Insect Pest Prevention & Control: Zinnias are excellent sources of pollen and nectar for many beneficial insects; therefore, it is not recommended that insecticides, other than insecticidal soap, be used to control damaging mite and insect pests. Rather than depending exclusively on pesticides, proper culture (soil preparation, fertilizers, irrigation, mulch, optimum plant spacing, and adequate sunlight) is the better method for preventing insect and disease problems.
Zinnias have few insect pests, but aphids, spider mites, and whiteflies may damage plants, especially during hot, dry weather. Aphids and spider mites can be hosed off the plants with a strong stream of water. Since spider mites thrive in hot, dry weather, reducing drought stress with supplemental irrigation will help prevent an infestation. Spray applications of insecticidal soap may be applied to control spider mites, aphids, and whiteflies, but always spray flowering plants in the late evening to prevent harming beneficial insects or hummingbirds. Insecticidal soap sprays must be directed at the lower leaf surfaces where many pests feed. See Table 1 for examples of insecticidal soap products. For more information about the use of insecticidal soap, see HGIC 2771, Insecticidal Soaps for Garden Pest Control.
Disease Prevention & Control: Most diseases may be prevented when zinnias are watered and cared for properly. Zinnias grow best in full sun all day; however, they may be more prone to leaf spot diseases in partial shade. It is important to keep the foliage dry to discourage bacterial and fungal diseases. Therefore, it is best to always water near the base of the plants. If overhead watering is used, irrigate during the early morning hours to allow the leaves to dry quickly in the morning sunlight. Disease incidence is greater if plants are watered in the late afternoon or evening, thus causing the leaves to stay wet longer. These conditions provide the perfect environment for both bacterial and fungal disease development.
Alternate using zinnias with different bedding plant species for a year or two to reduce disease incidence. Prompt removal of any highly pest-infested or disease-infected plants will reduce pest and disease spread. At the end of the growing season, remove all plant debris to reduce the carry-over of fungal disease inoculum (the pathogen responsible for the infection). This will lessen or delay the occurrence of diseases the following year. Infected seed may also be a source of these diseases; therefore, do not save seed for sowing the next year if there has been an outbreak of leaf spot diseases.
Powdery Mildew: Powdery mildew is a fungal disease (Golovinomyces cichoracearum) that appears primarily on plant foliage but may also infect flowers and stems. This disease appears as a grayish-white, powdery coating on zinnias. Practice good cultural methods. However, if fungicides are required, there are several that are very effective. These include copper-based fungicides, horticultural oil with baking soda, sulfur, myclobutanil, propiconazole, tebuconazole, and thiophanate methyl. Under conditions of disease development, spray plants weekly. Always spray fungicides in the late evening to reduce the impact on pollinating insects. See Table 1 for examples of brands and products. Follow label directions for mixing and use. For more information on powdery mildew and control, see HGIC 2049, Powdery Mildew.
Alternaria Leaf Spot: The fungus Alternaria zinnia causes a leaf spot on zinnias that starts as small, dark reddish, circular spots that rapidly expand and become irregularly shaped with grayish-white centers. As the spots become larger, they may develop dark concentric rings in the centers. The center of these leaf spots may eventually tear, fall out, and leave behind holes in the leaves. Serious infections cause the leaves to turn brown quickly and shrivel. Infections occur not only on foliage but also on ray flowers, which will wither and produce lesions on stems, which often become girdled and finally wilt. Seed from infected plants can spread this leaf spot disease. Spray plants following rainy periods, as rainy conditions increase disease occurrence and spread. The better fungicides to control Alternaria leaf spot are chlorothalonil, copper fungicides, and mancozeb. See Table 1 for examples of brands and products containing these fungicides. Follow label directions for mixing and use. Do not use thiophanate methyl fungicides, as these are ineffective. Always spray flowering ornamentals in the late evening.
Bacterial Leaf Spot: Xanthomonas campestris pv. zinniae is the causal agent of bacterial leaf spot of zinnias, and the symptoms of infection are purplish spots that usually remain small, but they become angular in shape. Spots may develop a yellow halo. Spray plants weekly with a copper fungicide following periods of leaf wetness to reduce disease incidence. See Table 1 for examples of brands and products containing copper. Follow label directions for mixing and use. Always spray plants in bloom during the late evening. Seed collected from infected plants may spread bacterial leaf spot.
Cercospora Leaf Spot: This fungal leaf spot, caused by Cercospora zinniae, occurs less commonly than Alternaria leaf spot, powdery mildew, or bacterial leaf spot, but it can be just as damaging. Cercospora leaf spots on foliage are typically reddish-purple, small, and round with light gray to almost white centers. These spots may coalesce (run together), and heavily infected leaves usually die. Spray as needed following rainy periods with chlorothalonil, copper fungicides, or mancozeb. See Table 1 for examples of brands and products with these fungicides and follow label directions for mixing and use. Always spray flowering ornamentals in the late evening.
Aster Yellows: Aster yellows is caused by a phytoplasma and is similar to a bacterium. It is carried by aster leafhoppers (Macrosteles quadrilineatus), which are insects that feed on the sap of the plants. As they feed, they inject the phytoplasma into the vascular system of the zinnia and cause the developing flowers to become deformed. There is no control method once the flowers have been infected, and plants with symptoms should be promptly removed and discarded in the trash. Fortunately, aster yellows is not extremely common.
Table 1. Fungicide & Insecticide Products for Disease & Pest Control on Zinnias.
|Active Ingredient||Examples of Products|
|Chlorothalonil||Bonide Fung-onil Concentrate; & RTU1
Ferti-lome Broad Spectrum Landscape & Garden Fungicide Concentrate; & RTU1
GardenTech Daconil Fungicide Concentrate
Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide Conc.
Ortho MAX Garden Disease Control Concentrate
Southern Ag Liquid Ornamental & Vegetable Fungicide Conc.
Tiger Brand Daconil Concentrate
|Copper-based Fungicides||Bonide Copper Fungicide Spray or Dust
Bonide Liquid Copper Concentrate; & RTU1
Camelot Fungicide/ Bactericide Concentrate
Dexol Bordeaux Powder Delete
Hi-Yield Bordeaux Mix Fungicide Delete
Monterey Liqui-Cop Fungicide Concentrate
Natural Guard Copper Soap Liquid Fungicide Concentrate
Southern Ag Liquid Copper Fungicide
|Horticultural Oil3||Ferti-lome Horticultural Oil Spray Concentrate
Bonide All Seasons Spray Oil Concentrate
Southern Ag ParaFine Horticultural Oil Concentrate
Espoma Earth-tone Horticultural Oil Concentrate; & RTS2
Monterey Horticultural Oil Concentrate
Summit Year Round Spray Oil Concentrate
|Insecticidal Soap||Bonide Insecticidal Soap Concentrate; & RTU1
Espoma Earth-tone Insecticidal Soap Concentrate; & RTU1
Miracle Gro Nature’s Care Insecticidal Soap RTU1
Natural Guard Insecticidal Soap Concentrate
Safer Brand Insect Killing Soap Concentrate
Safer Brand Rose & Flower Insect Killer RTU1
Garden Safe Insecticidal Soap Insect Killer RTU1
Whitney Farms Insecticidal Soap RTU1
|Myclobutanil||Spectracide Immunox Multi-Purpose Fungicide Concentrate
Ferti-lome F Stop Lawn & Garden Fungicide Concentrate
|Propiconazole||Banner Maxx Fungicide
Bonide Infuse Concentrate; & RTS2
Ferti-lome Liquid Systemic Fungicide II Concentrate; & RTS2
|Sulfur4||Bonide Sulfur Plant Fungicide (dust or spray)
Ferti-lome Dusting Sulfur (also wettable for spray)
Hi-Yield Wettable Dusting Sulfur
Safer Brand Garden Fungicide Concentrate
Southern Ag Wettable or Dusting Sulfur
|Tebuconazole||Bayer BioAdvanced Disease Control for Roses, Flowers & Shrubs Concentrate|
|Thiophanate Methyl||Cleary’s 3336-WP Turf & Ornamental Fungicide
Southern Ag Thiomyl Systemic Fungicide
|1 RTU = a pre-mixed spray bottle.
2 RTS = a hose-end spray bottle.
3 Three tablespoons of horticultural oil in a gallon of water plus 3 tablespoons of baking soda. Never apply an oil spray within 2 weeks of a sulfur spray, and do not apply oil sprays when temperatures are above 90 °F or to drought-stressed plants.
4 Do not apply sulfur if temperature is greater than 85 ºF or to drought stressed plants. Do not use sulfur in combination with, or within 2 weeks before or after the use of horticultural oil treatments. Sulfur will also control mites.