Evaluation of Control of Powdery Mildew and Alternaria and Bacterial Leaf Spot on Zinnia

The 2010 trial was set up at Farmhouse Flowers in Brookeville Maryland. The new low-risk materials tested were supplied by BioWorks Inc., Victor, New York. One product tested was the biofungicide Cease, which is an aqueous suspension of Bacillus subtilis. It has been reported to have multi-site modes of action on both fungi and bacterial diseases. SuffOil-X is a highly refined, high paraffinic, low aromatic oil. This product has shown efficacy on powdery mildew. The standard control was the copper-based product Phyton 27, which is effective on a broad spectrum of bacterial and fungal diseases.

Plants of Benary’s Giant series, mixed colors, were transplanted into the field in early June of 2010. On June 16 the trial was set up with 5 randomized treatments, including an non-sprayed control, replicated 6 times, sprayed along one meter of plant row for each product, alternating with one-meter buffer sections within 2 raised planting beds of zinnias.  Low levels of bacterial leaf spot were already observed at the start of the trial. Weekly sprays were applied starting on June 21 and continued until September 29. The trial was concluded on October 7.  We converted the labeled spray rates to metric and applied the treatments with 2-liter handheld Solo pump sprayers.

Five treatments were used:
1. Cease was applied at the labeled high rate of 8qt/100 gallons of water . We sprayed at the rate of 19ml Cease/liter of water.
2. SuffOil-X was applied at the mid-range rate of 1.5 gallons/100 gallons of water. We sprayed at the rate of 14ml SuffOil-X/liter of water.
3. Cease and SuffOil-X mixture was applied at the rate of 19mlCease and 14ml SuffOil-X per liter of water.
4. Phyton27 was applied at the rate of 1.8 ounces/10 gallons of water. We sprayed at the rate of 1.43ml/liter of water.
5. Control blocks were randomized untreated plants within the row.

We evaluated the trial 6 times starting on July 20 followed by August 10, 17, 31, September 20 and October 7. We used a 0-10 disease rating scale with 10 having 100% of the foliage infected. The most predominant leaf disease in 2010 was bacterial leaf spot. This disease is seed borne and was very severe in this field. Alternaria leaf spot was observed by mid August.

Powdery mildew was not observed until September 20, which was considered very late in the season.

Over the duration of the trial both the Cease and Phyton 27 treatments looked the best for leaf spot disease control. The average rating for both of those treatments ranged from 2.0 in July to 5.0 by August. By contrast the non-sprayed plants had a rating ranging from 3.5 in July to 5.9 in August. The statistical analysis of all the evaluation dates showed that the Cease and Phyton 27 treatments worked equally well for both leaf spots.
Powdery mildew was best controlled by the SuffOil-X treatment, followed closely by the  mixture of Cease and SuffOil-X. The average rating for those treatments in October were 0.5 and 2.0 respectively compared with the non sprayed rating of 5.8.

The 2010 season was unusually hot with many days above 90F. The only treatment where phytoxicity was observed was the Phyton 27 treatment. We did not include a cost analysis or study the differences between treatments and harvestable stems. The best treatments did give a noticeable improvement in foliage
quality and prevented diseases on the flowers. All treatments were relatively easy to apply with no sprayer issues and dried quickly with little or no foliage residue. Further studies need to be done to repeat these treatments.
Additionally, seed treatments for bacterial leaf spot control should be investigated since once this disease becomes established it is very difficult to stop in the field.

Thank you to the ASCFG Research Foundation for its financial support of this project.


Zinnias provide some of  the most popular summer blooms for specialty cut flower markets. Leaf spot diseases are common and often shorten zinnia production. The primary diseases examined in this trial were bacterial leaf spot caused by Xanthomonas campestris pv. zinniae, a fungal leaf spot caused by Alternaria zinnia, and powdery mildew caused by the fungus Golovinomyces cichoracearum (formerly Erysiphe).

Understanding Alternaria Leaf Spot
This disease is caused by the fungus Alternaria zinniae.  The symptoms are large, reddish brown or purple spots. The spots enlarge and become irregular in shape. They eventually develop gray centers that usually drop out, leaving a hole. Severely affected leaves turn brown, become dry and brittle. The fungus can overwinter in leaf litter in the soil, becoming a seedborne disease.  Seed treatments are time consuming and not completely effective.

Non-chemical Control of Alternaria Leaf Spot
Keep foliage dry and avoid overhead watering. Crop rotation may also reduce disease incidence.

Understanding Powdery Mildew
This disease is caused by the fungus Golovinomyces cichoracearum (formerly Erysiphe). The powdery mildew fungus requires living plant tissue to grow and reproduce, and overwinters on decomposing stems and leaf litter. It grows as thin layers of mycelium on the surface of the affected plant parts. Spores that you can see with a hand lens are part of the white, powdery appearance and are produced in chains on upper or lower leaf surfaces, flowers, or stems. Environmental conditions that favor the growth of powdery mildew include sunny warm days followed by cool nights.

Wind carries powdery mildew spores to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, water on plant surfaces for extended periods inhibits germination and kills the spores of most powdery mildew fungi. Moderate temperatures of 60F to 80F and shady conditions generally are the most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and leaf temperatures above 95 F may kill the fungus.

Non-chemical Control of Powdery Mildew
The best method of control is prevention. Avoiding the most susceptible cultivars, placing plants in full sun, and following good cultural practices will adequately control powdery mildew in many situations. Unfortunately, the most popular cut flower cultivars have little if any resistance to powdery mildew.

Understanding Bacterial Leaf Spot
This disease is caused by the bacterium Xanthomonas campestris pv. zinniae. It first appears as small (1 to 2 mm), diffuse, translucent spots surrounded by broad yellowish halos.  Under wet conditions the lesions slowly enlarge to about 5 mm in diameter. The spots become angular to irregularly circular and develop a reddish center. The lesions may merge to form irregular dead areas, 0.5 to 1.0 cm long, that may crack as they dry. During very humid weather, small brown spots may form on the ray flowers. If severe, the flower heads are seriously disfigured and may completely decay.

The bacterium will overwinter on decaying leaf litter and remain in the soil.  Unfortunately, this disease is also carried on seed and seed treatments are time consuming and not completely effective.

Non-chemical Control of Bacterial Leaf Spot
Keep foliage dry and avoid overhead watering.  Crop rotation may also reduce disease incidence.

David L. Clement

Extension Specialist

David L. Clement is Extension Specialist, Pathology, HGIC at the University of Maryland. Contact him at [email protected]

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]