What is all the Buzz about the Brown Spots on Corn?

With the wet weather, there seems to be more scouting occurring in fields, which is good.  Many have asked about the brown spots that they have been finding on corn stalks, leaf sheaths, and leaves. 

Purple leaf sheath (Picture taken by Matt Montgomery)

Purple leaf sheath (Picture taken by Matt Montgomery)

The pictures above show an abnormality called purple leaf sheath, which is a harmless discoloration caused saprophytic fungi and bacteria developing on pollen and particulate matter lodged between the stalk and leaf sheath.  The Compendium of Corn Diseases states the following about purple leaf sheath:
"Irregular, purplish brown blotches and spots of varying size form on the leaf sheaths, usually after silking.  This harmless discoloration occurs when saprophytic fungi, such as yeasts and Fusarium spp., and bacteria develop on pollen and other particulate matter lodged between the stalk and leaf sheath or after an infestation of aphids.  The stalk under the sheath is not discolored or infected.  This is not a new disease and there is NO NEED TO BE ALARMED!

Purple leaf sheath may be confused with Physoderma brown spot, which is a fungal disease that can infect corn stalks; however, Physoderma brown spot usually also causes symptoms on the leaves as seen in the pictures below.

Physoderma brown spot (Picture taken by Matt Montgomery)

Bands of lesions across the leaf caused by Physoderma brown spot (Picture taken by Matt Montgomery)

Physoderma brown spot caused by Physoderma maydis is NOT usually considered to be a threat to corn.  This disease is often seen after abundant rains and high temperatures, and infection takes place when spores are splashed into leaf whorls.  This disease might be more apt to show up in fields that consist of reduced tillage or corn on corn.  Some hybrids may consist of more resistance than others and of course, tillage can help bury the disease inoculum.  Again, there is not need for alarm if you are seeing this disease in your field.

Russet Mites found on Tomato

Recently, russet mites were discovered associated with injury on a tomato sample at the U of I Plant Clinic.  This is not a common pest found in Illinois, but serves as a reminder that we should be careful when diagnosing all plant problems.

Russet mites can be a very tiny pest of tomatoes and other plants. So tiny in fact, they can easily go without being noticed on plants and still be causing damage. They can be found in almost all areas where tomatoes are grown.  They are only about .2mm long, and if populations are high, they are capable of causing serious injury on tomatoes. Damage first becomes apparent in mid-summer when plants are half to fully grown and injury will generally start at the base, then move throughout the canopy. The highest population of mites will tend to be found where the damage is visible. A high infestation will cause leaves to start to curl and turn a russet brown color as the mites feed, sucking the juices out of any green part of the plant. Hot weather is conducive for high mite populations, and in turn, plants can even be defoliated. More information on russet mites can be found at: http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/PESTS/tomrusmite.html 

(Information gathered by Nick Pudhomme, U of I Plant Clinic student worker)

The russet mites on this tomato sample was discovered at the Plant Clinic by examining the leaves showing symptoms under magnification:

 

This pest was featured in the latest Illinois Fruit and Vegetable News (IFVN) found at the following link:  http://ipm.illinois.edu/ifvn/contents.php

Dr. Richard Weinzeirl, U of I Extension Fruit and Vegetable Entomologist stated the following about the russet mite: "Tomato russet mite is smaller than two-spotted spider mite and is somewhat wedge- or cigar-shaped.  The sample came from a home garden in northern Illinois, and the tomatoes were adjacent to petunias (often associated with russet mites), but it served as reminder to point out that most miticides are not very effective for russet mite control.  Agri-mek and wettable sulfur are generally most effective against this pest."

Why isn't my Fungicide Working in my Soybeans?

A farmer noticed some yellowing leaves in the lower canopy of his field.  They were 99.9999999999% sure that it was Septoria brown spot ( Septoria glycines), a common fungal disease of soybeans that occurs in warm and wet weather. This disease rarely causes significant yield loss; however, yield losses of 5 – 8 % may occur under severe conditions when much defoliation occurs.  This year has been very, wet, so many have chose to make a fungicide application. 

 

The above pictures all show the symptoms of the foliar disease that was appearing in the lower canopy of this farmer's soybean field.


As time passed, it appeared that another foliar disease, with different symptoms, started to show in the mid-canopy of this soybean field.  The spots were much darker and were more symptomatic of a bacterial disease.  The farmer wanted to identify the disease(s), because this would explain why the fungicide was "NOT WORKING".  A fungicide will have no effect on bacterial diseases.

As it turns out, the disease that was making it's debut in the mid-canopy was none other but, bacterial pustule (Xanthomonas axonopodis pv. glycines). If you examine the lesion on the underside of the leaf with magnification, you will be able to see pustules forming and this is how we distinguish bacterial pustule from bacterial blight.

However, the Plant Clinic also examined the lesions under the microscope from the leaves that were from the lower canopy of this soybean field.  If you remember, these were the yellowing leaves, with lesions that were assumed to be caused by Septoria brown spot.  But, when these lesions were cut and placed under the microscope, there were no fungal spores to be found!  We found bacterial ooze!  The disease in the lower canopy was Bacterial blight (Pseudomonas savastanoi).

Bacterial ooze seen streaming from the bacterial blight lesion of the soybean.

I am 99.999999% sure that both of the foliar diseases found in this soybean field were caused by bacteria and there was no need for a foliar fungicide application.  Oops!

Identification of fire blight on apple and pear

Blog post by Qiong (Jane) Liu - University of Illinois Plant Clinic Grad Student

The most common bacterial disease for apple and pear during spring is fire blight. Fire blight is one the most common and destructive diseases of apple and pear, which is caused by bacteria Erwinia amylovora. The name” Fire Blight” comes from the appearance of the diseased plant tissue, which looks as though it had been scorched by fire. Usually, twig shoot and blossom start showing symptoms in the spring. Blossoms will first be water soaked and grayish green in appearance, and then turn brown or even black. Twig blight starts at the tips of shoots, and turns darker, with disease development. Shoot symptoms develops rapidly. Leaves on the diseased shoot become fully, necrotic in a short period of time, thus giving the tree a burnt appearance. The end of the branch may resemble an upside down “J” or "shepherds crook", a distinctive symptom of fire blight (Fig.1).  Under high humid conditions, milky droplets of bacterial ooze, containing millions of Erwinia amylovora, may be observed from diseased plant tissue
(Fig.2 and 3).

Fig.1 Diseased pear branch showing upside down "J" symptoms

Fig.2 Bacterial ooze from diseased leaves.

 

Fig.3 Bacterial ooze on fruit

Once we receive a suspicious fire blight sample, the first step is to confirm that it is a bacterial disease. The bacterial ooze test is the simplest and quickest way to differentiate bacterial disease from other problems. If the symptoms are due to a bacterial disease, bacterial streaming or "ooze" can be seen from a cross section of a petiole under a microscope(Fig.3).

Fig.4 Bacteria streaming observed from cross section of leaf petiole under the microscope.

After the ooze test, we can isolate the bacteria from the diseased plant material. The key to successfully harvesting bacteria from the affected area on the plant is to select freshly infected tissue. Most of the time, it’s hard to isolate pathogenic bacteria from dried out samples. Petioles, especially those that are recently infected and still soft, are usually the best area to recover Erwina amylovora from the affected plant. The next step is to observe the bacterial colonies that grow on media from isolation.

Isolation of Erwinia amylovora on Kings B media.

Bacterial colonies suspected to be Erwinia amylovora in culture can be selected for further testing to determine the species of the isolated bacteria. The combination of PCR amplification and electrophoresis is an accurate method to confirm bacterial species on the molecular level. A bacterial isolate, which has been confirmed as Erwinia amylovora will be used as positive control.  This will provide a reference for the bacterial isolates taken from sample of unknown bacteria. Sample isolates with PCR products that show the same bright band as the positive control will be confirmed to be Erwinia amylovora (Fig. 5).

Fig.5 From left to right, first well is loaded with DNA ladder, the third well is loaded with Erwinia amylovora PCR products as positive control, and the rest of the wells are loaded with PCR products of the unknown sample isolates. All of the isolates in this picture have been confirmed to be Erwinia amylovora.

For further information on fire blight, you can refer to the following links:
http://hyg.ipm.illinois.edu/article.php?id=290
http://hyg.ipm.illinois.edu/article.php?id=349
http://hyg.ipm.illinois.edu/article.php?id=490