Crazy top of Corn
Excessive rain in much of the USA Corn Belt in 2019 with ponds of water in areas of fields can encourage infection by a fungus that has swimming spores. Corn Journal blog of 05/30/17 may be appropriate for this year as well.
One of the effects can be infection by an organism called Scleropthora macrospora. This is a fungus-like organism belonging to a group of organisms called Oomycetes. Also in this group are pathogens causing Downy Mildew and Pythium diseases of corn and other plants. Common among these are the ability to form thick walled spores to withstand stress environments that can release swimming spores when in water-saturated soil. S. macrospora infects more than 140 grass species in addition to corn.
The source of infection of corn is often grasses near a low spot or edge of a field. Oospores in the flooded living and dead leaves release swimming spores (zoospores) when close to the corn submerged leaf tissue these zoospores release a germ tube that infects the plant. The filaments (hyphae) grow towards the meristems throughout the life of the plant. This can initially be seen as fine stripes in the leaves but the most obvious symptom is proliferation of leafy aberrations of the tassel- the crazy top symptom. Scleropthora macrospora also can grow to the ear bud meristem, causing similar multiple ears from a single node- but no grain.
Related oomycetes occurring in warmer, subtropical and tropical environments can cause similar symptoms. These downy mildew diseases can also cause the proliferation of the tassels and ears. Susceptible genotypes can have severe grain loss from these diseases. Scleropthora macrospora infection is usually limited to a very small area near grass in a low part of the field.
Infection occurs when the plants have less than 6 leaves. Symptoms that show late in the season, but the problem began with excessive rain that occurred only a few weeks after planting. That early moisture that may contribute to large yields can allow this pathogen to form these unusual corn structures in a few spots of the field. In addition, it is just part of the interesting biology of corn.
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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.