Mid-summer in the flower field (Photo: Margaret Pickoff)

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This is the time of year when you may notice an uptick in foliar diseases. Several growers in our region have reached out with questions about bacterial leaf spot in zinnias, so this week’s update will focus on a few leaf spot diseases you might encounter in this stretch of stormy, humid summertime.

Brown, angular spots on yellow-green leaf — Bacterial leaf spot causes dark, angular leaf spots with yellow halos on zinnia (Photo: Diane Dallos)

The causal agent of bacterial leaf spot on zinnia, the bacterium Xanthomonas campestris sub Sp. zinniae, infects plants during periods when leaves are wet for an extended time. Initial symptoms are small, reddish or purplish spots that enlarge and become angular over time under wet conditions, developing a reddish center. The spots may develop a yellow halo around the edges, a common symptom of bacterial infection in plants.

Plant with dark purplish spots with tan centers on green leaves, and brown wilted leaves — Leaves with Alternaria leaf spot show purplish, round spots with light colored centers (Photo: Larry Osborne, Bugwood.org)

Bacterial leaf spot can be difficult to tell apart from other foliar diseases impacting zinnia, like Alternaria leaf spot and Cercospora leaf spot. Alternaria leaf spot (also called Alternaria leaf blight) is caused by a fungus, Alternaria zinniae, that overwinters on seeds and in soil. Similar to bacterial leaf spot, infection starts with small, reddish spots on the leaf surface. These spots often have grayish or tan centers.

Alternaria on zinnia — Dark purple leaf spots, characteristic of Alternaria leaf spot (Photo: Department of Plant Pathology, North Carolina State University, Bugwood.org)

As the spots expand, you may notice concentric rings inside the spot, and the tissue at the center of the spot may drop out, leaving a hole in the leaf. If leaves are heavily infected, they may shrivel and turn brown, as shown in the photos above.

Plant with green leaves and small, purple spots with white centers — Cercospora leaf spot on zinnia (Photo: Barbara H. Smith, Clemson University)

Cercospora leaf spot is also caused by a fungus, Cercospora zinniae. Infected leaves display round, reddish or purplish brown spots with white or light gray centers. Oftentimes, several of these leaf spot diseases are present on the same leaf surface.

Leaf spot diseases can't always be prevented completely, particularly when we have consistently wet and humid weather, as we have this season. However, there are some steps you can take to lessen the risk of infection. Always start with disease-free seed and transplants. Seeds can be pre-treated with hot water or chlorine bleach to kill pathogens prior to planting (more information on treating seeds). Plant zinnias in full sun, ideally with enough space between plants to provide air flow so that leaves dry off quickly after rain or dew. If possible, utilize drip irrigation, which delivers water directly to the root zone, rather than overhead irrigation, which creates more moisture on the leaf surface. Copper fungicides, sprayed weekly, can be used to prevent bacterial leaf spot during wet periods, whereas Alternaria and Cercospora leaf spot can be managed using fungicides with active ingredients like chlorothalonil and mancozeb, as well as copper products. You can check out our update from July 11 for a quick rundown on managing fungal diseases in the cut flower field.

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