Midwest corn belt planting delays will affect more than only normal yield dynamics but also corn diseases. Corn escapes damage from viruses such as maize dwarf mosaic because the damage occurs when the virus reaches the growing point before the v6 stage of development. Spread of the virus via aphids usually is limited close to the overwintering source of this virus as the aphids tend to spread further north as the temperatures become warmer. Although the June temperatures in the Midwest in 2019 have been cooler than normal, it seems reasonable to assume the aphids can feed on corn at very early development stages further north than usual. Frequent rains could mitigate this aphid survival affect, however. Other virus vectors likewise may have unusual distributions onto corn young plants.
Fungal leaf diseases, such as northern leaf blight, are usually favored by wet conditions. Sporulation on corn debris from the previous year is encouraged by the frequent rain, water in leaf whorls and high humidity allows quick invasion of leaves in the young plants. A few weeks later these lesions become sources of new spores that are easily distributed by winds to new fields. The dynamics of having later planted fields nearby could produce unusual northern leaf blight pressure, especially on those fields.
Gray leaf spot fungus also produces spores on infected leaves from the previous season, but infection of new crop is more favored by humidity than rain. Will excessive rain in June result in higher humidity than normal for the summer of 2019?
Common Rust and Southern Rust spores are carried in storms originating in the Southwest and Caribbean Islands. Corn is most vulnerable before flowering, as the leaf whorl provides an inoculation chamber for the spores to germinate and infect the leaves. The mix of planting dates within an area increases the potential for damage to the later plantings.
This unusual planting season will provide new disease pressures on corn that will require special attention this summer.
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.