Give Us Your Disease! Diseased Apple Leaves, That Is

2026 Leaf Blotch Survey

We have funding in 2026 to identify leaf blotch symptoms in apples. There are multiple pathogens responsible for leaf disease symptoms, and we want to get a better idea of which are the most common in Ontario. Contact Katie or Kristy if you would like samples collected from your orchard.

To participate in the survey, contact:

Kristy Grigg-McGuffin tel: 519-420-9422 or email: kristy.grigg-mcguffin@ontario.ca

Katie Goldenhar tel: 519-835-5792 or email: katie.goldenhar@ontario.ca

There has been significant reduction in multisite fungicide applications as a result of recent product cancellations, changes to application limits, and increased restricted entry intervals and pre-harvest intervals. These changes have resulted in increased use of single-site, highly specific active ingredients that are active on a narrow range of diseases, potentially leading to secondary or minor disease outbreaks.

In recent years, there has been increased levels of leaf blotching in apples, specifically the widely grown cultivar ‘Gala’. Leaf symptoms are characterized by brown blotches, sometimes with concentric rings and leaf yellowing that often leads to defoliation (Figures 1 & 2). In some cases, this can affect entire trees and has long-term plant health impacts.

Causes of Leaf Blotch

There has been no provincial survey to determine the cause of these leaf blotches. However, there are multiple possible causes of leaf blotching identified in the literature and previous studies, including;

• Colletotrichum (syn. Glomerella) species – commonly known as bitter rot
• Diplocarpon coronariae (syn. Marssonina coronaria) – commonly known as Marssonina (Diplocarpan) leaf blotch
• Botryosphaeria obtusa – commonly known as black rot or frog-eye leaf spot
• Alternaria species – commonly known as Alternaria leaf blotch
• Abiotic stress

Colletotrichum

In 2013, an OMAFA bitter rot survey also included 28 leaf blotch samples. Of those samples tested:

• 57% were diagnosed as Alternaria spp.
• 13% were identified as Colletotrichum spp.

Following that survey, a University of Guelph project focused on identifying Colletotrichum species in Ontario orchards. Researchers identified a small percentage of Colletotrichum species in asymptomatic, randomly collected leaves.

Diplocarpon

Marssonina (Diplocarpon) leaf blotch of apples is caused by the fungus Marssonina coronaria and was first identified in Ontario in the early 1900s. In recent years, eastern states including Pennsylvania and New York have identified an increase in this disease, causing premature defoliation, resulting in lower tree health and yield loss. This pathogen was confirmed in Ontario orchards but there has been no extensive survey to determine distribution of this pathogen.

Botryosphaeria

Botryosphaeria obtusa, commonly known as block rot, is a widespread and well-established pathogen in Ontario apple orchards. It is commonly managed on an annual basis due to its consistent presence and role in fruit rot and canker development. While typically considered in the context of fruit disease management, B. obtusa is also often associated with leaf blotch symptoms, known as frog-eye leaf spot.

Alternaria

Alternaria mali is the species reported in most US literature on Alternaria leaf blotch. Reports from Michigan, Pennsylvania and North Carolina all state A. mali as the cause of Alternaria leaf blotch. However, leaf blotch caused by A. arborescens has been reported in the Netherlands, France, Italy, Chile, Australia, and New Zealand (see references). In Australia, the main Alternaria species associated with 60% of leaf samples was A. arborescens, followed by A. alternata (16%), A. tenuissima (16%) and A. longipes (8%).

Abiotic

Leaf blotch symptoms may also arise from non-pathogenic causes, particularly under environmental stress or phytotoxicity. Necrotic leaf blotch associated with abiotics factors is often linked to periods of high temperature, drought stress, or rapid fluctuations in moisture availability, which can disrupt normal leaf physiology and lead to localized tissue collapse.

In addition, phytotoxicity from certain products (e.g., incompatible tank mix partners, applications under high heat or slow drying conditions, etc) has been associated with leaf necrosis resembling blotch symptoms. Nutrient imbalances – particularly deficiencies or excesses of potassium, magnesium or boron – can further predispose leaves to necrotic spotting or marginal scorch that may be mistaken for a pathogen.

Unlike biotic causes, abiotic leaf blotch symptoms are correspond to specific application timings or weather events, and do not typically spread within the canopy over time. However, prolonged or repeated stress events can still contribute to premature defoliation and reduced tree vigour.

Interested?

This survey hopes to provide insight into identification, especially early symptoms, and allow for targeted provincial management strategies to maintain tree health.

Let us know if you are interested in participating!