Fungal survival is dependent upon warding off competitors from potential organic sources of nutrition through the production of antibiotics and by moving to unoccupied new sources. Most of that movement is via spores. Each species has some distinct method. Pathogens gain advantage over those species that only can feed on dead material by attacking the living, dominating the newly killed area and then moving on. This certainly is the tactic of the leaf spot-causing fungi on corn as they usually infect a small area, kill the tissue and, before saprophytic fungi can invade and compete, they move on by producing spores to spread to new leaf material.
The fungi causing most leaf spots in corn belong to a group (phylum) called Ascomycetes. Among their commonality is the structure of the sexual stage in which haploid nuclei fuse and then undergo meiosis. These fungi live most of their life with only one set of chromosomes and thus as haploids. Widespread reproduction of these fungi comes through asexual production of spores, called conidia, dispersed often by wind. In fact, many fungi are only known by their unique conidia, the sexual stage not identified. This leads to confusion about the names of fungi as well because generally the name based upon the sexual reproduction structure according to fungal taxonomy rules should take precedent.
Most fungal leaf pathogens of corn produce huge numbers of conidia after the leaf tissue is wet for a very short time. It is not unusual for several hundred conidia to be produced from a single lesion of northern leaf blight. Within a few hours those that land in a moist area of leaf are able to germinate, produce specialized structures to enter the leaf tissue and within a few days begin feeding on fresh tissue. A few weeks later it produces more conidia and moves on. The fungus also remains in the dead leaf tissue after harvest and if not destroyed by competitors, such as when buried in soil, is ready to produce more conidia when temperatures and moisture are right the next spring.
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.