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The P&PDL Pictures of the Week
for 26 May 2003



Iris Leaf Spot

Gail Ruhl, Interim P&PDL Director, Senior Diagnostician, Dept. of Botany and Plant Pathology, Purdue University

Leaf spots caused by the fungus Didymellina macrospora (Heterosporium iridis) are a common sight on iris. Small, water-soaked lesions develop rather rapidly into 1/2-inch- long spots with brownish purple centers and yellow margins. The spots are usually observed more on the upper half of the leaves, however, if plants are crowded and wet weather prevails, the spotting may appear earlier in the season, covering more of the leaf and causing more damage. Premature leaf death will weaken the rhizomes or bulbs.

The fungus spreads from diseased to healthy leaves during the summer primarily via splashing water. For this reason, infected leaf tissue should be removed as soon as it is observed. In addition, it is important to avoid overhead watering of foliage, to work with plants only in dry weather, to space plants in order to promote good air circulation and to use more resistant species. Since the fungus over-winters on plant debris, it is especially important to remove infected leaf and flower stalks in the fall.

The fungicides registered for use against this disease are preventive sprays and thus perform best when used prior to infection. However, if leaf spotting is severe, you may help to control the spread of this fungus to healthy leaves by spraying with a registered fungicide at prescribed intervals, according to label directions.

Iris leaf spot
Close-up of iris leaf spot

Photos courtesy of Steve Goodwin


Wind and Frost Injury on Spruce, Fir and Deciduous Trees

Gail Ruhl, Interim P&PDL Director, Senior Diagnostician, Dept. of Botany and Plant Pathology, Purdue University

We have received numerous phone calls and samples questioning the dieback of the new growth on spruce trees as well as shoot dieback, blackening, shredding and scorching of foliage on various deciduous trees. The symptoms appear to be primarily on the west or windward side of trees. This type of uniform pattern appearance is suggestive of an abiotic (noninfectious) problem, such as might occur from environmental or chemical injury. Salt injury has been ruled out due to the recent timing of symptom development and non proximity of the trees and shrubs to sidewalks or roads. The most likely ‘culprits’ responsible for the symptoms observed on new tender plant growth are either excessive desiccation from recent severe winds or the delayed symptom expression of frost damage that may have occurred on April 23rd. Some of the leaf scorch observed may also possibly have been caused by windy and hot conditions (over 80 degrees) in early May when the leaves were developing.

If similar symptoms appear more randomly on the foliage of maple, ash, oak, walnut and sycamore, then a fungal disease known as anthracnose, may be responsible for the leaf blight. This fungal disease can cause darkened, necrotic areas on leaves, particularly in the lower canopy, following wet, springtime conditions. Another typical symptom of anthracnose on sycamores is the dieback of new shoots.

There is no need to spray anything on established trees for any of these problems. In order to prevent further stress to the trees this summer, maintain good cultural practices.

Click on the small image to view a larger image.

Noninfectious leaf scorch
on maple

Drooping candles (new growth)
due
to environmental injury

Noninfectious leaf scorch on ash
  Photos courtesy of Greg Bossaer  

 

Drooping candles (new growth)
due
to environmental injury
Close-up of drooping candles

Photos courtesy of Doug Akers

 

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Last updated: 27 May 2003/amd
The Plant & Pest Diagnostic Laboratory at Purdue University