Fungi and corn

​One of the intrigues of our grandchildren has been to put their fingerprints on the surface of the sterile culture agar in petri dishes in our lab.  After a few days, something always is growing on the surface of the agar. Even after washing their hands, there is growth. If they leave the petri dish lid off briefly, something contaminates and grows.  We are generally unaware of the prevalence of fungal spores around and on us.  We, like most other living organisms, successfully limit any potential harm from the fungi. 
 
The majority of fungal species surrounding a corn plant feed on dead tissues. Most grow within organic tissue by thin strands of filaments called hyphae.  A single hypha is visible to us only by a microscope but a mass of hyphae, also called mycelium, is more easily seen growing from dead kernels.  Hyphae of most fungi are composed of a string of single cells, each with its own nucleus.  Growth is mostly accomplished by cellular multiplication.  The single nucleus of a cell, which usually is haploid, divides by replicating the genetics, mitochondria also replicates, cell wall develops separating the new cell from the originating.  These cell walls, called septa, separating the individual cells have holes allowing the movement of ribosomes, mitochondria and, in some species, even nuclei to move between the cells. These pores in the septa allow transport of nutrients absorbed by the cells at tips of the hyphae to older cells within the strands.
 
Most of the nutrition of a fungus is dependent on hyphae growth within the dead or live tissue.  Survival of the species, however, depends upon spreading to new nutrition sources as the current source either is destroyed or, in the case of a pathogenic fungus, the host resistance limits the growth.  Spread to new nutrition sources is usually done through production of spores.  Fungi such as Exserohilum turcicum, cause of northern leaf blight of corn, kills corn leaf cells mostly be plugging vascular tissue within a few inches of leaf tissue, and receives nutrition from the dead leaf tissue.  With right temperature and moisture, the hyphae are stimulated to produce aerial structures called conidiophores terminated with spores (conidia) which are excised and carried by wind.  This fungus in a single northern leaf blight lesion can produce more than 100,000 conidia within a few days.  Survival of the fungus is dependent upon this large production because distribution is mostly random, although many will land on another corn plant within the corn field.  Even with this large production from a single lesion within a corn field, successful infection is infrequent.  We at PSR, Inc. artificially inoculate with this pathogen by placing about 1000 spores in the whorl of growing corn plants.  The more susceptible hybrids usually show 5-8 lesions developing from the inoculated tissue and more resistant hosts may show only 1 or 2 lesions.  Although most of the conidia germinate in this moist environment, successfully penetrating the leaf and overcoming the resistance systems of the plant limits the fungus.  Most fungi are dependent upon production of large numbers of conidia to overcome the environment limits and host plant resistance for continuation of the species.