Cost-Effective Organic Prevention of Downy Mildew on Zinnias


Zinnias are productive, profitable workhorses on many flower farms, but most of the popular tall cut flower varieties are sensitive to a host of fungal and bacterial diseases, especially powdery mildew. Disease infection affects zinnia vase life, reduces yields of salable stems, and forces many farmers to plant multiple successions or even eliminate this lovely flower from their planting schedules altogether.

To address these issues, we tested the efficacy of low-cost organic foliar sprays in preventing fungal infection over the zinnia-growing season. Specifically, we tested the application of pure neem oil or whey. Cold-pressed pure neem oil is a broad-based immune stimulant as well as a natural insect repellant, and is OMRI approved. Whey is an excellent low-cost probiotic high in calcium. The test sprays of pure neem oil and whey were compared to a control condition (no foliar spray), and Serenade, a commercially available
organic bio-fungicide already commonly used to combat powdery mildew.


Direct seeded a 152-foot row of Benary’s Giant BluePoint Series mixed colors on 4/10/2017. Due to insufficient germination, the row was reseeded on 5/9/2017 with good results. The row was divided into four 38-foot sections with landscape fabric on either side of the row (Image 1). Preventive spraying commenced three weeks after sowing according to the following plan listed in Table 1.

Table 1. Test Foliar Sprays

Foliar SprayCost per gallon pure productCost per gallon diluted test spray
Whey (5.3% concentration)$0.25$0.25
Control no spray  
Pure Neem Oil (applied 0.5% concentration)$176$1.03
Serenade (2% concentration)$56$1.31

Foliar sprays were applied every three weeks, for a total of 4 applications. Once the plants began to flower, salable stem counts (no disease observable on top 2 sets of leaves) were recorded for each trial condition.

Flowers were harvested into well water. Harvested stems were processed, then transferred into clean buckets and conditioned in water treated with Chrysal gerbera tablets. Each week 3 stems were randomly selected from each condition for a vase life study conducted at ambient basement temperature in plain well water.

Results and Observations

Harvest commenced six weeks after direct sowing seed and continued for a further 13 weeks (late June through mid September). Table 2 compiles the total harvest data for each test section. Figure 1 breaks down the harvest each week.

Table 2. Compiled Harvest Data

Foliar SprayTotal StemsPlants per SectionStems per PlantStems per Bed FootAvg. Plant Spacing
Neem Oil709838.5418.665.49

The middle of June brought a generally expected onslaught of Japanese beetles, and no foliar spray dampened their appetite for zinnia foliage and flower petals. The beetles moved on (to munch the dahlias!) and the zinnias outgrew the damage. Downy mildew was first observed on the lower leaves of some plants nine weeks into the study, with downy mildew present in all sections by week ten (Image 2). Two weeks later there was a general crash in production mostly due to the pervasive presence of downy mildew. Interestingly, the sections treated with whey and Serenade had a late summer rally with increased production of disease-free new growth. In the late summer the section treated with neem oil also had notably less downy mildew than the control, but a much higher rate of bacterial leaf spot, which still made the stems un-harvestable (Image 3). By the end of the study, the section treated with Serenade yielded the most salable stems with 836 stems, or 13.48 per plant. The section treated with whey was a close second with 808 stems. These yields were significantly greater than the control section with 575 stems.

Small differences in average plant spacing also correlated to less disease as expressed as more harvestable stems. Figure 2 illustrates this relationship.

There was no statistical difference in the average vase life of stems from different sections. However, there was a general trend through the season. The average vase life of stems from each week of the study is plotted in Figure 2. Average vase life peaked in mid July at about 8 days. The weather conditions in which the flowers were picked did effect the vase life. The end of August brought the remnants of a tropical storm and a long stretch of rainy weather and no choice but to harvest wet stems during week 16, these stems had an average vase life of 5 days and generally heralded the end of quality stems from the study row.

Figure 1. Zinnia Stem Harvest through the Season

Figure 2. Plant Spacing Compared to Total Harvest

Figure 3. Average Vase Life (Days) Through the Season

Image 2. Zinnia with powdery mildew on lower leaves, 7/21/2017, week 10 of study

Image 3. Zinnia with bacterial leaf spot, 9/1/2017, week 16 of study, from neem oil-treated section


As so often seems the case with a close look, this study yielded some very interesting trends without any great presumption of certainty. Compared with the control, preventative foliar applications of whey, neem oil, or Serenade all increased the number of harvestable stems over the 18 weeks of the study, but had no impact on vase life, and did not prevent a drastic reduction of harvestable stems after six weeks of harvest (week 13 of the study), although these products did stave off this fall in production by a week in comparison to the control. While foliar applications with neem oil did help decrease the impact of powdery mildew on the zinnia plants, it increased the impact of bacterial leaf spot in the late summer and early fall, greatly reducing flower yields for the last six weeks of the study.

Foliar sprays of whey, neem oil, or Serenade were all very cost effective. Prepared sprays of neem oil or Serenade cost about a dollar a gallon, and whey can often be sourced at very low cost or free from local dairies as the byproduct of cheese making.

This study did not control for bed spacing, since the row was direct seeded at the same seeding rate. The number of plants in each test condition were counted, though, and there was a correlation between greater average plant spacing and more harvestable stems (less disease). In a future study, it would be helpful to quantify bed spacing and remove this variable.

Foliar applications of whey or Serenade proved to be most beneficial in reducing powdery mildew and other diseases that reduce the productivity of zinnia plantings. They were not enough, however, to maintain a constant supply of quality cut stems through the season. Successive plantings would still be recommended.