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University of Illinois Plant Clinic ‘fungi-tastic’ season wrap-up
by Diane Plewa
December 22, 2015

It’s been an interesting year at the University of Illinois Plant Clinic, said U of I Extension diagnostic specialist Diane Plewa.

“With the excessively wet spring and mild summer temperatures, 2015 could be called the year of the fungi,” she said.

Bacterial and viral diseases were less common this year, most likely due to the relative lack of heavy summer storms (most bacteria infect plants through wound openings) and unfavorable environmental conditions (most viruses spread via insect vectors, and insect populations were low), Plewa explained. “Damping off, root rots, and crown rots were more common than normal this spring and persisted later into the growing season than normal while fungal leaf spots were everywhere to be seen,” she said.

Rhizoctonia, Pythium, and Phytophthora are common fungal and fungal-like root pathogens that were even more prevalent this year. Pythium and Phytophthora cause damping off and root rots and are favored by cool, wet weather, of which we had plenty, Plewa said. These pathogens were observed on woody and herbaceous ornamental plants, fruits, vegetables, and field crops. Rhizoctonia is most famous for causing brown patch and yellow patch in turfgrass.

“This year we saw a lot of those diseases, and we saw Rhizoctonia affecting ornamentals such as bachelor buttons and coneflowers,” she added.

Fungal foliar spots were everywhere this year. Of 400 woody tree samples, 41 samples were confirmed with anthracnose, a common leaf spot caused by various fungi. “While we normally see anthracnose in spring, we saw more samples with it this year, and we were finding it later into the season,” Plewa said.

“For most trees, this disease is not considered a serious problem. However, in sycamore and dogwood trees, anthracnose can pose a threat to the overall health of the tree,” she said. “Anthracnose was also found on herbaceous ornamentals, including hosta and English ivy.”

Cercospora and Septoria, two common fungi known for causing leaf symptoms on field crops and vegetables, were found affecting herbaceous and woody ornamentals, including creeping jenny and garden mums (Septoria) and peonies and dogwood (Cercospora). Unusual foliar issues also showed up as aerial blights caused by both Rhizoctonia and Pythium were identified. These blights only occur in situations with very high humidity and low air movement. The Rhizoctonia was found on an ornamental ground cover, whereas the Pythium was affecting turfgrass.

Plewa said another odd turf problem seen this year was nigrospora blight. “It’s rarely seen in Illinois, and only one sample was identified,” she said. “More common problems on turf included rust, anthracnose, powdery mildew, ascochyta blight, and dollar spot.”

Boxwood blight is a federally regulated pathogen that was a concern this year due to the movement of plants and the favorable weather conditions. Plewa said it was not found in Illinois, although a number of common pests were, including the insects boxwood leafminer and boxwood psyllid, and the diseases Volutella blight and Fusarium blight.

“We saw a large number of stress issues in woody plants. Fungal cankers, nutrient chlorosis in deciduous trees, and tip blights in conifers were the most common stress-related issues seen in 2015,” Plewa explained. “Many trees are still suffering the effects of the two back-to-back summers of drought a few years ago, and the recent harsh winters and excessively wet spring have caused substantial amounts of stress to both newly established and mature trees.

“Depending on the problem, management may be available that targets just the symptom or the pathogen, but the best management will include reducing stress when possible,” she said.

Both pine and spruce trees are affected with needle blights, which are also considered stress diseases. Diplodia and Dothistroma fungal diseases on pine needles were very common this year. Also common were Rhizospaera and Stigmina, two fungi that affect spruce needles. Of the 15 samples confirmed with Rhizosphaera and 14 samples confirmed with Stigmina, eight samples were confirmed with both.

Plewa said that oak wilt continues to be a disease of concern. This year, samples from eight counties tested positive for the fungus. “The Plant Clinic has confirmed samples from 35 counties across Illinois with oak wilt over the last several years. A proper diagnosis is important as the disease can quickly spread from tree to tree, and it is considered a fatal disease,” she said. “While infected trees should be removed to reduce the potential spread of the fungus, it is best to wait until the tree is dormant.

“Proper packaging and shipping is the key to an accurate diagnosis. If you suspect oak wilt, please refer to the Plant Clinic website for sampling guidelines or contact us,” she added.

Samples from two new counties were confirmed with Bur Oak blight, an emerging fungal disease of Bur Oaks. A sample from a third new county is suspected of being infected with Bur Oak blight. “So far, the disease appears to be limited to northern and western Illinois, though the pathogen was only recognized in 2012, and we continue to keep an eye on this disease,” Plewa said.

For more information about the U of I Plant Clinic, including where to send samples, please visit http://web.extension.illinois.edu/plantclinic.

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