Potential Control for Powdery Mildew and Leaf Spot on Zinnias

From 2000 to 2009 the University of Maryland conducted summer field days for cut flower growers. We distributed written surveys to establish the most popular cut flowers grown, and what insect and disease problems occur on these plants. For the last nine years zinnia have been included in the top five. The major insect that has reduced yields and caused economic loss has been Japanese beetle. The major diseases have been a complex of foliar leafspots caused by fungi and some caused by bacteria and powdery mildew.

There are two important zinnia leaf spots: one bacterial, caused by Xanthomonas zinnia, and the other fungal, caused by Alternaria zinnae. Bacterial leaf spots are small, angular, brown spots often surrounded by a yellow margin. Fungal leaf spots are larger and reddish-brown with grayish centers. Both diseases result in leaf blighting and death of foliage, resulting in the death of the plant.

Powdery mildew, caused by the fungus Erysiphe cichoracearum, covers the foliage with grayish-white superficial growth and often causes foliage to die.

Leaf spots and powdery mildew significantly shorten the production time for zinnias. Pathologists like to say avoiding overhead watering helps reduce the incidence of bacterial leafspot and Alternaria leaf spot. This is true, but in wet years, frequent rainfall can spread these diseases rapidly, causing a shortening of the life of zinnias. In 2009, the United States from the Midwest to the East Coast experienced one of its highest rain incidences in the last 3 decades. In Maryland we had over 50″ of rain by early December.

Materials to Control Foliar Diseases

Marla Faver, of BioWorks, Inc. asked if we would be willing to test two disease control products. BioWorks, Inc. is a biological formulating company based in Victor, New York which specializes in low-risk pesticides for insect and disease control. One product, called Cease, is an aqueous suspension biofungicide that contains a bacterium called Bacillus subtilis. Bacillus subtilis is reported to have efficacy against both fungal and bacterial pathogens. Bioworks claims Bacillus controls disease using multi-site modes of action. This product has a OMRI listing and a 4-hour REI.

SuffOil-X is a pre-emulsified, highly refined, high paraffinic, low aromatic oil. The pre-emulsification process breaks down the oil particle size, reducing the droplet size form 700 microns to approximately 50 microns, which is 1/14th its original size. The small droplet size assures that a thin, uniform coating of oil is applied to the foliage. This product is OMRI listed and has a 4-hour REI.

Objective of Our Trial

To evaluate the efficacy of Cease, SuffOil-X and combinations of the two in controlling leafspot and powdery mildew on zinnias growing in a commercial cut flower operation. This is a preliminary trial to decide if it is worth pursuing these products for further testing.

Materials and Methods

Zinnia elegans were transplanted into the field in early July of 2009. On August 5, Gill, Clement, Klick and Kenny observed the planting blocks and noted low levels of bacterial leaf spot, fungal leaf spot and powdery mildew on the lower foliage of the zinnia plants. The plots were set up as three randomized blocks, with a block consisting of 10 feet of zinnia plants with a 3-foot buffer zone between each treatment block. Each treatment block contained 8 plants.

Four treatments were administered:

1. Cease applied at the labeled high rate of 8 qt/100 gallons of water. We applied 1 liter of water with 19 ml of Cease mixed in 1 liter of water.
2. SuffOil-X applied at the mid-range rate of 1.5 gallons /100 gallons of water. We mixed 14 ml of SuffOil-X in 1 liter of water and applied to 10 ft. of growing area.
3. Cease and SuffOil-X mixture using 19 ml of Cease and 14 ml of SuffOil-X in 1 liter of water and applied to 10 ft of growing row.4. Untreated control.


On two dates in August we evaluated the foliage of the plants for presence of powdery mildew and bacterial leaf spot, with a rating scale of 0-10 with 10 having 100% of the foliage infested.

Cease provided approximately 90% control of powdery mildew, Cease and SuffOil-X gave around 98% level of control and SuffOil-X gave about 98% level of control of powdery mildew.

Bacterial leaf spot was established on plants when we started the treatment; Cease gave under 50% level. Cease would probably provide better levels of control if applied before bacterial leaf spot was established.

Where Do We Go From Here?

We would like to expand this trial in 2010 with additional replication. We would also like to start treatment before bacterial leaf spot is established on the foliage as it was in the 2009 trial. We would like to see if these treatments increased the number of harvestable cut stems for the zinnia plants. A cost analysis will be included in trials in 2010. Both Cease and SuffOil-X look like they have strong potential for a safe, relatively inexpensive treatment for foliar diseases of zinnias.