Today's newsletter is a special edition focused on leaf spots. Leaf spots are relatively widespread this year, likely due to the many episodes of heavy rainfall and fungicide wash off. We discuss a leaf spot disorder and three other leaf spot diseases that are caused by pathogens. We have several photos to share from our recent set of samples to help with your field identification. Finally we offer recommendations on fall sanitation practices.
Leaf Spots
This year I have been asked a few times about leaf spots that are showing up fairly widespread throughout orchards. With several orchard volunteers we took samples showing common leaf spot symptoms and have concluded several different issues. Fungicide wash off during this year's heavy rainfall events have provided opportunities for weak pathogens. The symptoms can be difficult to differentiate so let us review each possibility below.
Disorder - Necrotic Leaf Blotch
Necrotic leaf blotch is a disorder that is specific to Golden Delicious and its strains and seedlings (Gala, Sunrise). The symptoms first show up in the mature leaves in the middle of the shoot. A hormonal imbalance in the tree is thought to contribute to the symptoms of irregular brown blotches surrounded by a green halo (Figure 1). The affected leaves then turn yellow and fall off within one week.
The symptoms are believed to be caused by environmental conditions in a combination of air temperature, light intensity, and soil moisture. The disorder is most likely when 5 days of cool and rainy weather precedes a stretch of hot and sunny weather. The disorder appears mid- to late-season and may defoliate trees by up to 50%. The disorder does not usually affect tree health unless the leaf drop is substantial.
Figure 1: Necrotic leaf blotch symptoms on Golden Delicious in early September of this year. Irregular brown blotches are surrounded by a green halo.
Disease - Frogeye Leaf Spot (Black Rot)
Frogeye leaf spot is a leaf infection caused by the fungus that is also responsible for black rot infections on fruit. The leaf infection resembles a frog eye because what begins as a purple dot enlarges into a circular lesion with a dark border and a tan coloured centre (Figure 2). A black pinpoint will often develop in the centre of the spot. By summer and late in the season the lesion is brown, small, round, and shiny.
Frogeye leaf spot does not tend to affect tree health, however the infections can be a source of spores that allow the organism to spread, survive, and overwinter in cankers and mummified apples. With more inoculum present the risk of fruit rot would increase over time.
Figure 2: Symptoms of frogeye leaf spot on Ambrosia (left) and Honeycrisp (right). Lesions are circular with dark borders and tan centres.
Disease - Marssonina Leaf Blotch
Species of Marssonina have been reported in Nova Scotia for many years and its distribution is widespread throughout eastern Canada and North America on many different tree species. Lately the pathogen has caused early apple tree defoliation in regions like Pennsylvania and New York because heavy rainfall washed off fungicide protection. Marssonina is otherwise considered a minor pathogen because it is controlled with common fungicides.
Samples this fall from several orchards in Nova Scotia have revealed leaf infection by Marssonina. New infections are small with a dark grey centre and a purple border. As infections progress the lesions enlarge, darken, and merge together. A distinct sign of Marssonina is the raised black dots/bumps within the lesion that are visible by the naked eye (Figure 3) or using a hand lens (Figure 4). The black dots are the fruiting bodies of the fungus.
Severe infections can cause early defoliation and fruit infection (Figure 5) but it is unlikely. If Marssonina becomes more prevalent we may need to reconsider the full impact of using less mancozeb and especially less captan.
Figure 3: Marssonina leaf blotch symptoms in October of this year. Note the purple border and the black dots on the far right photo.
Figure 4: Magnified fruiting structures (acervuli) of the Marssonina fungus. The raised black dots viewed under a microscope with 10x magnification (left). The same fruiting body viewed under 40x magnification (right).
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Figure 5: Fruit infection by the Marssonina fungus. Photo added Nov 3, 2023.
Disease - Glomerella Leaf Spot
Glomerella leaf spot is caused by the same fungus that leads to bitter rot of apple fruit. The leaf infection has lesions that are irregular in shape and the tissue inside the lesion is tan coloured and dead (Figure 6). As you can see, the tan coloured lesions do not have the black dots/bumps that are present in the Marssonina infections. Severe infections can lead to defoliation but it is uncommon.
Figure 6: Glomerella leaf spot symptoms with irregular tan coloured tissue on a Honeycrisp leaf in October of this year. Please note that the leaf is a few days old in this photo and due to age it has lost its green pigment in the leaf surface surrounding the lesions.
Pathogen Party
On our samples from this fall where Marssonina leaf blotch was found, Glomerella leaf spot was also found in tandem on all samples. Marssonina was much more prevalent (~80%), suggesting it may be the primary infection and Glomerella blotch the secondary infection. The sample shown in Figure 6 has prominent Glomerella infections but may also have small lesions of Marssonina, outside of the tan coloured blotches. There are other possible leaf infections but this post shows what we found in a few representative samples taken this fall.Management
Just like apple scab, the pathogens that cause frogeye, Marssonina, and Glomerella also overwinter on leaves and dead wood on the orchard floor. Spores can be reduced for the next growing season by accelerating the decay of infected leaves in the fall of the current season if possible. Where disease pressure is high this year, efforts to clean up the orchard floor will help to reduce primary inoculum for next year.Recommendations:
- Spray urea (46-0-0) onto leaves on the ground at a rate of 50 kg/ha in 1000 L/ha of water. The solution can alternatively be applied to full trees as leaf fall begins. Urea should be dissolved in warm water before putting it in the tank. The 50 kg/ha rate will supply approximately 23 kg/ha of nitrogen to the ground, so nitrogen application next spring should be adjusted accordingly.
- Flail chop all plant matter on the orchard floor in November so the leaves are more easily broken down by bacteria and molds and consumed by earthworms.
- Using a combination of both shredding and urea applications can produce the best results.
Perennia Food and Agriculture Corp. by Michelle Cortens, Tree Fruit Specialist and Dustin MacLean, Field Plant Pathologist.
Thanks to Danny Davison for discussions on the topic and to orchard volunteers for samples.
