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Posted on: 4/14/21

Maple Tar Spot Disease

Maple tar spot is a very visible problem for maple trees. It starts with small yellow spots on growing leaves, and by late summer these yellow spots expand into large black blotches that look just like tar has been dropped on the leaves. This is because a fungal pathogen in the genus Rhytisma has taken hold.

When the fungus initially infects a leaf, it causes a small 1/8-inch-wide yellow spot. As the season progresses, that spot spreads, eventually growing up to 3/4 inches wide. The spreading yellow spot also changes colors as it grows, slowly turning from a yellow-green to a deep, tarry black.

The tar spots do not emerge right away but are typically obvious by mid to late summer. By the end of September, those black spots are at full size and may even appear to be rippled or deeply grooved like fingerprints. Don’t worry, though, the fungus only attacks the leaves, leaving the rest of your maple tree alone.

The black spots are unsightly, but they do not do any harm to your trees and will be shed when the leaves fall. Unfortunately, maple tree tar spot is spread on the wind, which means that your tree can get reinfected next year if spores happen to hitch a ride on the right breeze.

Maple Tar Spot Treatment Because of the way maple tar spot disease is transmitted, complete control of maple tar spot is virtually impossible on mature trees. Prevention is the key with this disease, but if nearby trees are infected, you can’t reasonably expect to destroy this fungus without community support.

Start by raking all your maples fallen leaves and burning, bagging, or composting them to eliminate the closest source of tar spot spores. If you leave the fallen leaves on the ground until spring, the spores on them will likely reinfect the new foliage and start the cycle again. Trees that have trouble with tar spots year after year may also be struggling with excessive moisture. You will do them a great favor if you increase the grade around them to eliminate standing water and prevent moisture build-up.

Trees may require treatment, especially if other trees have had a lot of their leaf surfaces covered by tar spots in the recent past. If you’re planting a younger maple in an area prone to maple tar spot, treat tar spot by applying a fungicide, like bayleton and mancozeb, at bud break and twice again in 7- to 14-day intervals is recommended.


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Posted on: 7/14/20

Magnolia scale eggs hatch internally and the crawlers emerge from the mother insect. Crawler emergence occurs late summer into early fall. Insect life cycles are dictated by heat so emergence of crawlers will vary from year to year. On average crawler emergence occurs from late August through the end of September. This would also be the best time to treat with insecticides. The crawlers move around until they find a suitable feeding site, usually on branches, where they settle down and remain through the winter. The adult female dies after reproducing, but may remain attached to the stem for many weeks, making the population seem larger than it really is.

Damage
Scale insects have sucking mouth parts and feed on sap from the tree. They can remove large quantities of sap and can stress the host tree. Trees can usually tolerate small populations of scale. The extensive feeding by a larger population will stress the trees and often leads to yellowing of leaves and twig dieback. Over time, an untreated population of magnolia scale may lead to decline of the tree.

Excess sap is excreted by the insects as honeydew. Honeydew is sticky and will coat plant parts and often drip onto surfaces under the tree. A black fungus called sooty mold will grow on the honeydew, but does little actual damage to the plant. The sticky honeydew and black sooty mold are often noticed before the insects are seen. The honeydew may also draw other insects like ants and wasps to the tree.

Hosts
Magnolia scale will attack magnolia trees and tulip tree. Star magnolia, saucer magnolia and many magnolia hybrids are most commonly and severely affected.


Posted on: 6/9/20

Rain has been over-abundant in the last couple of weeks.  So, of course, it is no surprise that anthracnose is already showing up.  Anthracnose is primarily a foliar disease affecting many deciduous trees including ash, elm, oak, and maple. So far, all the reports we have received have been on maples, but it is likely other species are showing symptoms and we just have not heard about it. Leaves are already heavily spotted. Often, we don’t see a lot of defoliation with anthracnose (except for sycamore anthracnose), but infections on maple seem to be severe enough this year to be causing some defoliation. This will not be fatal, but it will put some additional stress on trees as their “food factories”, the leaves, drop off prematurely.  The food that trees make for themselves is different from what fertilizers provide, so extra fertilization is not warranted.

The fungi are able to infect the young, tender leaves, especially during cool and wet springs, like we’ve been having this year. The disease is caused by several different fungi. The fungi are host specific, so the maple fungus doesn’t infect oak trees, and so on.    Symptoms vary with the plant host, weather, and time of year when infection occurs, but this disease often produces brown or black blotches (fig. 10) and curled or twisted leaves. Infection is more severe when prolonged spring rains occur after new growth is produced. Although the symptoms appear in late spring into the summer, the initial infection took place in the early spring at bud break and before the leaves hardened off. Once the symptoms show up, it is too late for any chemical applications to be effective.

Management: Cultural methods are usually sufficient to reduce the severity of anthracnose in our region. These include:

  • Pruning trees to open up the canopy for better air circulation.
  • Maintaining tree vigor with proper watering during times when rain is inadequate.
  • In the fall, cleaning up and destroying fallen leaves to reduce the source of inoculum.

Posted on: 5/07/20

We have received photographic evidence in our Plant Clinic email this week to show that the cedar rust galls are producing their telial spore horns and producing spores. I went to my back window to check my neighbor’s poor juniper. On Tuesday morning, I could not see any spore horns, but by Wednesday noon, I did not need the binoculars to confirm the horns are expanded. They are fully expanded due to the rain this week. All three cedar rusts (cedar-apple, cedar hawthorn and cedar quince) are active now. If you have not started protective sprays on the deciduous host, it time to get going!

Source: Morton Arboretum


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