What is Guignardia leaf spot? Guignardia leaf spot is a common fungal leaf disease that affects vining plants such as Boston ivy and Virginia-creeper. The fungus that causes Guignardia leaf spot also causes a leaf spot and fruit rot of grape called black rot.
What does Guignardia leaf spot look like? Symptoms of Guignardia leaf spot include roughly circular, or sometimes angular (i.e., straight-edged), ¼ to ½ inch diameter dead spots on affected leaves. Spots often have a purple-red border, and the centers may eventually fall out. Young leaves are more susceptible to infection than mature leaves. If infections occur before leaves have fully expanded, leaves can become puckered and distorted. Within the spots, a diffuse ring of black dots (reproductive structures of the fungus) is typically visible.
Where does Guignardia leaf spot come from? Guignardia leaf spot is caused by the fungus Phyllosticta ampelicida (formerly Guignardia bidwellii), which survives in leaf litter. Spores of the fungus are produced under cool, moist conditions and can be dispersed by wind or splashing water.
How do I save a plant with Guignardia leaf spot? DO NOT panic! Guignardia leaf spot is most often a cosmetic disease, making an affected plant look a little ragged, but not killing the plant. Only occasionally will the disease be more severe, resulting in defoliation.
How do I avoid problems with Guignardia leaf spot in the future? Remove and burn (where allowed by local ordinance), bury or hot compost fallen, infected leaves. Thin plants to increase airflow and promote rapid drying of foliage. This drier environment is less favorable for disease developments. Water plants at the base using a soaker or drip hose to minimize wetting of leaves and reduce the movement of spores. If a plant has been severely defoliated by Guignardia leaf spot for several years, preventative fungicide treatments may be necessary. Make an initial fungicide application as leaves first begin to emerge. If the weather is cool and wet, make additional applications at seven to 14 day intervals until hotter, drier weather develops or until leaves are fully expanded and mature. Chlorothalonil, copper, mancozeb and thiophanate-methyl are labeled for Guignardia leaf spot control. DO NOT use the same active ingredient for all treatments (particularly if you are using thiophanate-methyl). Instead, alternate the use of at least two active ingredients. This strategy will help minimize problems with fungicide-resistant strains of Phyllosticta ampelicida. Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.
For more information on Guignardia leaf spot: Contact your county Extension agent.
© 2003-2021 the Board of Regents of the University of Wisconsin System doing business as University of Wisconsin-Madison Division of Extension.
An EEO/Affirmative Action employer, University of Wisconsin-Madison Division of Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. This document can be provided in an alternative format by calling Brian Hudelson at (608) 262-2863 (711 for Wisconsin Relay).
References to pesticide products in this publication are for your convenience and are not an endorsement or criticism of one product over similar products. You are responsible for using pesticides according to the manufacturer’s current label directions. Follow directions exactly to protect the environment and people from pesticide exposure. Failure to do so violates the law.
Thanks to Lisa Johnson, Ann Joy and Patti Nagai for reviewing this document.
A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website: https://pddc.wisc.edu.