Based on a limited number of cases we saw last week, there are two unusual injuries growers should be on the watch for.

"Popped kernel" is the name given to kernels where the seedcoat on the crown of the kernel breaks open, giving it the appearance of a partially popped popcorn kernel (Figure 1). Little is known about this condition, although it has been associated with irregular rainfall, especially when very hot and dry conditions are common. We speculate that the injury occurs when very dry weather during grain fill is followed by a flush of rainfall, causing the endosperm to outgrow the pericarp. However, this is speculation, since scientific research on this is lacking.

"Silk cut" is a different condition resulting in a rupture of the seedcoat where it is in contact with unpollinated silk. While the genesis of "silk cut" is not well-understood, it is thought that contact with unpollinated silk (which can remain alive for days) interferes with normal expansion of the developing seedcoat, resulting in a rupture of the kernel. Unpollinated kernels are certainly common this year, because of heat and drought during the pollination of many corn crops.

Because both of these conditions result in a rupture of the seedcoat, each opens the kernels to invasion by ear and kernel rot fungi. In particular, this condition raises concern about possible avenues of infection by mycotoxin-producing fungi, especially those that produce fumonisins or aflatoxins.

Given the widespread occurrence of very hot, dry conditions during pollination, we suggest the following:

1. Scout fields for these and other forms of kernel injury.

2. For affected fields, be prepared to harvest in a timely way and dry grain promptly.

3. Provide good storage conditions (aeration, monitoring for mustiness, etc.) for all grain but especially for corn from fields showing kernel injury.

4. Market affected grain promptly.

Last week, southern corn rust was detected by Dr. Don Hershman on the University of Kentucky Research Station in Caldwell County (Figure 1). Dr. Hershman reported that the incidence in several corn blocks was about one leaf with some pustules for every 30-40 leaves examined. These infected leaves were generally in the mid-canopy. In one block, he found two plants that were severely blighted due to southern corn rust (which is probably where the first windblown spores arrived in the field). Based on the disease levels observed, I am guessing that the fungus blew into the state sometime in the first ten days of July.

As a reminder, southern rust is distinct from common rust, which is also present at low levels in scattered corn fields. Southern rust is a much more aggressive disease under warm, wet conditions than is common rust, especially given rust and common rust can be found in a June 22, 2010 article in this newsletter. The UK Plant Diagnostic Labs can also be of assistance in documenting cases of southern rust.

Southern rust is not yet prevalent in our corn-production areas. I say this because, on a tour last week through the Purchase Area, I didn‟t see any southern rust in the corn fields I visited. Furthermore, several Extension agents in western Kentucky are specifically monitoring for southern rust, and they have not yet found any. However, with the very warm, humid air masses that are predominating lately, conditions are favorable for continued spread.

I am not trying to sound an alarm bell, especially since the disease is not yet prevalent. However, it is a disease worth watching for, particularly for late-planted fields.

By Paul Vincelli, Kentucky Pest News

Between field visits and reports I am receiving, gray leaf spot levels vary from minimal to moderate (with scattered lesions up to the top of the plant at the milk stage). Fungicide sprays have been made in some fields, justifiably in some cases. In some fields, disease levels were high enough that a spray may have protected against as much as a 10-15% yield loss. In other fields, disease levels were so low that I expect no more than 1-2% yield loss due to the disease. Because of the drought many fields suffered at silking, pollination levels are variable from field to field, and a low yield potential makes it tougher to recover fungicide costs.

There is enough gray leaf spot activity to warrant checking stalk strength as fields mature. Even in fields with no measurable yield loss from gray leaf spot, there may be enough disease to reduce stalk strength.

Producers should make records of where significant gray leaf spot damage is occurring, since the crop residue from those fields will serve as a source of inoculum for next year. Of course, that residue poses the greatest risk if a corn crop is no-tilled into it next year, but even if the field is rotated to soybean, the corn residue will provide some windborne spores to attack adjacent corn crops. Thus, making notes this year of where the gray leaf spot damage is, will help in selecting hybrids for nearby fields next year.