Keep Watch for Marssonina Leaf Blotch

Marssonina leaf blotch could become a concern for Ontario apple growers. With recent maximum application reductions of several broad-spectrum fungicides used in apple disease management, this fungus may be able to build up in orchards.

Marssonina leaf blotch of apples is caused by the fungus Marssonina coronaria and was first identified in Ontario in the early 1900s. Despite this, Marssonina leaf blotch has been sporadic in Ontario apple orchards. In recent years, eastern states including Pennsylvania and New York have identified an increase in Marssonina leaf blotch in apples causing premature defoliation, resulting in lower tree health and yield loss. When foliage infection is severe, fruit lesions can occur, although they are uncommon. There is little known about the presence and impact of this disease in Ontario apple orchards.

Symptoms typically appear mid-summer on the upper surface of mature leaves. Lesions start small (5-10mm), have a greyish, brown centre with a darkened, purple border (Figure 1). As the infection progresses, lesions darken and coalesce, with the surrounding tissue turning yellow, resulting in premature defoliation (Figure 2).

At any stage of infection, when examined closely with a hand lens or dissecting scope lesions contain raised black dots or bumps which are the fruiting bodies of the fungus (Figure 3).

To distinguish this disease from scab lesions, Figure 4 shows the two diseases on a mature leaf. Apple scab lesions are more web-like and do not contain the black bumps. Fruit lesions are less common but can be found on trees with severe foliar infection. Fruit spots are small, dark brown/black and stay on the fruit surface with minor indentation (Figure 5). This fungus needs a long duration of leaf wetness to infect, and symptoms can take as long as 40-45 days after infection to appear.

This fungus overwinters in infected leaves in the orchard. Sanitization is important in limiting the disease, reducing the inoculum for disease occurrence. There are no registered fungicides in Canada, however research shows that Marssonina leaf blotch is susceptible to most conventional apple scab fungicides. Captan and mancozeb have shown excellent efficacy against this disease, but both have been recently re-evaluated and application numbers reduced (see 2020 Orchard Network Newsletter article, Re-Evaluation Round-Up: Updates on Recent Registration Decisions Impacting Apple Growers), which could lead to an increase of this disease in Ontario. Cultivars differ in their susceptibility to Marssonina leaf blotch (Table 1).

Table 1. Apple cultivar susceptibility based on observations and research in Pennsylvania experimental orchard (Kari Peter, Penn State University)

Cultivar
Susceptibility (to date)
Rome
Very susceptible1
Honeycrisp
Very susceptible1
Empire
Very susceptible1
Scab-resistant varieties
Very susceptible1
Cameo
Susceptible
Fuji
Susceptible
Red Delicious
Susceptible
Gala
Susceptible to moderately resistant
Golden Delicious
Moderately resistant
Stayman
Moderately resistant
Cortland
Moderately resistant
1 – if no fungicides are applied early season

OMAFRA is planning a survey of susceptible apple cultivars in the 2021 growing season. If you are interested in participating in this survey, please email or call/text Kristy Grigg-McGuffin (kristy.grigg-mcguffin@ontario.ca, 519-420-9422) or Katie Goldenhar (katie.goldenhar@ontario.ca, 519-835-5792).

References

Lee, B. (2011). Biological Characterization of Marssonina coronaria Associated with Apple Blotch Disease. Mycobiology, 39(3), 200–205. https://doi.org/10.5941/MYCO.2011.39.3.200

Parmalee, J.A. (1971). Marssonina leaf spot on apples. Canadian Plant Disease Survey, 51(2), 91-92. Retrieved online: http://phytopath.ca/wp-content/uploads/2014/10/cpds-archive/vol51/CPDS_Vol_51_No_2_(91-92)1971.pdf

Katie Goldenhar
Katie Goldenhar

Pathologist – Horticulture, OMAFRA

Kristy Grigg-McGuffin
Kristy Grigg-McGuffin

Horticulture IPM Specialist, OMAFRA