June 5, 2017

Shade Tree Anthracnose

Wet spring weather brought on an outbreak of a disease on familiar shade trees. Anthracnose has affected certain species of maple, sycamore and oak trees.

Anthracnose is caused by fungi and can only infect leaves when there is consistent moisture and cool temperatures at the time the leaves expand in the spring.

: Sliver Maple leaf with anthracnose lesions.

: Silver maple canopy with affected foliage

: Blighted leaves on lawn.

Raking and removing the leaves, now or in the fall, can help remove some of the fungal spores so it is a good practice to help reduce the disease pressure for next year. The leaves can be composted (as long as the compost bin is not right below the tree) without risk of spreading back to the tree.

The good news is with warmer and drier conditions the disease will be no longer be a problem. No treatment is necessary and the overall health of shade trees is not affected by anthracnose.

: Anthracnose can lead to dieback especially on sycamore.

: Healthy new growth on sycamore.

Due to the rainy season we had, this disease is prevalent but if we have more normal spring weather next year it is quite possible the disease will not be seen at all.

: Anthracnose blighted bur oak leaves.

: Anthracnose on older leaves. New emerging foliage is healthy.

The spotted leaves will remain that way and some of those affected leaves may fall off early. However, any new growth that emerges in June and through the summer will be healthy.

September 2, 2015

Tar Spot of Norway Maple

Premature leaf drop due to Tar Spot.

Norway maple leaves on the ground in August and early September? Noticing black spots on the green leaves? The Tar Spot fungal disease of Norway maples is prevalent this year.

Close up view of the lesions

There are black spots and the leaves are falling. We’re noticing the symptoms now but it all started in the spring as the new young leaves were expanding. At that time of year they can become infected by spores that are released by from spots on last year’s leaves. Those spores can drift via air currents and if they reach a Norway maple leaf and it there’s enough moisture then it will grow inside the leaf. It is a slow process and the first symptoms aren’t noticed until mid summer, but by late summer and early fall we can see the results of those spring time airborne spores.

Leaf drop occurs when the Tar Spot lesion develops on the leaf stem or petiole and cuts off the pipework to the rest of the leaf.

Will this cause my tree to decline? For concerned homeowners the good news is that this disease does not affect the overall health of Norway maples and they will produce a full canopy of leaves again next spring. Will it help to remove the fallen leaves with spots this fall? Yes, somewhat, but since the spores can drift through the air the infection can still take place from airborne spore originating nearby. With the variations in spring moisture from year to year the amount of the disease also varies annually.

Tar spot on Norway maple tree branch foliage.

Composting is okay! A Cornell study by Dr. George Hudler concluded that the leaves can be composted with no danger of spreading the disease as long as those leaves are not exposed to air at the top of the pile the following spring when they would be releasing their spores.

Leaf lesions on Norway maple.

Although once popularly planted as a landscape tree, the Norway maple has a bad habit of shedding a copious number of seeds each year. The subsequent seedlings have invaded wild forests and crowded out some of our native plant species. In the spring of 2015 they became regulated and now any Norway maple trees sold in NY state must be accompanied by a label with the words “Invasive Species- Harmful to the Environment”.

July 8, 2014

Tree Diversity Minimizes Pest Problems

A diverse Christmas tree planting in Upstate New York.

A large planting of a single species is easier to manage and harvest but can be a windfall for pests. Monocultures allow insects and disease to expand rapidly as the pest can move unimpeded from one plant to another. After the devastation that occurred in community forests with streets lined exclusively with American Chestnuts (Chestnut Blight) and American Elms (Dutch Elm Disease), planting a diversified mixture of trees is now the standard practice among municipalities.

Dead Fraser fir with healthy Canaan Fir foreground and Turkish fir behind.

Many nursery and Christmas tree growers also know the advantages of growing a variety of species. They’ll choose tree species to fit their different environments on their farm. Then just like the smart investor who has diverse portfolio to minimize risks in the future, diverse plantings can prevent pests from moving in and wiping out all the trees. Even landscape designers when working with customers who envision a planting of a single species will often try to specify a mixture of similar species to increase resilience.

There’s increased interest in Turkish firs such as this example of Abies bornmuelleriana because of it’s root rot resistance.

Having a diverse planting works because most insects and diseases have preferences for the host species they feed upon. Many simply cannot live on a different genus, for example it is rare to find an insect or disease that lives on both fir and spruce trees. In addition certain sites on a farm or landscape may be more conducive to one species over another. For example if a species is mis-matched on a site with soil conditions it cannot tolerate, then the whole planting may succumb to a root rot.

Growers taking notes on new tree species they may want to add to their plantings at Cornell Plantations.

Growers taking notes on new tree species they may want to add to their plantings while at Cornell Plantations.

Cornell Plantations is one place where you can experience and compare a wide variety of deciduous trees and conifers. Although their plantings extend over several acres on campus, a concentration of conifers can be found at the Kienzle Overlook. http://www.cornellplantations.org/our-gardens/botanical/kienzle-overlook . At this website you can find a list of the 80 + conifers species that are in that planting.

Phil Syphrit

Phil Syphrit pms26@cornell.edu is the curator of this collection and can help answer questions on some of these unusual conifers if you are interested in adding them to your plantings.

October 4, 2013

Check for Fir Rust Disease

Fir Broom rust is becoming more common in New York as the number of fir tree plantings increase. This disease produces a proliferation of shoots “witches brooms” that can make the tree unsalable.

Severe Fir Broom Rust can cause losses for growers.

The fall and the upcoming sale season, are a good times to see if it is present in the field. When taking off the bottom limbs of trees be sure to take notice if any brooms are present. Also when the trees come in during harvest season keep an eye out.

It can be difficult to detect the small “brooms” at first

Because the affected growth is shorter than the surrounding branches it is often hidden.

The disease often shows up on the lower branches first where the rust fungus benefits from increased moisture in the shaded lower canopy. The lower branches are also to the source of the spores that are coming from the chickweed plants. The infection of the firs happens in the spring when rust spores travel from chickweed to fir tree needles at bud break. With enough moisture those spores can germinate, penetrating the needles and later in the season producing spores on the needles that can only infect chickweed, continuing the cycle. Both mouseear and common chickweed are susceptible to this disease. These weeds often grow unnoticed amongst grasses in Christmas tree fields.

Mouseear & common chickweed. (Click for expanded view.)

There are no fungicides registered to control this disease on fir trees. Fortunately it can be managed by controlling the chickweed in the field with broadleaf herbicides.

For more information see this factsheet: http://nysipm.cornell.edu/factsheets/n_gh/fir_broom_rust.pdf

May 21, 2013

Quince Rust on Junipers

Another rust disease in the cedar rust group, quince rust is the most likely to cause serious dieback on junipers.

: Upright blue juniper with dieback from quince rust.

: Orange rust gall lesions on affected juniper.

: Nearby hawthorn trees visible at the left of this image.

With this disease the bright orange “galls” are actually just slightly swollen lesions on the stems. Widespread lesions can lead to significant browning of the branch tips as pictured above. Spores which spread from these galls in the spring infect the alternate hosts of quince, hawthorn, crabapple or apple tree leaves. On hawthorn trees this disease causes causes symptoms on the leaves, white fringes on the fruit and swellings and distortion of the branch tips.

As with similar rust diseases separating the juniper from the alternate host (the further apart the better) can help keep this disease in check.

For lists of resistant varieties and fungicides labeled for managing this disease refer to the Disease Section of Cornell Guidelines for Commercial Production and Maintenance of Trees and Shrubs.

May 14, 2013

Hawthorn Rust

Bright orange swellings can be seen at this time of year on some Eastern redcedars and other junipers. After a rain those small, translucent-orange galls with gel-like projections appear. These weird galls are caused by an intriguing fungus that needs two different host plants to live!

Hawthorn rust galls on Eastern redcedar.
Photo take in May, Onondaga County, NY.

Close up of galls. (Click for expanded view.)
Photo take in May, Monroe County, NY.

In the spring the orange masses expand after a rainfall and release spores that can travel though the air and only infect the leaves of a hawthorn, apple tree or similar host (see below). Then in the fall, from spots that formed on the hawthorn or apple leaves, spores are produced that can only infect a juniper/cedar. The cycle continues in the spring when you can see new galls on the juniper/cedar. Although a problem for apple growers this rust disease does not cause serious harm to the junipers. There are other rusts that can cause problems with junipers such as quince rust.

In addition to the foliage of hawthorn and apples this disease can occasionally affect the leaves of affect crabapple, service berry or quince or pear. However this disease does not affect any other evergreen tree species. Only redcedars and junipers can become infected.

April 26, 2013

Armillaria Root Rot (Shoestring Rot)

Fraser fir killed by Armillaria

This disease is caused by several species of the fungus Armillaria. It is also known as shoestring rot due to the dark wiry strands it produces. These fungal “shoe strings” can spread the disease. It most frequently attacks trees weakened by environmental stress or other factors.

Unfortunately the shoestrings are often confused with roots. One of the easiest ways to recognize the disease is the white layer of mycelium that can be found under the outer surface of the roots or just below the bark at the base of the tree.

This fungal disease will sometimes produce small groups tan colored mushrooms near the base of affected trees. Although not present every year the mushrooms most often appear in the early fall following rainy weather.

White mycelial fan on trunk

Fungal “shoestrings” on rotting stump next to infected tree

Disease Management:

· Keep tree vigor as high as possible to avoid tree stress

o Avoid planting trees in droughty or poor planting sites.

o Water trees, if possible, during extended periods of drought.

· If found remove and destroy stumps and roots of affected trees.

· Plant new trees as far as practical away from where an affected tree was removed.

· Avoid planting new trees in a recently cleared woodlot that had a problem with Armillaria.

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Category Archives: Plant Disease