Gardeners and farmers are some of the most astute folks around. They pay close attention to the health of their plants, and anything out of place immediately raises warning flags.
One of the most common and obvious symptoms a plant can show is spots on its leaves. A once-pristine green leaf changing color or developing dark blotches usually lands right on the desk of the horticulture agent or Extension Master Gardener volunteers. With the weather going from hot and dry to wet and cool always signals the uptick in incidence of leaf spot diseases. Just this week, the hydrangeas are starting to pile up!
Understanding Fungi in the Landscape
It is important to remember that fungi are all around us all the time, and the vast majority are not pathogenic to our plants. Fungi are essential decomposers in the landscape (building healthy soil and compost!), serve as a food source for countless organisms, and keep natural ecosystems functioning.
When a fungal leaf spot does pop up, it is usually just taking advantage of the weather. Plant health and weather are inextricably linked—even the most attentive gardeners doing all the right maintenance practices can be stumped when temperature and moisture align perfectly for fungal spores!
Spring's Spot-Maker: Phyllosticta
Phyllosticta species are widespread fungal plant pathogens that cause brown spotting or splotching with a dark black, brown, or yellow rim (a symptom known as necrosis). Depending on the plant species and how bad the infection is, the pattern of these blotches can vary across the canopy.
These species occur worldwide and infect a wide variety of plants. Some species of Phyllosticta are host-specific—such as Phyllosticta hamamelidis, which only attacks witch hazel—while others are generalists that cross over to multiple plant families. Even cacti can become susceptible to them!
Phyllosticta is an opportunist that takes advantage of cool, wet winter and spring weather. According to UMass Extension, the fungus produces tiny, black fruiting bodies within the dead leaf tissue, and its spores are easily splashed to healthy foliage during rain events. Because it relies heavily on ideal weather, a Phyllosticta outbreak might look terrible one spring and completely disappear the next.
Summer Spots: Cercospora
While Phyllosticta makes its grand entrance during damp spring weather, its close look-alike in the leaf-spotting business, Cercospora, loves our sweltering, humid North Carolina summers.
You may start noticing small, circular spots with dark purple, red, or maroon borders surrounding tan or grayish centers on your hydrangeas, crape myrtles, redbuds, or garden vegetables in July and August. Cercospora is likely the culprit. On some plants, these distinct purplish borders give the leaf spots a "frog eye" appearance.
Much like Phyllosticta, Cercospora spores spread through splashing rain or overhead irrigation. In severe cases, infected leaves will turn yellow and drop off prematurely, leaving plants looking a bit ratty by late summer. However, it rarely kills an established plant outright; it is primarily an aesthetic issue driven by summer heat and humidity.
Late Summer's Bullseye: Corynespora
If you notice spots on your foliage that resemble miniature archery targets during the muggiest weeks of August, you are likely meeting a third fungal culprit: Corynespora cassiicola, the pathogen responsible for Corynespora leaf spot (commonly known as Target Spot).
Unlike the irregular splotches of Phyllosticta or the purplish rings of Cercospora, Corynespora often forms circular, necrotic spots with distinct concentric rings—giving mature leaf lesions a classic "bullseye" or target-like pattern.
This fungus has an astonishingly broad appetite, capable of infecting over 500 plant species worldwide! In home landscapes and gardens, it frequently turns up on ornamentals like hydrangeas, ligustrum, and hibiscus, as well as veggie patch favorites like tomatoes, cucumbers, and peppers.
Corynespora thrives in warm, humid weather with prolonged leaf wetness. Infection usually starts on lower leaves inside dense canopies where air circulation is poor. If left unmanaged during wet, muggy weather, it can cause rapid yellowing and heavy defoliation from the bottom of the plant upward.
What Can You Do?
Whether you are dealing with spring Phyllosticta, mid-summer Cercospora, or late-summer Corynespora, the playbook for managing fungal leaf spots remains largely the same.
- The first step is to practice sanitation. Rake up and destroy any fallen, infected leaf litter beneath your plants. Do not add these leaves to your home compost pile, as fungal spores can easily overwinter in plant debris.
- The next step is to improve the airflow in your plants’ canopy. Thin out crowded branches or pick off small, early leaf infestations by hand to improve air movement through the canopy and help foliage dry quickly after rain.
- While we can’t control the rain, we can control irrigation! Avoid overhead sprinkler irrigation, when possible. Drip irrigation or soaking plants directly at the soil level prevents water droplets from splashing fungal spores up onto lower leaves.
- Finally, sanitize your tools! A pair of pruners is no different than a hypodermic needle, which you shouldn’t share with friends! Wipe down or spritz pruners with rubbing alcohol (at least 60 percent) between cuts so you don't inadvertently spread spores to healthy stems.
If you suspect you have a leaf spot pathogen in your landscape and are concerned about your plant’s long-term health, contact your local Cooperative Extension Office for a confirmed diagnosis! Fungicides are best used as a preventative and are available for severe cases, but proper identification is key before applying any treatment, as not all fungicides are effective against all fungal pathogens.
Get In Touch!
Amanda Bratcher is the horticulture agent with N.C. Cooperative Extension- Lee County Center. You can reach out to the office at 919-775-5624 for more information or questions regarding this article, but also any other questions about your garden, farm, land or plants! We’re here to help! You can check out our website's events page for more information and to register for upcoming programs.
This article originally appeared in The Sanford Herald. You can read more articles from the Herald on their website.
Resources
NCSU Plant Disease and Insect Clinic July BOLO
NCSU Plant Disease and Insect Clinic August BOLO
Washington State University Hortsense Hydrangea Leaf Spots
Clemson Extension Hydrangea Care in South Carolina
Louisiana State University Cercospora Leaf Spot on Hydrangea
University of Georgia Wet weather can lead to fungal leaf spot disease in hydrangeas
IPM Images Phyllosticta leaf spots (Genus Phyllosticta Pers.) Images
Royal Horticultural Society Phyllosticta leaf spot and dieback
University of Tennessee Phyllosticta leaf spot on ornamental plants
A phylogenetic re-evaluation of Phyllosticta (Botryosphaeriales) Wikee, et al. 2013
