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."