Bacteria are single cell forms that continually surround corn above and below the soil. Defense systems of corn, and all other forms of life, generally is effective in keeping potential pathogens from invading cells, while tolerating the saprophytic nature of most bacteria species. Bacteria are vulnerable to desiccation and generally most successful in moist environments, including those inside corn plants.
Corn plant defense systems against bacterial infection includes the tight epidermal layer of cells with a waxy covering that inhibits invasion in leaves. Vulnerability to entrance through stomata is limited by the anti-microbial fumes emitted through the stomata. If this outer defense is avoided, some pathogenic bacteria thrive on the moist internal leaf environment until other host resistance systems stop its spread.
Goss wilt bacteria (Clavibacter michiganensissp. nebraskensis) was first recognized causing a corn disease in 1969. The disease was strongly linked to physical damage to leaves by hail. Breaking the leaf epidermis allowed the bacterial to enter and thrive in susceptible corn genotypes. Spread elsewhere in the USA corn belt has implied that less obvious damage, perhaps from wind in rain storms provide sufficient injury for the bacterial to enter plants. Structural damage to leaves and moisture are essential to successful invasion by this bacterium.
Stewart’s wilt bacteria (Pantoea stewartii) cause corn disease by avoiding desiccation by surviving in an insect vector, primarily the corn flea beetle (Chaetocnema pulicaria). This insect feeds on corn and other grass leaves, penetrating the epidermis while inserting the bacteria into the leaf. This pathogen multiplies and spreads especially through the vascular system in susceptible genotypes.
Bacterial leaf streak is caused by bacterium (Xanthomonas vasicola pv. vasculorum) that appears to invade through stomata. Appearance of linear lesions mostly limited on sides by vascular bundle cells implies that these bacterial mostly digest the mesophyll cells in susceptible genotypes. There are some indications that it is associated with warm, rainy weather, perhaps allowing the bacteria to increase in the moisture of the corn whorl and eventually penetrating through the stomata.
A few other Xanthomonas and Pseudomonas species have been associated with contaminated irrigation water from ponds with infections causing leaf blights. Bacterial stalk rot likewise is associated with heavy concentration of a bacterium (Erwinia carotovora) in flooded soils, allowing penetration in to the lower shoot area of the plants.
Although most corn inbreds and hybrids have good resistance to most of these bacteria, inconsistent weather patterns, genetic changes in potential pathogens, factors influencing vectors and unexpected susceptibility in new corn genetics have and will continue to allow emergence of bacterial diseases.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.