5 Zinnia Diseases to Watch for This Season
Zinnias bring bounce to the display with their vibrant ray petals, sunny yellow centers, and the buzz of busy pollinators. While reliable, quick, and easy to grow, they’re not immune to common diseases, with a couple of pathogens all their own. Garden expert Kathierine Rowe targets zinnia diseases and how to treat and prevent them.
Contents
Zinnias bring a sweep of color as a fast-growing, easy summer flower. The workhorse annuals are low-maintenance, fast-growing, and bloom profusely in the warm season. Their vibrant ray petals in single and double forms range from pink to red to lime green and delight pollinators with their nectar-rich disk florets. Zinnias are also a joy in fresh florals; the more you cut, the more they produce.
Zinnia elegans becomes less carefree when cultural conditions lead to disease. The favorite blooms are sometimes prone to fungal problems that pop up in humid, damp situations. Detecting problems early is helpful in their efficient treatment, and knowing how to prevent them before they even begin helps ward off problems.
Setting Zinnias Up For Success
The Mexico and Central America natives thrive in heat and full sun. They need at least six hours of daily sunlight but appreciate eight or more. Well-drained soils are essential, and combined with sun exposure, help prevent pests and fungal diseases, especially in humid climates.
Loose, organically rich soils are best for showy blooms, though zinnias tolerate various types. Amend poor soils with compost to boost aeration and drainage capacity (heavy clay soils are challenging for zinnias).
The vibrant annuals prefer consistent moisture, especially as young seedlings and transplants. Once established, the adaptable bloomers tolerate short periods of drought and drying out slightly between waterings. If the soil is dry to the depth of an inch or two, it’s time to water.
To reduce competition with developing roots, pluck weeds before planting. Zinnias work well around existing specimens or in mixing with other seeds at sowing, as long as they have room for mature growth.
For fresh blooms all season, consider succession planting. Sow rounds of zinnia seeds every two to three weeks until early summer for a nonstop display. Younger seedlings take the place of those that may slow from all their booming in the summer heat.
Powdery Mildew
Powdery mildew is one of the most common diseases for zinnias and many others, from vegetables to ornamentals. It thrives where there’s a lack of air circulation. Erysiphe cichoracearum appears as a grayish-white powdery coating on leaves, stems, and buds. Leaves turn yellow, distort, and drop during severe infections.
Powdery mildew spores spread easily. They travel on the wind, with infected plants or overlapping tool use, and by splashing up from the soil via raindrops and irrigation. The spores begin on the leaves and spread to the stems, depleting the plant of energy and nutrition.
The fungus spreads quickly, and if left untreated, zinnias wither and eventually die. Fortunately, powdery mildew is treatable with early detection.
Treatment
Handpick and remove the infected leaves when possible. Discard them in a garbage bag or burn pile to keep them away from the garden and compost pile.
For more extensive outbreaks, horticultural oils like neem or insecticidal soaps can treat powdery mildew early on and prevent its spread to healthy tissue. Follow application directions, as these treatments impact beneficial insects that visit the ray blooms for nectar and pollen. The time of day is important for applying products effectively.
Prevention
Keep powdery mildew at bay by not splashing water on the leaves. Water at the base of the plant to minimize damp conditions. If overhead watering is the best means available, time it for early in the day so foliage has a chance to dry in the sun and breeze.
Alternaria Blight
Alternaria blight joins powdery mildew as one of the most common zinnia diseases. The fungus Alternaria zinniae causes leaf spots with a reddish-brown outline and gray-white centers. The irregular circles spread quickly, merging to become larger, brown, dry patches on leaf surfaces.
The blight also impacts the blooms with spotting on petals, followed by darkening and wilting. Spots and cankers form on stems, causing them to wilt completely. The fungus also spreads to the roots, which rot on the tips and are unable to uptake moisture and nutrients to support upper growth.
Scout especially for Alternaria in midsummer and early fall, the most likely times for the fungus to crop up.
Treatment
Remove affected leaves as soon as they show spotting to minimize the spread. The disease has a long survival period of two years or more, even without the host. If widespread, remove the whole zinnia.
Preventative fungicides like copper and mancozeb may be pre-treatment options. Though they aren’t often necessary, they can delay the spread if caught early. Apply them at the onset and every week to 10 days during the growing season. Spray after rainy periods, too, as prolonged rain promotes the spread.
Prevention
Alternaria survives on seeds, in debris, and in soil around infected plants. Practice good garden cleanup at season’s end by removing plant parts to prevent harboring the spores. If you’ve had an outbreak, avoid planting zinnias in the same spot the next two seasons. The long crop rotation gives the spores time to die out.
Avoid overhead irrigation when feasible, watering at the base. Choose varieties that show increased resistance to the blight (though none are likely to be completely resistant).
Botrytis
Like powdery mildew, botrytis thrives in high humidity and intensifies with limited air circulation. Botrytis cinerea is a blight (also called gray mold) that presents with light grayish-brown watery spots on petals. Leaves and flowers discolor, and “fluffy” mold pops up. Stems may develop tan-brown cankers that cause them to collapse.
Botrytis thrives in the cool, damp conditions of spring and fall. It affects a wide range of plants, from ornamentals to food crops.
Treatment
Botrytis is an adaptable fungus that adjusts to fungicides. Use both organic foliar applications and added soil microbes to keep the spores from developing immunity.
Neem oil is effective early on, as is potassium bicarbonate. A powdered formula mixed with water is useful as a regular spray treatment. Mycorrhizae and beneficial bacteria added to the soil are suitable countermeasures. Look for Trichoderma and Cladosporium to help ward off botrytis.
Prevention
Limit prolonged moisture to prevent botrytis. Take care not to overwater or overfertilize, which causes weak growth and increases susceptibility. Proper spacing and avoiding wet leaves when feasible is a good defense.
Deadhead spent blooms to keep them from fostering the spores when they drop. Remove fallen debris, too, to minimize optimal fungal conditions.
Bacterial Leaf Spot
Bacterial leaf spot is a common infection that grows on leaf surfaces. Zinnias have their own specific leaf spot disease to watch for: Xanthomonas campestris pv. zinniae. Small, purple, angular spots appear on the leaves and may develop a yellow halo.
The incidence of fungus increases in wet conditions, with spots appearing when leaves stay wet from prolonged moisture from rain or overhead watering.
Treatment
Early detection is the best control for leaf spot. Cut off diseased leaves and remove any that drop to avoid overwintering for the next season. Destroy or discard the impacted parts away from the garden to prevent the spread.
Sanitize pruners while moving between plants. An alcohol wipe or a dip in a 70% isopropyl alcohol solution is handy while working.
As with Alternaria, organic copper fungicides may prevent the spread at early onset and with repeat applications.
Prevention
Ample air circulation through plant spacing is the best preventative for leaf spot. Also, ground-level watering rather than spraying the leaves reduces damp conditions. Add a mulch layer to reduce soil splash during watering. Mulch also offers other benefits like temperature regulation, weed suppression, and moisture retention.
Aster Yellows
Aster yellows is a disease that affects zinnias and spreads via leafhoppers. The sap-sucking pests spread the phytoplasma (similar to a bacterium) as they feed on plant parts and travel between specimens.
The phytoplasma causes weak, distorted leaves and shoots, overall yellowing, and malformed, greenish flowers.
Treatment
Unfortunately, aster yellows doesn’t have a working treatment. Spot the signs early and remove infected zinnias to minimize the impact on surrounding plants.
Prevention
Treat severe leafhopper infestations to avoid their introduction of the phytoplasma. Remove fallen leaves and debris to prevent the spread, and disinfect tools. Provide the best cultural practices of consistent watering, proper spacing, and nutrient-rich soils with good drainage to boost resistance to insect and disease damage.
