Scouting Corn for Aspergillus Ear Rot
Paul Vincelli, Extension Plant Pathologist, University of Kentucky
The content of this article previously occurred in the Kentucky Pest News, number 1315.
Aflatoxins are potent, naturally occurring toxins that sometimes develop in corn and certain other crops. Aflatoxins are also among the most carcinogenic substances known. There are well-defined limits on how much aflatoxin is allowable in corn, based on the intended end use. For example, corn intended for human consumption must have less than 20 ppb (parts-per-billion!). More information on aflatoxin can be found in the UK Extension publication, Aflatoxins in Corn, ID-59, at http://www.ca.uky.edu/agc/pubs/id/id59/id59.pdf.
The prevalence of drought conditions this growing season is resulting in aflatoxin contamination in at least a few sporadic instances. Some Extension agents have expressed interest in informally scouting for the problem in advance of harvest. This is an excellent idea. The problem is, it is very difficult to assess standing corn crops for aflatoxin contamination. Some of the reasons for this include:
• The mold associated with aflatoxin may not always be visible in contaminated grain.
• The presence of the mold does not mean aflatoxin is present.
• There are several green molds on corn, and they can be hard to tell apart.
• Even when it occurs, aflatoxin is contamination is very sporadic for lots of reasons, so a grower may have very high levels in one field and low levels in another.
Having now lowered expectations, I do think scouting is a good idea, especially this year. It might help producers manage their risks a little more wisely.
The scouting approach that makes the most sense to me is to visually inspect the ears for mold typical of Aspergillus ear rot. Scouting can be done any time after black layer. Pay special attention to the driest parts of the field. Usually the disease appears as olive-green mold (Figures 1-4). The moldy growth appears powdery, the result of the massive numbers of spores produced by the fungus. The spores are dispersed easily in the air, and they may appear as fine dust when the husk is pulled back. The mold is commonly found at the tip of the ear, but as you can see from the photos, it may be found anywhere on the ear, especially where physical injury occurs to the kernels.
Other ear molds of corn are caused by fungi that produce various powdery molds, including green molds. Trichoderma ear rot is more greenish than the olive-green typical of Aspergillus (Figure 5). Penicillium kernel rot can be quite common on injured kernels (Figure 6), but that mold tends towards a blue-green color than olive-green.
What should you do if you suspect a case of Aspergillus ear rot? We can confirm the causal fungus in the UK diagnostic labs, which may be useful in deciding whether to test for aflatoxins. But please bag the sample and inform the lab that it is a suspect case of Aspergillus ear rot, so that the staff can protect themselves by handling the sample in a biosafety cabinet.
UK doesn’t offer a routine testing service for aflatoxin, but there are numerous laboratories that are equipped to analyze corn samples. A list of laboratories is available at http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-MISC-1.pdf. This is an incomplete list; other competent labs exist, but this will serve as a handy reference of some of the available laboratories.
This is a really important point: If you properly collect 10 samples from a moving stream of grain (see ID-59), aflatoxin quantity will vary quite a bit among those 10 samples. That is the nature of sampling for a chemical present at parts-per-billion levels. This variability will be even worse if you are submitted ears collected while scouting. Look at Table 1 in ID-59 (http://www.ca.uky.edu/agc/pubs/id/id59/id59.pdf) to understand just how variable sampling for aflatoxin can be, even when done according to best practices.
As far as to do with the information from scouting, see ID-59 for helpful ideas.
Fig. 2 Aspergillus ear rot, Alison Robertson, Iowa State University,
http://www.ipm.iastate.edu/ipm/icm/2005/9-19/aflatoxin.html
Fig. 3 Aspergillus ear rot, from Iowa State University,
http://www.ipm.iastate.edu/ipm/icm/node/226
Fig. 4 Aspergillus ear rot, Purdue University, http://www.extension.purdue.edu/extmedia/BP/BP-83-W.pdf
Fig. 5 Trichoderma ear rot.
Fig. 6 Penicillium ear rot, photo University of Illinois,
http://bulletin.ipm.illinois.edu/article.php?id=1244