Understanding and Controlling Powdery Mildew and Fungal Leaf Spot Diseases
by Melinda Heigel, EMGV
When most of us hear the word fungi, we instantly think about mushrooms. And while that is indeed the right classification, fungi come in myriad forms, from mushrooms to microscopic molds, and affect the landscape in many ways. There are over 100,000 identified fungal species, and experts estimate there are 5 million or more world-wide. Fungi are amazing and perform some pretty vital tasks. Unlike plants, fungi have no chlorophyll and cannot produce their own food, so they produce enzymes that break down organic matter like plant material and dead animals for energy. Known as saprophytic fungi, this type of fungi play an important role as master decomposers in the soil-food web. And the byproduct of their digestive work is nutrient-rich organic matter that promotes the healthy soil–the kind gardeners covet. By the nature of their parts (specifically threadlike structures called hyphae), some fungi help soil particles bind together which promotes good water penetration of the soil, while also improving the soil’s ability to hold onto precious water, too. Some, like mycorrhizal fungi, invade vegetable, shrub, and plant root systems and work cooperatively with plants to help deliver nutrients from the soil. Other mycorrhizae grow on the surface of roots and are best known for their symbiotic relationship with trees. So these fungi in your soil are natural, beneficial, and wondrous things. They signify healthy living soil.
But, alas, some types of fungi pose problems for gardeners, as roughly 85% of all plant diseases are fungal. Some of these pathogens can present challenges to our desire for picture-perfect gardens. There are a host of fungal diseases that affect plants differently–cankers, wilts, and galls. But two common and often innocuous fungal diseases gardeners will see in the late summer and fall on leaves are powdery mildew and leaf spot diseases.
Timing and Causes
Late summer to fall is high season for powdery mildew. One major factor in this timing is humidity–something our region knows well. Most fungi need water to thrive, but powdery mildew can get all the moisture it needs from high humidity, aided by cooler nights. These conditions are most favorable to the development of the pathogens’ air borne spores, the means of reproduction. The infection process starts when spores land on leaves. These spores can overwinter in the soil and persist on living or dead infected plant material. So winter temperatures or a hard frost aren’t a slam dunk for eliminating this problem. Fallen leaves and debris allow for overwintering.
Powdery mildew is very common and easily recognizable. There are many different but related fungi that cause powdery mildew, and many plants are susceptible. You’ll often find it on crepe myrtle, zinnias, phlox varieties, and cucurbits like squash and cucumbers, roses, bee balm, and dogwoods. No need to dive into each of them as many are host-specific pathogens. Just know that if the leaves of your annual, perennial, shrub, tree, or even vegetable plant appear to be dusted in white talcum powder or have white or gray spots, as they say, “A fungus is among us.” What you are actually seeing are the hyphae structures of the fungus growing on the leaves. Sometimes, the pathogen initially hangs out on the underside of some hosts’ leaves where it’s cooler. That’s a great place to scout for early infestations. Use a jeweler’s loupe or magnification and look for the start of colonies on lower leaves where the problem often begins. Sometimes you will find discoloration on the tops of the leaves (yellow or purple spots) prior to the powdery spotting spreading to all the leaves.
Leaf Spot Diseases
Timing and Causes
Just like powdery mildew, the fungi that cause leaf spot diseases rely on moisture. Generally initial infections begin unbeknownst to the gardener in the spring and often become most evident with wet weather and/or the start of fall. There are numerous fungal pathogens that cause leaf spot on plants including those found in the genera Cercospora, Alternaria, Anthracnose, Corynespora, Cylindrocladium, Cylindrosporium, Fabraea, Marssonina, Phyllosticta, Pleospora, and Septoria–just to name just a few. Some are equal-opportunity ‘infectors’ and can affect many types of plants while others only target one particular plant. The organisms responsible for these diseases survive on infected leaves and twigs that have fallen to the ground and can become soil-bound. Easily spread by wind and water that splashes up onto new growth, the reproductive fungal spores germinate, these leaves become impacted, and the cycle begins again.
Leaf spot diseases pretty much look like their name suggests: they are concentric spotty lesions on leaves. Typically they have light brown centers ringed with darker borders, ranging from dark brown to black. If you’ve ever heard of the term blight, that’s simply what happens when there are so many lesions close together that unite and cause a large dead patch on leaves. Often leaf spot diseases are found on ornamental shrubs like acuba, mountain laurel, hydrangea, rhododendron, roses, and peonies. Many other plants like trees and vegetables are also susceptible.
Control and Mitigation
While there are scores of fungicides on the market that gardeners might be tempted to employ to address foliar fungi, the good news is that’s probably not necessary in the home garden. Unless you have a high-value plant, like a long-lived cutting from your grandfather’s garden, using chemicals is generally not necessary.1 Gardeners can control both these fungal pathogens surprising well with some simple cultural and mechanical practices instead of chemical ones, two pillars of integrated pest management (IPM). 2 Here are a few tips to keep in mind.
(1) Plant and prune to ensure good air flow. When pants, shrubs, vegetables, and trees are planted too close together or the plant itself is too dense, this creates a perfect environment for the introduction of diseases as well as insects. Plants in these environments become stressed, have to fight for water, and become more susceptible to infection. Likewise good air circulation speeds up drying and that helps reduce excessive moisture on leaves. Proper spacing and especially with vegetables, trellising and staking, will help keep lower leaves from touching the soil where these fungal pathogens live.
(2) Buy healthy plants that are disease free and choose disease-resistant cultivars. Some plants are more susceptible than others to powdery mildew and leaf spot diseases. Do your research and rely on plant pros at your local nursery for guidance. Clemson University’s Home and Garden Center has great list of ornamental cultivars that are resistant to powdery mildew:
Plant Varieties Resistant to Powdery Mildew
|Kousa dogwood, Cornus kousa||‘Milky Way’, ‘Milky Way Select’, ‘National’|
|Cornus florida x kousa hybrids||‘Aurora’, ‘Constellation’, ‘Celestial’, ‘Stellar Pink’|
|Flowering dogwood, Cornus florida||‘Cherokee Brave’, ‘Springtime’, ‘Pygmy’, ‘Jean’s Appalachian Snow’, ‘Karen’s Appalachian Blush’, ‘Kay’s Appalachian Mist’|
|Crepe Myrtle: The Lagerstroemia indica x faurieri hybrids||‘Apalachee’, ‘Fantasy’, ‘Hopi’, ‘Miami’, ‘Osage’, ‘Tonto’, ‘Tuscarora’, ‘Tuskegee’, ‘Wichita’, ‘Acoma’, ‘Sioux’, ‘Natchez’|
|Phlox||‘David’, ‘Delta Snow’, ‘Natascha’, ‘Robert Poore’, ‘Shortwood’, ‘Katherine’, ‘Glamour Girl’|
|Zinnia||Pulcino and African varieties, Zinnia angustifolia, Profusion Cherry, Profusion Orange|
|Hybrid Tea Rose||‘Duet’, ‘Eiffel Tower’, ‘Grand Slam’, ‘Mister Lincoln’, ‘Tiffany’, ‘Jamaica’, ‘Matterhorn’|
|Floribunda Rose||‘Golden Slipper’|
|Grandiflora Rose||‘Camelot’, ‘Queen Elizabeth’, ‘John S. Armstrong’, ‘Pink Parfait’|
|Rugosa Rose||‘Rugosa Alba’, ‘Blanc Double de Coubert’, ‘Fru Dagmar Hastrup’, ‘Topez Jewel’, ‘Alba’, ‘Alba Semi-Plena’|
|Monarda||‘Marshall’s Delight’, ‘Cambridge Scarlet’|
(3) Keep it tidy. Gather all fallen leaves in the fall and once more in the spring (when the disease cycle starts again). This helps reduce the number of spores that are in the soil. If you notice some early infected leaves, remove them to increase control. You can also prune out any infected parts of the plant as well. Clean your pruners well afterward and remove and dispose of the infected plant parts in a plastic bag. Also, keeping plants mulched helps as this can keep spores from splashing up onto your plants during rainfall.
(4) Keep plants happy. Proper pruning, watering, weeding, light conditions, and healthy soil (fertilizing based on soil test results) are all important factors for vigorous plants. An added bonus is that healthy plants are often less susceptible to disease and, if infected, can bounce back.
(5) Avoid overhead watering. For both these pathogens, excessive moisture encourages infection. If at all possible, water plants at the soil line and keep the leaves dry. Drip irrigation is great for this. If you must irrigate overhead, try reducing the frequency and do it very early in the day so the leaves and stems have plenty of time to dry.
(6) Rotating crops. For the home vegetable gardener, crop rotation matters when thinking about fungal pathogens, especially leaf spot diseases. Many fungal diseases attack vegetables within the same family (such as Mustards (Barsssicaceae) like kale and collards or Nightshades (Solanaceae) like tomatoes and peppers). In addition to planting disease-resistant cultivars, moving crops to different locations in the garden helps to alleviate the build up of family-specific fungal pathogens in the soil.
Powdery mildew and leaf spot diseases are easy to spot in your landscape. As you tend your garden this fall, hopefully these simple measures can pay off in the spring to keep these fungi at bay.
1–If considering the use of fungicides, proper identification of the pathogen and strict adherence to label instructions are musts. Chemical controls typically need to be applied prior to the first sign of infection and followed-up with a disciplined treatment regime to be effective. It’s always good practice to consult the NC Agricultural Chemicals Manual first (https://content.ces.ncsu.edu/north-carolina-agricultural-chemicals-manual).
2–Integrated pest management is a method of controlling pests like these two fungal pathogens. The main idea is that multiple forms of mitigation (cultural, mechanical, biological, and chemical) work best when performed in concert.
Resources and Additional Information
Did you know? North Carolina State University has a Plant Disease and Insect Clinic that works in conjunction with local extension offices to help home gardeners diagnose plant problems. To learn more and how to submit questions, see their website
For more information about soil fungi, this USDA site offers a good overview
Clemson University’s Home and Garden Information Center has a thorough factsheet on powdery mildew
University of Maryland Extension’s website talks in depth about fungi and shrubs
Check out Missouri Botanical Garden’s excellent piece on leaf spot diseases for the home gardener
North Carolina State University has a comprehensive vegetable pathology
Article Short Link https://wp.me/p2nIr1-2bK