Affect of Early Rust Infection (posted October 2008)
Frequent storms into the Midwest USA in June 2008 brought spores of Puccinia sorghi, cause of common rust, from South Texas and Mexico into the whorls of corn plants. Our disease nursery in Northern Illinois was especially affected at about the same time that we normally apply other pathogens to the plants. PSR, Inc. uses this method to evaluate the resistance to common diseases for seed companies. Normally, infection occurs within 48 hours after spraying the spores into the whorls and symptoms of the disease show in 1-2 weeks, with some variability among pathogens.

This summer, as in another heavy rust year of 1993, the symptoms of the diseases did not significantly show until late September. How could the rust fungus affect the other diseases?

Plant disease resistance is generally a three-part system. First, the plant cells must recognize the presence of the invader. This usually involves recognizing a single protein in the pathogen and therefore is simply inherited.

Second, the plant communicates the need to turn on (elicit) the resistance system. The elicitor is usually not transmitted great distances in the plant. Interestingly, this system often involves salicylic acid (the major component of aspirin). Involves more genetics.

Third, a general anti-microbial resistance system is produced. Actual components of this resistance are not completely defined and may include several compounds. Genetics may be complex.

Evolution has presumably favored this system because energy is not wasted when no pathogen has invaded. It also is reasonable to assume that vigorous growing plants are quicker to respond to a pathogen than plants with cells that are senescing, consistent with observations of many maize diseases.

Rust fungi differ from most other corn leaf pathogens in that the fungus can only grow in living plant cells. It is favored when the invaded cells do not die. The observation of this season, and of the 1993 season, is consistent with the hypothesis that infection by the rust fungi results in the plant producing general resistance compounds. This may not stop other pathogens from invading the plant but does inhibit the growth of the pathogens in the plant. Eventually the resistance compounds are diluted, or decompose, allowing the other pathogens to progress to producing symptoms. Among the observations from the 2008 plots was the eventual symptoms were on the same leaves that were initially infected with the other pathogens, as if initial infection occurred by the fungus but couldn't progress for several weeks.

Making a hypothesis is usually the easy part of science. It will be up to a vigorous, young scientist to refine and test it.

2008 Stalk Rot in Summary Form (posted September 2008)
Acknowledging the concerns of those text messengers, I offer the following:

Extra high kernel numbers, late season stresses of factors influencing photosynthesis (cloudy weather, extra high plant densities, leaf diseases, drought, hail) cause a shortage of sugars for the corn roots. This leads to root senescence and invasion by soil organisms. As the root decay, and water transpires through the leaves, individual corn plants reach a point of wilt in which the ear droops and leaves 'suddenly' turn gray. Stalk pith tissue pulls away from the rind, and the rind turns yellow and then brown. It is now officially called stalk rot, usually with a preface naming the most easily identified fungus (Diplodia, Gibberella, Fusarium, Anthracnose).

This has to be the most compressed description of stalk rot I have ever written! Including the details with the evidence to back up the claims in this brief summary is a lot more fun for me, and perhaps some of you. For more complete, referenced report click here.

Corn Maturity (posted July 2008)
Late spring and early summer of 2008 featured lots of rain and cooler than normal temperatures. Not only did this result in delayed planting but also will cause a late pollination and harvest.

Corn originated in Central America, a semitropical environment that had year around sunlight from about 11-13 hours per day. As the new crop was dispersed into more temperate hemispheres it would pollinate too late for the corn to mature. Fortunately, over a few thousand years, there were humans adept enough to select for the few variants that responded to heat instead of length of day in determining flowering time in corn. Because of the genetic variability, and the ingenuity of our species, corn was adapted to northern Canada by the time the Europeans arrived in North America.

Adaptation to the longer day summers of the temperate zones required a different mechanism for determining maturity. The change was possible because the major factor influencing time to flowering and pollination became accumulated heat, independent of the length of day.

At some time in the growth of a young corn plant, the apical meristem (growing point) cells quit producing more leaves and switches to producing tassel cells. Although this meristem is only a few inches above the soil surface, the change is visible with a microscope. Many years ago, I compared the heat unit accumulation to that point of apical meristem differentiation with all the hybrids of a major U.S. company. All plants were from a single location in northern Illinois with the same planting date. There was a perfect correlation between the heat units to tassel differentiation in the meristem and the published maturity of the corn hybrids. In other words, the main differences in maturity were determined by the heat units needed to cause the growing point to switch from producing leaves to tassels.

The time between differentiation and pollen shedding is mostly independent of heat; it is mostly a time factor. The next phase, the time from pollination to formation of black layer is about 55 days for all hybrids. T

his means that the biggest variable in determining the time from planting to black layer is the heat accumulation during the first 30-50 days of the season. Cool late may and early June delayed pollination to the latter part of July and early August for much of the Corn Belt. Black layer will not be here until Sept. 15-30. That determines safety from frost. Other genetic factors such as husk thickness, numbers and opening, grain features, cob thickness and environmental factors such as humidity, wind, and heat affect the actual acceptable time to harvest. Except for these drying factors, the corn maturity timing for 2008 is set.

Fusarium & Corn (posted June 2008)
The fungus genus Fusarium has a confusing relationship with corn. Not only are there several species, they are found in practically all parts of a corn plant. Sometimes the fungus produces a toxin (Fumonisin) that is potentially dangerous to animals including humans. The fungus is so prevalent in corn plants that it is easily isolated from nearly all plants showing disease symptoms or no disease. It can be found in healthy, and unhealthy, germinating seeds, seedlings, leaves and stalks. In fact, it becomes easy for a plant pathologist to label the causal agent of a disease as Fusarium, especially when no other pathogen is obvious. Stalk rot is an example of a corn condition that is mostly physiological (not enough photosynthate to support both the grain filling and the root’s viability). If no Diplodia, Colletotrichum, or Gibberella species is obvious, but a Fusarium species is isolated, it is called Fusarium Stalk Rot. If seedlings are experiencing poor growth in cool wet soils, or herbicide damage, then it is easy to find the Fusarium fungus and the disease becomes Fusarium Seedling disease. In both cases, it would be more helpful to the grower if one would look for the environmental affect rather than imply that the plant fell victim to an aggressive pathogen.

This is not a new conundrum. When I was a student at Iowa State in 1960 , I was a technician in the lab of Dr. Dean Foley. Little did I know that he was showing that Fusarium species were essentially residents of a corn plant and not the aggressive pathogens that others had implied. I understand that publication of his paper (Foley, D.C. 1962. Systemic infection of corn by Fusarium moniliforme. Phytopathology 52:870-872.) caused considerable consternation among pathologists. Perhaps I picked up some bad habit from Dr. Foley, because there was some similar reaction to my stalk rot theories in the early 80’s. Several have studied Fusarium and corn in the last 45 years and seem to go through a pattern of first assuming the fungus must be an aggressive pathogen and then finally accepting that it is some sort of inhabitant of corn that seems most aggressive on dying and dead tissue.

Although mostly innocent of damage in most of the plant, Fusarium in the ear is significant, because of its affect on grain quality and the production of toxin. It is of interest, therefore, as to the origin of the ear rot Fusarium. Did it come from infected seed? Munkvold and co-authors used more modern techniques to trace the Fusarium fungus from the seed to see if it was the source of infection of other parts of the plant. It was found in the seedlings but rarely in the stalk and ear. Consistent with most studies, the main source of ear rot infection was through the silks. Most ear rots occur when silks are exposed to a barrage of fungal spores (Fusarium, Gibberella, Diplodia, Aspergillus, Penicillium species) before pollination. This is most frequent when silks emerge but no pollen is present such as silking on rainy days or severe drought in which silks emerge after most pollen is gone. Probably most infections by Fusarium species comes from corn and other debris in the field.

When corn is diagnosed with Fusarium ear rot, stalk rot or seedling rot, it is best to look for some environmental cause rather than assume the misfortunate presence of an aggressive pathogen.

A recent reference on this topic is: Wilke, A.L., C.R. Bronson, A. Tomas and G.P. Munkvold.2007. Seed Transmission of Fusarium verticilloides in Maize Plants Grown Under Three Different Temperaure Regimes. Plant Dis. 91:1109-1115.

ADDED CONFUSION
Specialists studying Fusarium species have realized that the fungus known as Fusarium moniliforme is now named Fusarium verticilloides. Probably many earlier references to Fusarium moniliforme were actually one of two other species, Fusarium subglutinans and Fusarium proliferatum. Fungi like these are named according to the asexually produced spores, called conidia. Sexual reproduction for these kinds of fungi results in completely different structures, historically leading scientist to believe that they were distinct species. As it becomes clear that an asexual species is the same as one named based upon the sexual structures, the sexual-based name takes precedence. The sexual stage of Fusarium species belongs to the genus Gibberella. Consequently, Fusarium verticilloides, Fusarium subglutinans and Fusarium proliferatum actually have a very similar sexual stage named Gibberella fujikuroi.

Another Gibberella species, Gibberella zea has the asexual stage called Fusarium graminearum. This fungus is associated with Gibberella ear rot and stalk rot of corn and head scab of wheat.

Life is not simple, especially in the world of fungi and crops.

Energy Relationships in Corn (posted May 2008)
I became interested in the energy relationships in corn when studying stalk rot of corn. Most of my thoughts then and now were built upon the research concerning photosynthesis in corn and the translocation of its products in corn. Current interests in biofuels brings the concepts related to corn’s capacity to convert light energy to stored energy to the front.

Stalk rot of corn is the result of predisposition of roots to destruction by soil microorganisms and eventual invasion of wilted stalk tissue by fungi (most notably species of the genera Fusarium, Gibberella, Diplodia and Colletotrichum). Predisposition was caused by the lack of carbohydrate in the root tissue to maintain metabolic defense to the multiples of organisms ready to invade root tissue. Plants with predisposition did not produce enough carbohydrate to meet the storage commitments in the grain and the metabolic needs of the roots and the rests of the plant.

As this interaction became apparent in the mid 70’s, it was necessary to contemplate how to achieve both higher grain yields and acceptable stalk quality. During that time frame, there were studies done showing that corn varieties varied for the photosynthetic rate per square centimeter of leaf space, leaf area per plant and architectural differences affecting number of leaves exposed to maximum light. My conclusion from some private studies was that corn breeders were accomplishing gains in total photosynthesis by simply testing many hybrids for both standability and yield, and not worrying about how they got the photosynthetic gain. The other conclusion that I reached was that there remained plenty of variability for each of the factors (photosynthetic rate, leaf area and architecture) and that gains should continue. In fact, the gains have continued probably mostly through increased leaf area per acre as plant density increases for the past 25 years have been dramatic.

It is easy to imagine that other physiological and structural differences between corn genotypes also affect the plants ability to capture and store energy. Efficiencies of moving carbohydrates from the leaves to grain and plant structures must be complex; size of vascular bundles, hormone concentrations, number of ovules in ears come to mind. How about the energy requirements for movement of molecules across membranes, tissue development, whole plant growth, avoidance of senescence?

What we do know is that corn is wonderfully diverse. Even among genotypes that are currently adapted to a geographic area, every corn breeder is amazed with the diversity that is visible in the breeding nursery. And that is only what we see!

The future of food and fuel use of corn will be met by identifying the genotypes with best ability to capture the most energy per acre in a form that we can efficiently convert to our use. Biotech will help but a lot will come from effort to identifying those varieties with the combinations of complex factors (some unknown to us) that give us more food and fuel. For the most part I think the method of measurement is called yield testing.