infection. Once the pathogen has infected the host plant, the relative humidity is no longer a factor as powdery mildew can prosper regardless of the humidity. Most epidemics develop when the humidity levels are high.

Spores of this pathogen are spread by air movement. Once the spores land on plants, it takes 3 to 7 days for an infection to develop and form visible colonies, provided favorable conditions are present. The spores release, germinate, and cause infections without a film of water on the plant surface.

The occurrence of powdery mildew on Gerbera can be reduced if high humidity levels or dramatic swings in humidity are avoided. When production occurs within enclosed structures, it is beneficial to provide humidity purges during night to reduce the humidity (<70%) inside the structure as the humidity usually rises at night. Maintaining sufficient plant spacing and increasing the air circulation are also useful to reduce the humidity levels near the plant canopy where infections occur. 

Scout crops of Gerbera at least once a week, more if favorable conditions for this disease have occurred. The first mildew colonies can be difficult to detect; look for small colonies on the lower and middle portions of the plants. Examine the upper and lower leaf surfaces. Detecting powdery mildew early and implementing control strategies upon detection will help growers greatly reduce the severity of the infections. It is very difficult to eradicate existing, well-established mildew colonies. 

Powdery mildew can be controlled using products containing kresoxim-methyl, myclobutanil, petroleum oil (horticultural oil), potassium bicarbonate, piperalin, pyraclostrobin, pyraclostrobin + boscalid, and triflumizole. To avoid fungicide resistance, rotate with fungicides from different chemical classes.

Using non-ionic spreaders with many of the above fungicides often helps these products to penetrate through the established mycelium and improves the efficacy of the application; refer to each product’s label for information regarding the use of spreaders. 

Preventative strategies are usually not necessary unless routine scouting is not being performed. Control strategies should be implemented as soon as powdery mildew is detected. When detected and treated early, powdery mildew colonies can be eradicated with no reduction in plant appearance or quality. 

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

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