Mild winter, Pathogen Survival, and Early Disease Development

When the winter is as mild as it was this past year, pathogens that would have otherwise been killed or at least suppressed by the cold temperatures end up surviving and getting an early start to infect our crops. With the exception of the rust fungi, most of the pathogens that cause leaf spots and blights and ear rots and molds have developed strategies to survive our usually harsh winters. But even so, cold, dry winter conditions do contribute to reducing pathogen survival is a “normal” year. However, when temperatures during the winter months are not consistently low, even pathogens like the rust fungi that do not usually survive well here in the Midwest are able to overwinter on volunteer plants and in cultivated fields.

This was certainly the case in wheat this year. Very late in the fall and during the first few weeks of the winter, pustules of leaf rust and powdery mildew were seen in several fields across the state. This is not uncommon, but the fact that these diseases are already being reported at fairly high levels now that the winter is “over” suggests that those fall infections carried over into the spring. Although Septoria and Stagonospora leaf blotches have not yet been reported, it is highly likely that the fungi causing these diseases also survived the winter in higher-than-usual numbers and will likely infect the crop as conditions begin to warm up, especially is it stays wet over the next few weeks. For powdery mildew, a disease that thrives in dense wheat stands with high nitrogen, severity will likely increase after the wheat is top-dressed.

Rust, powdery mildew, Septoria and Stagonospora are all polycyclic diseases, meaning that once they get an early start, several batched of spores will be produced between now and grain-fill, if not managed and conditions are favorable. Spores produced early in the season will spread up the plant and to neighboring plants, causing new lesions or pustules to develop, and these new lesions will themselves produce new spores within 7-14 days that will again infect new leaves and plants, causing the diseased to spread rapidly, particularly if the variety is susceptible. The more cycles we have, the more disease we will see and the greater the grain yield and quality reduction. Keep monitoring the progress of these diseases and be prepared to apply a fungicide. We typically do not recommend fungicide application at green-up for foliar disease control, since we rarely see high levels of disease this early, however, this may be the year for a green-up application if it stays cool and your variety is susceptible. Remember to always use label-recommend rates; half-rates do not provide adequate protection and may increase the risk of fungicide resistance. Also, early applications will not protect the flag leaves and spikes against infections later in the season.

Here is a word for our corn producers as well. As you get ready to plant corn, make sure you select hybrids with resistance to northern corn leaf blight (NCLB) and gray leaf spot (GLS). Just like the wheat pathogens, the NCLB and GLS fungi likely survived the mild winter and will be producing lots of spore by the time the crop is planted. If the hybrid is susceptible and the weather is as wet and humid as it was last year, we will again see NCLB and GLS (as well as other diseases such as eyespot caused by residue-borne pathogens) developing well before pollination. Again, if the hybrid is susceptible and the weather is favorable, early disease = more disease = greater yield reduction, if not managed. NCLB and GLS are also polycyclic. Start by selecting resistant hybrids! Click on the link below for more on NCLB resistance: https://agcrops.osu.edu/newsletter/corn-newsletter/select-hybrids-resist...

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About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.