Spiroplasmas and phytoplasmas are single celled organisms grouped as Mollicutes and classified as bacteria but lack cell walls and much of the cytoplasm common to other bacteria cells. Most infect insects in which they multiply by using their own DNA to cause the host to produce the replication of the organism. The spiroplasma species, Spiroplasma kunkelii, causes the disease corn stunt, found in semi-tropical and tropical areas.
This species infects a few leaf hopper species (Dalbulus maidis, Dalbulus elimatus and Graminella nigrifrons), existing all year in the warmer climates. The spiroplasma replicates in the gut and saliva of these insects and consequently is injected into the corn plant upon feeding by the insects. From there the organism becomes concentrated on the phloem tissue, perhaps aided its own mobility. Ultimately, they must move to the growing point affecting cell elongation and thus stunting of the plant. The net effect, especially if the plant is infected while young, is yellowing of leaf tissue, stunting of the plant and eventual reddening of the leaf tissue perhaps as a result of anthocyanin accumulation because of no translocation to grain. This also can result in multi-branching in affected plants.
There are indications that occurrence of corn stunt in temperate zones is mostly related to the ability of the insects to withstand winter weather and thus related to winter temperatures as reported in an Argentine study (Journal of Economic Entomology, Volume 106, Issue 4, 1 August 2013, Pages 1574–1581). It is likely that the vectors are moved by winds from warmer areas during summers but infection into corn plants by midsummer does not allow sufficient movement to the growing point to cause meaningful symptom development. Late planted sweet corn has some vulnerability to this disease if conditions are right.
Another Mullicute causing a corn disease is a phytoplasma. This disease is called maize bushy stunt. It is found in Southern USA, Mexico, Central and South America. Although the organism differs from the cause of corn stunt, it is vectored by the same insects and causes similar symptoms. Both organisms may even be found in the same stunted plants.
Extreme small size of the organisms, complicated relationship with vectors, overlapping symptoms and interactions with host plant biology has resulted in limited information about nature of resistance. They deserve increasing attention in temperate zones if warmer winters allow expanding distribution of the vectors.
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.