Genetic challenges

​Constant interactions with the corn plant biology, affected by its genetics, and pathogen biology, affected by its genetics, requires us to carefully observe potential changes.  Each participant in these interactions has potential for reducing our final product from the corn crop.
 
Bacterial pathogenesis of corn differs from fungal pathogenesis mostly because of biological differences in the two types of organisms.  Bacteria are single-celled organisms with a single chromosome without a nucleus in its cytoplasm.  Cell division in bacteria results in a new, distinct individual cell, whereas fungal cells divide to form a filament, frequently maintaining cell to cell communication and specialization.  Bacteria cell single chromosome simply replicates, then separates as a cell wall divides the cell into two halves, each with its own single chromosome.  Fungal chromosomes are enclosed in a nuclear membrane, divide by mitosis during cell division but also can undergo meiosis forming single strands of the chromosome and thus exist as haploids until the nuclei fuse to form diploids.  These structural and functional differences between bacteria and fungi results in different ‘strategies’ concerning adaptations of these organisms to changing environments.  Mutations occur in both, but those in the single bacterium chromosome immediately results in a changed new individual bacterium.  Mutation in fungal DNA may not take effect until recombination to form the homozygous recessive.  Bacterial biology allowing rapid reproduction of the new forms allows adjustment to new hosts.
 
Bacteria are ubiquitous, distributed by wind, water, insects and animals.  Rapid cell reproduction allows quick spread when in a nutritional environment.  Despite their widespread presence, most species cannot infect living corn leaves.  Leaves have evolved epidermal cells tightly held together and with a waxy covering, repelling water and prohibiting bacterial infection. Pathogenic fungi establish a multicellular mat (appresorium) from which it produces a penetration peg that enzymatically drills through the wax and epidermal walls to enter the leaf.  Bacteria cells can enter through open stomata-some even have flagella to help swim to the stomata. Plant resistance systems can produce antibacterial fumigants to inhibit the bacteria from replicating and there is some evidence stomata close when faced with bacteria invasion.
 
Small size and few visible structural features has made it difficult to identify a bacterium causing a plant disease.  Although some disease symptoms do become associated with specific species, confirmation usually requires culturing and chemical tests. 
 
It often takes a simple mutation in a potential pathogen, or in the corn host, for a relatively minor disease to become major.  Good for everyone in corn production to be aware as the crop grows.