Many shade trees are prone to one or more fungi that cause scattered, rather definite, round to oval or irregular spots on the leaves. The symptoms usually occur from late June through August.
Few spots on the leaves here and there do little or no significant harm to the tree and are far more unsightly than perilous. Nevertheless, leaf spots that start early in the growing season usually lead to premature defoliation. This is the first year ever I have noticed such occurence on my beloved Magnolia tree. Apart from it's age (its over 30 years old), it has an unusual and interesting trunk shape and a foliage that brings shelter in times of mental summer weather. It is surrounded by old Pine, Fir and Cedar tress, but through the years and many battles for a better share of sun rays, it has positioned and formed its lavish crown in the best possible way so that it gets its fair share of afternoon sun. Since I could remember, it has always displayed a lavish show of blooms each spring and the rich foliage that followed, creates a sort of a towering umbrella above that particular part of the garden and enough shade for a garden table, some benches and a pond, making it a unique place with a refreshing climate even during the hottest days. These days it all boils down to removing excessive foliage loss and cleaning the pond underneath clogged with dead leaves. The only lucky nuggets making profit in this story are the floating pond plants finally getting some reasonable amount of light. I don't mind any of that as long as my Princess Magnolia stays safe and sound. Losing such a relic tree would be inestimable.
This year the bloom quantity was noticeably in decrease but I thought it was to do with the crazy and unusual weather that seems to surprise and affect just about everyone these days. Which is definitely one of the favorable factors for such a disease. For over a month now it has been losing its lush foliage. Considering the dry period that we've had, that wouldn't surprise me either, as most magnolias are intolerant of drought conditions. But then the following symptoms would alert even a nonchalant and unexperienced gardener like me.
As far as the leaf spot goes, if it occurs for over two or more successive years, it can seriously weaken the tree, minimize its growth and increase its liability to winter injury and other diseases.
Leaf spots commonly increase in number and size in late summer and early autumn
as the leaves begin to fall. The occurrence of a leaf spot disease late in the growing season generally does not seriously affect the health of a tree. From the many varieties of the leaf spot disease, I believe I have uncovered the one in question here - PHYLLOSTICTA LEAF SPOT (if you think I might be wrong, do not hesitate to broaden my horizons ;-) I'm sure if Gil Grissom was here to help, he'd say There's always a clue...
I have found some useful info from the University of Nevada factsheet,
where they studied the effects of such disease on Maples.
Phyllosticta leaf spot symptoms range from a few round spots or lesions that do not affect the overall tree, to early loss of leaves by mid to late summer from a severe infestation that, overtime, can debilitate the tree.
The irregular, round, yellowish brown lesions are usually less than 5mm in diameter. Under the right conditions, tiny black fruiting bodies of the pathogen (which can be seen with a hand lens or microscope) are produced, usually they form a circle. The center of these spots is dead tissue that easily breaks away leaving a hole.
Wet weather in spring and early summer in successive years contributes to infectious development and growth of Phyllosticta minima leaf spots. Infection occurs on wet leaves by water-splashed spores. Black fruiting structures (pycnidia) develop in the center of the spots. Infection is most severe in the lower third of the tree, where there is more moisture, as well as on soft tissues of newly emerging leaves. A severe infestation of this Phyllosticta species kills a large portion of the leaves which leads to premature leaf loss.
CONTROL & DISEASE MANAGEMENT
There are many strategies to minimize or eliminate damage from this disease. These include:
- Removing and destroying leaves from trees infected with leaf spot disease prevents fungal pathogens from overwintering in fallen leaves on the ground
- Maintaining uniform soil moisture by regularly irrigating and mulching around young trees to prevent stress
- Avoiding overhead irrigation. If it is necessary to use overhead irrigation, water when foliage can dry quickly
- Spraying the tree, when necessary, with appropriately labeled fungicides to curtail or prevent further disease spread. Fungicide sprays may be used to control diseases of trees with a history of the disease. Fungicides must be applied before or during the early stages of infection, in early spring and summer, to obtain the best results. Fungicides act as protectants. Little control can be expected after symptoms develop. If fungicides are necessary, use a product containing mancozeb (Dithane) or chlorothalonil (Bravo, Daconil)
- Removing, as a last resort, heavily infested branches to reduce the number of spores available to infest the nearby healthy foliage
So then, early next spring, I will try with good old copper sulfate spray before the tree puts out the new growth and we'll see how it goes. If anyone's got a piece of advice or experience with such an issue, that odious blogger spell check shouldn't stop you from kindly helping the Princess Magnolia vs Leaf spot ;-)