Corn made up 65 of the 168 total field crop plant samples submitted to the MU Plant Clinic in 2016, only second to soybean. The incidence and severity of diseases and disorders followed the wild swings in weather pattern. With the mild winter and early spring heat, many farmers started planting early. It was a great start, until the cool May weather slowed plant vigor and promoted disease development. Samples of seedlings and young plants were submitted with symptoms of damping-off and/or early season root rots. Typical pathogens involved in these diseases include species of Pythium, Rhizoctonia and Fusarium. Symptoms include stunting, foliar tip necrosis or nutrient deficiency type symptoms (Picture 1).
Damping off causes a rot at the soil line. While root rots will cause symptomatic roots to include dark lesions, root pruning and rot of the cortex tissues (Picture 2). For diagnosis, washed roots are observed under the microscope to examine for pathogen structures used to determine the causal agent(s) (Picture 3). Early season root rots do not always kill plants, but can impact plant health throughout the season and ultimately affect yield. Additionally, these soilborne pathogens can move from the roots into the stem and cause stalk rots. Correlated with the abundance of early season root rots, a fair number of stalk roots were diagnosed later in the season.
From May we had a hot, dry June. For many the thoughts of drought loomed large, as drought-type stress symptoms were common. During this time, symptoms started showing up in fields that had been fallow in 2015. Plants were stunted and expressed symptoms of nutrient deficiency. Fallow corn syndrome is a disorder that causes low vigor plants due to poor soil microbial communities. More information can be found on this issue in the June 7, 2016 cropPest newsletter (Stunted Corn Following Prevented Planting-Fallow Syndrome).
After a scorching June, the skies opened up over a lot of Missouri. High amounts of rainfall were reported over the 4th of July holiday. For some regions the rain continued off and on into August. High humidity caused an abundance of foliar diseases to start popping in July. Common, but not always problematic, foliar diseases such as anthracnose, Diplodia leaf streak, gray leaf spot and rust were diagnosed repeatedly. Both common and southern rust were prevalent around the state.
A new, to the USA, foliar disease of corn was announced in August of 2016. Bacterial leaf streak caused by Xanthomonas vasicola pv. vasculorum was confirmed by the USDA-APHIS-PPQ in July 2016 from corn samples originating in Nebraska. Following confirmation the USDA surveyed the Corn Belt and confirmed it in 9 other states. USDA inspectors along with MU Extension Agronomists inspected over 200 fields in Missouri and the disease was not found. However, it is probable that the disease could be found in 2017. The bacterial pathogen survives in crop debris and is most problematic in irrigated, corn on corn fields. Keep an eye out for this disease, the main symptom to look for are interveinal streaks on the leaves. Streaks (lesions) can be yellow, orange or brown in color; these can be very long and coalesce. Lesions can look similar to gray leaf spot, however, they tend to have wavy margins (Picture 4). Foliar fungicides will not prevent this disease. There is no evidence that the bacteria moves systemically or carries over in seed. Diagnosis of this disease can be made at the Plant Diagnostic Clinic. A sample should include several (2-5) symptomatic leaves, wrapped in newspaper and mailed in a crushproof box.
Ear rots were also popping by the end of the season. In the clinic we saw quite a bit of Diplodia, Penicillium, Trichoderma and other types of Fungi on symptomatic ears. Another attester to the wet weather, were examples of seeds sprouting in the husks (Picture 5).
Besides diseases and environmental related disorders, there were also numerous insect issues. Japanese beetles were problematic (Picture 6) and there were numerous incidences of spider mites, thrips or rootworm damage.
A list of confirmed diseases diagnosed from corn samples:
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REVISED: February 21, 2017