The UW Lab Lens summarizes current pests, diseases, and problems occurring throughout Wisconsin to help you stay ahead of pests in your garden or landscape.
Weekly Plant Disease Update
By Brian Hudelson, UW Plant Disease Diagnostics Clinic Director
Website: pddc.wisc.edu
Brian Hudelson reported a busy week with a wide range of diagnoses, particularly root rots and leaf diseases. Below is a summary of notable cases:
Diseases of Herbaceous Ornamentals
- Hosta: Diagnosed with Southern Blight (Agroathelia rolfsii, formerly Sclerotium rolfsii). Brian emphasized its aggressiveness and long-term persistence, especially under warm, wet conditions. Management includes sanitation, fungicide rotation, and avoiding cocoa bean mulch.
- Veronica: Diagnosed with Septoria leaf spot, confirmed via physical sample. Symptoms included yellowing and necrotic spots on lower leaves.
- Coneflower: Classic symptoms of Aster Yellows observed, but not confirmed in coneflower growing near the above mentioned veronica.
Diseases of Conifers and Needled Ornamentals
- Arborvitae, Junipers: Continued cases of Phytophthora root rot, favored by wet soil conditions.
- Larch: Exhibited symptoms consistent with exposure to growth regulator herbicide.
- Various Evergreens: Diagnosed with Cytospora canker, a common fungal disease exacerbated by environmental stress.
Diseases of Deciduous Trees and Shrubs
- Magnolia: Diagnosed with Rhizoctonia root rot, an unusual case for woody ornamentals.
- Oak Trees: Multiple leaf diseases identified, including:
- Anthracnose (Discula)
- Tubakia leaf spot
- Monographella leaf browning
- Septoria leaf spot
- Chlorosis widespread.
- Catalpa: Diagnosed with Verticillium wilt, a common cause of branch dieback in this plant.
- Elm: Confirmed case of Dutch Elm Disease, identified by characteristic “lollipops of death” fruiting structures.
- Hydrangea: Diagnosed with mechanical girdling due to weighted strings tied to branches to stabilize the plant in a windy site.
- Aspen:
- Venturia leaf and shoot blight: Caused black blotches and shoot dieback.
- Marssonina leaf blotch: Caused by a fungus similar to the one that causes apple scab. Typical symptoms are blotchy leaf necrosis.
- Ninebark: Powdery mildew observed on a red-foliaged variety.
Diseases of Vegetables and Fruit Crops
- Cucumbers: Diagnosed with Anthracnose, presenting as irregular necrotic spots.
- Peppers: Bacterial spot (Xanthomonas spp.) confirmed, likely seed-borne.
- Potatoes: Symptoms mimicking late blight were confirmed as Phytophthora nicotianae via DNA sequencing.
- Tomatoes: Case of walnut toxicity due to juglone exposure from nearby walnut trees.
Other Diagnoses
- Slime Molds: Multiple sightings of dog vomit slime mold in lawns and flower beds.
Additional Plant Disease Resources
- Anthracnose – UW Plant Disease Factsheet
- Aster Yellows – UW Plant Disease Factsheet
- Bacterial Spot on Tomato – UW Plant Disease Factsheet
- Black Walnut Toxicity – UW Plant Disease Factsheet
- Chlorosis – UW Plant Disease Factsheet
- Cytospora Canker – UW Plant Disease Factsheet
- Dutch Elm Disease – UW Plant Disease Factsheet
- Herbicide Damage – UW Plant Disease Factsheet
- Root and Crown Rots – UW Plant Disease Factsheet
- Southern Blight – UW Plant Disease Factsheet
- Slime Molds – UW Plant Disease Factsheet
- Tubakia Leaf Spot – UW Plant Disease Factsheet
- Verticillium Wilt of Trees and Shrubs – UW Plant Disease Factsheet
Weekly Insect Pest Summary
By PJ Liesch, UW Insect Diagnostic Lab Director
Website: insectlab.russell.wisc.edu
PJ Liesch reported a mix of landscape, vegetable, and fruit crop insect activity:
Landscape Pests
- Cicada Killer Wasps, Great Black Wasps, and Great Golden Digger Wasps – active and hunting cicadas and other respective prey insects. Large, but generally harmless.
- Elm Zigzag Sawfly: Increasing reports, including 35–40% defoliation on American elms in Madison. Multiple generations due to asexual reproduction.
- Goldenrod Soldier Beetles: Common on flowering plants; harmless pollinators.
- Walnut Caterpillars: Gregarious molting behavior; minor defoliation.
- Fall Webworm: Visible silken nests on branch tips, especially black walnuts. Can be common this time of the year. Management is usually not needed as impacts to plant health tend to be minor.
- Two-Line Chestnut Borer: Found in stressed oaks; careful bark excavation needed to reveal tunnels and larvae.
- Lily Leaf Beetle, Viburnum Leaf Beetle: Ongoing sightings.
Vegetable Garden Pests
- Caterpillars on Cole Crops: Imported cabbageworm, diamondback moth, cabbage looper active.
- Purple Carrot Seed Moth: Distinctive green caterpillars with white dots; minor damage unless growing for seed. This insect tends to mostly be a nuisance as it can impact the use of herbs for culinary purposes.
- Squash Vine Borer, Cucumber Beetles: Recent reports, active in gardens.
Fruit Crop Pests
- Codling Moth, Apple Maggot: Recent samples at the Insect Diagnostic Lab. Mostly from home fruit growers that had not managed these pests. Reports common through the time of harvest.
- Stink Bugs: Juveniles present for many types; adults expected soon.
Structural & Miscellaneous Insects
- Bumblebee, Yellow Jacket, Hornet Nests: Increasing reports; management options available.
- Jagged Ambush Bugs: Common on flowers; harmless ambush predators with excellent camouflage.
PJ wrapped up with a reminder to watch for Spotted Lanternfly emergence.
Additional Insect Pest Resources
- Apple Maggot – UW Horticulture
- Assassin Bugs and Ambush Bugs – UW Horticulture
- Brown Marmorated Stink Bug – UW Horticulture
- Cabbage Looper – UW Horticulture
- Codling Moth – UW Horticulture
- Cucumber Beetle – UW Horticulture
- Diamondback Moth – UW Horticulture
- Elm Zigzag Sawfly – UW Horticulture
- Fall Webworm – UW Horticulture
- Golden Rod Soldier Beetle – UW Horticulture
- Imported Cabbage Worm – UW Horticulture
- Lily Leaf Beetle – UW Horticulture
- Spotted Lanternfly – UW Horticulture
- Squash Vine Borer – UW Horticulture
- Two Lined Chestnut Borer – UW Horticulture
- Viburnum Leaf Beetle – UW Horticulture
- Wasps and Bees – University of MN Extension
- Wisconsin Bee Identification Guide – UW Horticulture
