Friday, May 25, 2012

Downy Mildew of Roses (Peronospora sparsa)

(Written by Sean Mullahy, U of I Plant Clinic Student Worker)

Downy mildew of roses is generally not a problem in the home garden; it mostly affects roses grown in the greenhouse environment. Most rose growers are familiar with its similar (in symptoms) cousin powdery mildew, although the two are very different and can be differentiated.
                Downy mildew first appeared in the 1860’s in England, and from that point on, it was reported all throughout Europe. After making its way through Europe in the early 1900’s, it made its way over to Scandinavia and the Soviet Union. Downy mildew made its way stateside in 1880, first appearing in the Midwest. Since then, it has made its way all around the United States, and to almost every corner of the globe.
Downy mildew can be identified through a number of symptoms. Watch out for purplish red to dark brown spots developing on leaves (these will be more angular in shape) and the yellowing of leaflets. These yellow leaflets may have green tissue sections of up to 1cm2 in size. Check beneath lesions for mycelia and conidia, especially under cool humid conditions. Abscission of leaves may also be quite severe. Purple to black colored spots may also appear on stems and peduncles. Downy mildew can be differentiated from powdery mildew because the grayish spores will be produced on the underside of the leaf as opposed to the top. 
               For greenhouse management, lowering the humidity should be the first step, this can be done with ventilation and aeration. Also increasing temperatures to 27C / 80F will help in controlling it; as spores are killed at this temperature. In both cases, keeping the plants aerated and dry is a great idea. Prune roses to keep them open, and try to water them at the base. This will help prevent germination of the downy mildew. For field or garden grown roses, a fungicide application is recommended. This should be done in a preventative manner, when environmental conditions are favorable for developments (cool, humid). If you already see the symptoms, it’s probably too late! The growing of a resistant cultivar is as always recommended. For example, the rugosa family of roses is resistant to downy mildew.

To prevent spread and seasonal carry over, all suspected plant parts should be removed and all infected plant material should be destroyed. 

Additional resources:

Friday, May 18, 2012

Rhizoctonia Root Rot of Soybeans

(Written by U of I Plant Clinic Student Volunteer Zu Dienle)

We have recently received a sample of soybean seedlings infected with root rot. This root rot disease is caused by a common fungus known as Rhizoctonia solani. Watch out for symptoms such as lesions and reddish discolorations near the roots and stems. Soybean seedlings may also experience stunted or irregular growth as a result of girdling at the base of the stem. The rot on the root differs from those caused by oomycetes (Pythium or Phytophthora) in the sense that it is dry. 

Rhizoctonia only causes problems if conditions are favorable and some yield losses can be observed.  Commonly in Illinois, the disease makes it presence known around the early spring when weather conditions are wet. During severe conditions, R. solani can cause up to 50 percent stand losses!  This disease is especially prevalent under stressful conditions – injuries, herbicide applications, nutrient imbalances are all possible precursors to root rot disease development.  

In order to diagnose Rhizoctonia, we place rotted root tissue under the microscope and identify the characteristic right-angled hyphae. Some other common characteristics to look out for are segmentations in the hyphae near the branching points and constrictions near these segmentations. In certain cases, the hyphae can be pigmented. 

If you find your soybean infected with R.solani, below are some management strategies that you can employ:

There is no better way to stop a problem than avoiding it at the start! Ensure that the seeds come from a disease-free, certified stock. Make sure that the seeds are all “healthy” with no visible cracks or discolorations. Certain fungicide treatments can be applied to seeds can help to control the Rhizoctonia pathogen.

When planting these seeds, make sure that the area is not a previously infected site, as R.solani can survive up to many years in the soil. Crop rotation from time-to-time to non-host is recommended.  An ideal planting site is a warm (60 Fahrenheit or 15.5 Celsius and above) seedbed with proper tillage. Ensure that the other components of the soil such as its nutrient levels and pH are adequate.
Other useful links:

Friday, May 11, 2012

Early Planted Corn with "fused coleoptile or bursting on side"

In 2011, the U of I Plant Clinic received several corn seedling samples that appeared to have a "fused coleoptile or bursting on side".  In addition, the roots appeared to be "bottle-brushed". This corn had been planted the first week of May and unfortunately, 2 weeks later, experienced cold temperatures.

I am starting to see similar symptoms again this year in 2012:

The plants did not appear to have any disease or insect damage.  The root tissue appeared healthy, but some of the roots of the plants were sparse and appeared to have "bottle-brushed" symptoms.
There are several factors or a combination of conditions that could be causing these symptoms:

   1.)  Cold imbibhition can cause a "cork screw symptoms".  Cold injury has also been linked to ""fused coleoptile or bursting on side".  Here is a link that describes cold injury to corn in depth:

2.)  Cloroacetamide herbicide (Dual, Harness, Outlook, Surpass, Bicep) can also cause twisting of the seedling shoot. However, these herbicides are not widely used at this time and these herbicides were not known to be applied in these corn fields.

I suspected that cold temperatures and wet soils could be part of this problem, but I did not like the look of the roots of some of the corn seedlings in the sample.

Pictures of one of the corn samples (similar to those of the pictures above) were sent to Dr. Emerson Nafziger.  Here are comments from Dr. Emerson Nafziger:
"While I think cool temperatures may have contributed, seed mechanical damage and herbicide can both cause symptoms like this, and it makes sense that these factors might have worked together. Checking other fields with this same seed lot planted at the same time may indicate if seed was part of the problem.
The roots do look somewhat odd, and seem to lack the proliferation we would normally see. That could be partly due to herbicide, but I suspect that cool-then-warm soil temperatures might have contributed as well. Comparing nearby fields planted at the same time might indicate temperature effects more clearly, but I doubt that by itself this contributes much to the problem."

Could genetics along with the cold/wet temperatures play a role in this problem?