TechNotes Sept. 5 - Rust, Powdery Mildew on Ornamentals, Soil Temperatures

Rust Showing

As cool season turfgrass slows, rust becomes a major problem.  Leaves that are showing rust symptoms have orange pustules present on the leaf surface.  If symptoms are severe enough, walking through an infested lawn may result in shoes and pant legs turning orange.  With the arrival of drier conditions, rust symptoms could become more severe resulting in a withering of leaf blades due to eruption of the leaf epidermis by the fungus causing moisture loss.  Reducing the severity of the disease involves promoting turfgrass growth.  Irrigating areas showing rust symptoms can reduce the withering caused by moisture loss form the plant.  Fertilization applications helps to promote growth, which in turns reduces rust severity.

Controlling Powdery Mildew on Ornamentals  

Powdery mildew greatly reduces the aesthetic appeal of numerous herbaceous and woody ornamentals, but also impacts plant vigor, the number of blooms produced, and overall plant quality.  Infections are most commonly identified by the presence of whitish-grey powdery patches (¼ to ½ inch in diameter) of fungal growth usually on the upper leaf surfaces.  These patches, often referred to as colonies, may also appear on the lower leaf surfaces, stems, and flowers with severe infestations.  The first colonies seem to appear ‘overnight’, and with favorable conditions develop and quickly spread over a larger area, often covering much of the plant.  

Powdery mildew diseases are caused by several different fungi and are usually host specific; meaning the powdery mildew that infects garden phlox is different from the mildew that attacks lilac.  The primary mildew genera found on ornamentals are Erysiphe Leveillula, Microsphaera, Podosphaera, and Spaerotheca; these genera each consist of several species. 

The occurrence of powdery mildew is prevalent during warm, dry spells or during cool to warm, humid periods.  Spores of this fungus 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. A fairly high relative humidity (greater than 70%) and moderate temperatures (68 to 86º F) will promote the initial infection.  Once the pathogen begins to infect the host plant, the relative humidity is no longer a factor as it can prosper regardless of the humidity. 

The presence of powdery mildew can be reduced if optimum plant growing temperatures are maintained and high humidity levels or dramatic swings in humidity are avoided.  Maintaining sufficient plant spacing and increasing the air circulation are useful to reduce the humidity levels near the plant canopy where infections occur.  Routine scouting on susceptible crops should occur at least on a weekly basis, more if favorable conditions for this disease have occurred. 

To control powdery mildew, it is recommended to apply biorational compounds or fungicides during, or just after, periods which are optimum for the development of these fungi.  Detecting powdery mildews early and implementing control strategies upon detection will help growers greatly reduce the severity of mildew infections.  It is very difficult to eradicate existing well established mildew colonies.

Potassium bicarbonate and neem oil are effective biorational products.  Some of the most effective chemical controls include: azoxystrobin, kresoxim-methyl, myclobutanil, piperalin, propiconazole, triadimefon, trifloxystrobin, and triflumizole.  Be sure to rotate between chemical classes when applying fungicides to reduce the likelihood of these mildews from developing resistance to these products.

With the host specific nature of powdery mildews and several different species of mildews attacking ornamentals, there is not a single fungicide that works equally well on all mildew species.  One fungicide may provide excellent control on one mildew species, but provide little, if any, control on another. Although the powdery colonies look similar between species, they each have different fungicide sensitivities.

-Paul Pilon, Perennial Solutions Consulting, paul@perennial-solutions.com

The mention of specific active ingredients does not constitute an endorsement or recommendation of, or discrimination against similar products not mentioned. ALWAYS READ PRODUCT LABELS AND USE THEM AS DIRECTED ON THE LABEL.

Soil Temperature Map