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I love snipping zinnias as cut flowers in the summer. However, it can be frustrating when unsightly spots on zinnia leaves diminish their wow factor. From the most common white powdery spots to more serious cases of brown lesions and wilting, these and other zinnia problems are usually manageable if you catch them early. In this article, learn what to look for, what different symptoms likely mean, and what you can do to help your zinnia plants thrive. You’ll also find tips for preventing some of the conditions that cause the various types of spots on zinnia leaves that sometimes show up.
There’s a lot of variation between the different kinds of spots on zinnia leaves that you may see. For instance, spider mites cause light-colored stippling on the upper leaf surface. Other maladies start out as yellow or brown spots which darken to purple-black. Tracking the color, shape, size, and location of zinnia spots can help you diagnose specific problems.
The importance of identifying the problem
Although some zinnia issues fall into the “minor nuisance” category, there are others that can spread to nearby plants and can even remain in the soil to affect new plantings over subsequent years. That’s why it’s important to identify the problem and get it treated as quickly as you can.
Common causes of spots on zinnia leaves
Some of the most common culprits behind those spots on zinnia leaves are so small that you need a microscope to see them. Most often a fungus is to blame. Bacterial pathogens, insects feeding on your zinnias, or a simple nutrient deficiency can also contribute to spots on zinnia leaves. Read on for a little more about each possible issue:
- Fungal diseases
- Bacterial infections
- Possible insect damage
- Nutrient deficiencies
1. Fungal diseases that cause spots on zinnia leaves
Cercospora zinniae and Alternaria zinniae fungi cause diseases that look similar and can infect zinnias simultaneously. Cercospora zinniae causes Cercospora leaf spot and blight. Alternaria zinniae causes Alternaria leaf spot and Alternaria blight. While Cercospora zinniae mostly affects leaves, Alternaria zinniae also damages leaf petioles, nodes, and even flowers.
These infections start with small brown spots with grayish-white centers. Over time, the spots can grow up to half an inch in diameter. They’ll morph into a purplish-brown with sunken centers that turn brittle and dry. End-stage disease results in black cankers and plant rot.
Featuring fuzzy, gray spots which gradually spread, powdery mildew is another fungal disease that affects zinnias. Although this one’s more common, it’s not always as damaging as Alternaria and Cercospora leaf spot.
2. Bacterial infections
Zinnias infected with a specific strain of Xanthomonas campestris bacteria can develop bacterial leaf spot. Left untreated, this can advance to full-blown bacterial blight.
Bacterial leaf spot starts with small, reddish-brown lesions which enlarge and eventually join to cover entire leaves. Your zinnias are more likely to develop this disease if their leaves get wet and remain wet for long periods.
3. Possible insect damage
Literally sucking the life out of plant leaves and spinning delicate webs as they go, spider mites definitely can do a number on zinnias. Still, leafhoppers are much worse since they’re a vector for the phytoplasma—a parasitic bacteria-like organism—which causes aster yellows disease. Badly deforming plant stems and flowers, aster yellows is such a deal-breaker that you must unearth and trash any infected plants.
4. Nutrient deficiencies that cause spots and splotches
Sometimes the tips of new leaves—especially those near developing flower heads—begin to look crunchy and brown. This is often the mark of a calcium deficiency. That doesn’t necessarily mean you need to add calcium to your soil. Environmental conditions like high humidity, inadequate airflow, or a soil pH below 5.5 all can contribute to the problem.
Preventing spots on zinnia foliage
Take these preventative measures to keep your zinnias relatively spot-free:
- Avoid overhead irrigation: Watering plants from overhead isn’t ideal since it can cause soil-dwelling pathogens to splash up onto plant leaves. This, in turn, can contribute to the spread of certain kinds of disease. Instead, use a soaker hose at ground-level to slowly direct water straight to the root zone.
- Space plants adequately: Zinnias are less likely to become diseased if they have good air circulation and access to full sun. Overcrowded plants shade one another and make conditions more favorable for insect pests and disease.
- Practice crop rotation: Don’t plant zinnias in the same spot year after year. Instead, switch between different plant types in order to mitigate the build-up of zinnia-targeting pathogens.
Choose resistant varieties
Horticulturists have crossed Zinnia elegans (sometimes also called Zinnia violacea) with Zinnia angustifolia to breed zinnias which are more disease-resistant. Classified as Zinnia marylandica, these hybrids are commonly marketed as “Pinwheel” and “Profusion” zinnias. Some specifics to try include Zinnia Pinwheel mix and Raspberry Lemonade zinnias. Prefer open-pollinated heirloom varieties? Old Mexico zinnias (Zinnia haageana) are less disease-prone than others.
Good garden hygiene
Practicing good garden hygiene is one of the simplest and most effective actions you can take. Sanitize any tools that may have come into contact with plant pathogens before and after using them around your zinnias. Have diseased zinnia foliage? Remove and throw away—do not compost—any affected leaves.
Using organic product controls
If you have spots on zinnia leaves from insect pests or disease, there are a couple of organic products that can help as a last resort. First, if you have a severe mite infestation or your zinnias are overrun with aphids, leafhoppers, or similar intruders, you might try an insecticidal soap. (Do keep in mind that, like all pesticides, these also will affect beneficial insects.)
Fighting disease-causing bacteria or fungal spores? Copper-based fungicide or sulfur-containing spray are some of the organic bacterial and fungicidal treatments available.
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