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What are those large black spots on maple leaves this year? People all over the Internet seem to be asking, so perhaps you are wondering too.
The black spots are a fungus called tar spot. This disease – which affects only maple trees, especially Norways, silvers and reds – looks ominous, but it is cosmetic only and does not cause harm in the long run.
The fungus grows when there is a warm spring, from 60 degrees to 70 degrees Fahrenheit, in which the leaves stay wet for seven or more hours a day. (Trees growing in the shade are more susceptible to tar spot disease than trees in sunlight, too.) Remember how rainy and wet it was this spring? The 7.25 inches of rain recorded locally in May made this our fifth-wettest May on record. You may also remember that we had measurable rain on 13 spring weekends in a row. The perfect weather for tar spot fungi to grow.
The spores germinate and penetrate into the leaf, infecting it and causing the leaves to not produce enough chlorophyll. The disease begins to develop as pale yellow spots in mid-June, not very noticeable. In mid-July to August, a black spot develops in the center of each yellow spot. The black spots grows larger and thicker, until by late summer they begin to resemble a spot of tar.
Infected leaves tend to fall with the fruiting structures facing up; tar spot fungi overwinter on these fallen leaves and, when spring arrives, ripens, splits open and ejects tiny needlelike spores. (Spores are somewhat analogous to seeds that can grow into a new organism.) The wind carries these sticky spores to trees, where they attach to newly emerging maples.
The best way to prevent the disease from spreading is to rake and remove fallen, spotted leaves and put them out as yard waste for collection. Even simply raking the leaves into a pile is helpful: Only the leaves on top of the pile will be able to eject their spores into the air. Mulching the leaves destroys many of the tar spots before they ripen, and it is especially helpful to cover a mulch pile before maple trees leaf out in the spring.
Even if you manage to remove all the infected leaves from your property, there are probably infected leaves overwintering in nearby parks and wooded areas. Unless the leaves of all the other infected maples are also removed, reinfection will probably reoccur in favorable spring conditions. Mostly you just have to hope that the weather this coming spring is not wet enough for the fungus to grow.
It was once thought that the air pollution of urban or industrial areas kept tar spot fungus from developing. Research has shown this is not true – the disease is less common in urban areas only because leaves in cities are more likely to be raked up and removed each year, thus inhibiting transmission.
Christiaan Hendrik Persoon
Tar spot disease was first recorded in 1794 by Christiaan Hendrik Persoon (1761-1836), an unemployed, unmarried, poverty-stricken, unhappy recluse but a prominent mycologist who described 2,269 new fungal species and classified them in a rational and understandable way – a taxonomy that mostly survives today.
Though born in South Africa, after corresponding with botanists throughout Europe he rented an upper-floor apartment in a poor part of Paris – called the souffrant faubourg, “the suffering suburb” – and built up a substantial library and herbarium of dried plants and fungi. He lived much of his adult life in Paris.
His biographer described Persoon as a large, bony man with an enormous mouth containing a small number of shaky teeth, always full of saliva, which escaped in jets with each word he spoke. He described Persoon’s clothes as “bizarre of form and strange of color, worn through by long service. He often lacked the most common necessities.”
Persoon wrote to the king of the Netherlands in 1824:
Devoid of all inheritance and not seeing any employment at all which will give me a fixed income, I am filled with anxiety for the future; I fear that an unforeseen illness would compel me to be inactive. Because of these considerations, and also through foresight, I am taking the liberty of addressing myself to the paternal government of H.M. the King of Netherlands who protects and encourages the sciences and the arts, in the end of obtaining a modest, but sufficient, pension to uphold the dignity of my name; a pension which, moreover, would not be paid for very long because I am old and infirm.
The king granted him a pension of about $350 per year – much more in today’s dollars, but hardly a fortune. To get this money, Persoon had to give the government his library and his herbarium, a collection of 14,000 specimens collected over about 50 years.
As he wrote to relatives in South Africa:
You will be wanting to know how I am living here in Europe … Although I do not enjoy the most robust health, at the same time, thanks be to God! I have not had any severe illness. My pursuits consist in cultivation of the sciences, or, more specifically, in the study of nature. I have published a number of books on botany that have been received by the public with approval. I am a fellow of various learned societies and a Doctor of Philosophy. But although these things bring honor and satisfaction, they produce few earnings, so that, did I not receive from Amsterdam … the wherewithal for my keep, I should not be able to live here very comfortably … I once more hope to see again my beloved mother country and my relatives and friends, and in their friendly midst to lead a much happier life than here [in Paris].
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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