Dave Pike, Ph.D.
The tropical storm that passed up through Alabama to Michigan on the 29th and 30th of May could have had a message for the crops along its path. Dense and fast moving storms like Alberto have winds capable of dislodging insects and disease spores from host crops or plants in the Gulf Coast states and then depositing them in the heavy rains dropped on corn and soybean fields as it moved north. There are a number of diseases and insects that overwinter along the Gulf Coast and, if moved northward early in the season, can result in serious epidemics or infestations throughout the Midwest. The two diseases we are most concerned about at this time of the year are Asian soybean rust and Southern corn rust. With this most recent storm there is both bad news and good news about the possibilities for these diseases to become Midwestern problems.
Part of the bad news is that Tropical Storm Alberto occurred early in the season. An early season storm sweeping up the Eastern side of the Corn Belt is likely to have deposited disease spores on young susceptible crops, providing plenty of time for infections to become well established by the time the crop is at sensitive reproductive stages. Even small and localized infections at early vegetative stages could be a bellwether of more serious infections to come. Another caution associated with this storm is the speed at which it moved. A fast moving storm like Alberto has a likelihood that it moved disease spores great distances. More typically winds will move rust spores a few feet to a mile at most. But there is no way to predict how far rust spores may have been moved by Alberto. Perhaps 30 miles, maybe 300 miles.
If there is good news it would be that there hasn't been a great deal of disease across the Gulf Coast states to provide a large spore reservoir. These states experienced a rather cool winter in early 2018 which eliminated most of the spores and many of the plants that are disease hosts. However, the few spores that did survive to create infections on host plants might be enough to be a concern to growers along the storm's track. Actually more good news could come to us by way of drier weather. Rust spores that survived their trip up from the south must have damp conditions over the next couple weeks to germinate and establish infections. On the other hand, regular rains, heavy dews, and high humidity may create favorable conditions for both these migratory diseases that could have come from the south, as well as many of the common diseases that are capable of overwintering in the Midwest and that we see regularly.
Going back over the last 20 tropical seasons to the year 1998, we have seen named tropical systems track from the Gulf of Mexico through the Corn Belt 12 times. The most active season saw four different named systems track through the Corn Belt in 2005. Prior to Alberto, the last time a named storm had made this journey was in 2015, when the remnants of Tropical Storm Bill tracked from Texas through the Ohio River Valley.
Alberto was a rarity in that it made this trip so early in the season. The overwhelming majority of storms that decay across the Midwest occur during the months of August and September. There is a sweet spot during the late summer when wind shear is weaker across the Central US which allows these systems to migrate northward while they decay. During the spring and fall months a stronger jet stream often pushes Gulf of Mexico storms east toward the Atlantic Coast.
Storms like Alberto will be a threat each year, but they'll remain a relatively low threat. It took that rare combination of a very early season storm in Alberto during a spring that featured an unseasonably strong ridge of high pressure that forced stronger jet stream winds northward into Canada.
While we are not able to tell you how far diseases have been moved by Tropical Storm Alberto, we can tell you that there is some risk that Southern corn rust and Asian soybean rust have moved to the north as a result. We recommend you scout your crops on a periodic basis and we would love to hear about what you are finding in your fields. We at Agrible will keep you up-to-date on weather, crop, and soil conditions in your field and will raise alerts as conditions dictate.