Scab (apple and pear)
Hosts: Apple, crabapple, pear, mountain-ash
Pathogens: Venturia inaequalis, Venturia pirina
Signs/Symptoms: Feathery-edged spots on leaves and fruits often leading to leaf loss and tree defoliation
More information on scab
Septoria Leaf Spot of Lilac
Host: Lilac
Pathogen: Septoria sp.
Signs/Symptoms: Dead spots on leaves, potentially leading to complete leaf browning
Aster Yellows
Hosts: Many herbaceous ornamentals and vegetables
Pathogens: Aster yellows phytoplasma
Signs/Symptoms: Yellow/orange/purple leaves, stunted and distorted growth, leafy flowers, brooming
More information on aster yellows
Gymnosporangium Rusts
Hosts: Juniper, apple, crabapple, hawthorn, quince
Pathogen: Gymnosporangium spp.
Signs/Symptoms: Brown blobs with orange gelatinous masses (juniper); yellow/orange leaf spots (other hosts)
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Winter Injury/Winter Burn
Hosts: All conifers, particularly yew and juniper
Cause: Insufficient water
Signs/Symptoms: Needle browning/bleaching over winter or in spring as plants come out of dormancy
Find more information on winter burn
Bacterial Canker
Hosts: Stone fruits (cherry, peach, plum)
Pathogen: Pseudomonas syringae
Signs/Symptoms: Masses of sap on affected branches/trunks, branch die back, tree death
More information on bacterial canker
Powdery Mildew
Hosts: Any vegetable, particularly vine crops, peas
Pathogens: Miscellaneous powdery mildew fungi
Signs/Symptoms: Powdery white growth on leaves
More powdery mildew information for…
Septoria Leaf Spot and Early Blight
Host: Tomato
Pathogens: Septoria lycopersicia and Alternaria solani
Signs/Symptoms: Spotting and eventual total collapse of leaves working from the bottom of the plant up
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Rhizosphaera Needle Cast
Hosts: Colorado blue spruce, other spruces
Pathogen: Rhizosphaera kalkhoffii
Signs/Symptoms: Browning/purpling of interior needles of lower branches, followed by needle drop
Find more information on Rhizosphaera needle cast
Diplodia Shoot Blight and Canker
Hosts: Austrian pine, other pines
Pathogen: Diplodia spp.
Signs/Symptoms: Dieback of brand tips with dead needles showing uneven lengths
More information on diplodia shoot blight and canker
Chlorosis
Hosts: Pin oak, red maple, birch, azalea, white pine, blueberry
Pathogen: None (Abiotic)
Signs/Symptoms: Yellow leaves with dark green veins
Find more information on chlorosis
Authors: Brian Hudelson, UW-Madison Plant Pathology
Last Revised: 04/18/2025
D-number: D0136
For more information on plant problems to watch for in 2025: Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or pddc@wisc.edu.
Thanks to Julie Hill, Kathy Johnson and Margaret Murphy 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.
Send a Plant Sample for Analysis
Be cautious when self-diagnosing plant health issues. Very few diseases can accurately be diagnosed by eye.
Contact the UW Plant Disease Diagnostics Clinic (PDDC), and for a small fee, clinic staff can examine a plant, determine the cause of the disease/disorder, and provide advice on how to control or prevent the issue.
