Among the functions of leaf cuticle is providing protection from potential invasion by microbes. This waxy layer does ward off many microbes as the rain washes them off the leaf surface. However, successful pathogens develop mechanisms to adhere to the hydrophobic cuticle until the germinating spore produces hyphae to successfully invade the leaf. Many pathogenic species such as Exserohilum turcicum, cause of Northern Leave Blight of corn, has a thin gelatinous outer layer that sticks to the leaf cuticle long enough for the germinating spore to set up ‘drilling station’ called an appresorium to enzymatically forced entrance with a penetration peg into a leaf epidermal cell.
Another approach is to develop an adhesive pad that includes enzymes that partially dissolve the cuticle wax. Spores (uredospores) of corn rust the fungi, Puccinia sorghi and Puccinia polysora, produce such a pad when moistened on a corn leaf. After securing itself the cuticle, it germinates to grow thin hyphae across the leaf surface until it finds a stomatal pore, apparently by touch, to enter the leaf between cells. After reaching a living mesophyll cell, it produces an appresorium to attach to the cell and then a peg into the cell wall. It does not penetrate the cell membrane but instead produces a membrane adjacent to the host cell membrane called an haustorium. This allows this obligate parasite to keep the host cell alive as it provides nutrients to the rust fungus. Eventually the rust fungus produces more uredospores to spread the disease. These spores are very easily picked up in winds for long distance distribution.
The gray leaf spot fungus Cercospora zeae-maydis has a slightly different tactic to overcoming the potential defense mechanism of the leaf cuticle. Whereas most pathogens must enter to leaf tissue within a few hours or the spores will dehydrate after exposure to the dry air, C. zeae-maydis progresses slowly on the leaf surface as it requires nearly 100 hours of 90-100% humidity to slowly grow on the leaf surface until it finds a stomatal opening from which it enters the mesophyll.
Leaf cuticle layers offer a system to prevent many fungi from entering living plant tissue. Relatively few of the many fungal species exposed to the plant have mechanisms to overcome this first line of defense. After entry into the plant, other corn plant methods must be employed to limit the disease.
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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.