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One of five fungal diseases that causes the majority of foliage disease on aspen, cottonwoods and other poplar species, Ink Spot is caused by the fungus Ciborinia whetzelii.

Pest of the Month: Ink Spot of Aspen

By Jodi Zirbel


What does it look like?

One of five fungal diseases that causes the majority of foliage disease on aspen, cottonwoods and other poplar species, Ink Spot is caused by the fungus Ciborinia whetzelii. Often initially confused with leafminer attacks, the leafspot results in large, black fungal spots that resemble drops of ink. These spots eventually drop out, leaving behind large shotholes in the leaves.


Host material and range

While many of the foliar diseases that affect aspen trees occur when these trees are planted in mountain valleys, Ink Spot is primarily found in higher-elevation stands such as those in the mountains of Colorado. The fungal disease affects trees in the genus Populus, which includes aspen and cottonwoods.


Current threat

Because it’s rarely associated with tree mortality, Ink Spot is thought to be more aesthetically damaging than fatal. It is most common in young aspen stands and will often cause premature defoliation. Although some localized outbreaks may cause 25 percent of foliage loss, more severe outbreaks can see 100 percent of the leaves to drop early.

Leaf spots — arriving in the form of tan or brown patches on the upper leaf surfaces — will appear in late spring to early summer. As the fungus moves through the leaf, the discoloration may continue until the entire leaf is brown by midsummer. Soon thereafter, oval black masses that resemble ink or tar spots appear on the infected brown leaves. These fungal bodies are often 1/4–inch long. By late summer, these spots fall out, leaving behind shotholes, while the remainder of the leaf remains intact. This early defoliation, especially in young aspen trees, may reduce overall tree growth.


Prevention tips

Ink Spot overwinters when the black fungal tissue falls from infected leaves to the ground. Cool and wet spring weather produces fruiting, stalk-like structures that stimulate spore production. These fungal spores are then blown from the ground to nearby trees.

Although it’s not always feasible in large stands of trees, the removal of infected leaves from the forest floor will help to reduce spore production in the spring. In addition, if trees are provided adequate space during planting, it will help to reduce the possibility of spores reaching new leaves.


Treatment tips

If applied early enough, fungicides can prevent foliar diseases. It is important to note, however, that fungicides can only prevent new infections, so a fungicide treatment will not cure infected leaves. Treatments, especially on trees partial to fungal infections, should be sprayed at bud break followed by three additional treatments at 12- to 14-day intervals. Look for fungicides with active ingredients including mancozeb chlorothalonil and basic cupric sulfate.


What can you do?

When making recommendations on new poplar trees to plant, suggest hybrid options that are resistant to common foliar diseases. During tree planting, be sure to space trees adequately apart to reduce the likelihood of spore-to-leaf contact, as well as reduce humidity. In addition, prevent future spore production by raking and destroying infected leaves.


Jodi Zirbel is with Epic Creative, Wis. Article provided by Mauget, a leader in micro-injection and micro-infusion tree care. Contact Mauget or visit www.Mauget.com to learn more about Ink Spot.

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