One of the hardest things to do as a plant pathologist can be to diagnose a disease through a photograph. Over the past two weeks I’ve received several images of what would appear to be sudden death syndrome (SDS), at least at first glance. But, upon some serious phone and field time it appears that we have an interesting situation developing in Mississippi this soybean growing season. SDS has historically only been an issue in lighter soil classes (see more in next blog posting). So when someone calls with a case of SDS from a heavy soil class, or from across an entire field I am immediately interested in the situation. SDS will more typically occur in small patches, a few random scattered plants, or when soybean cyst nematode occurs in the same field in a large oval area. SDS does not normally occur across an entire field nor will it be observed until soybean plants have almost reached R6. But, with that said, there are some situations that will manifest themselves with leaf symptoms that appear to be exactly the same as SDS, but with some subtle differences. The list is quite long in fact:
-Phytophthora root rot
-shock from lack of water or another root associated stress (typically in vegetative or extremely early reproductive stages)
-black root rot (or as it has been referred to most recently, the mystery disease)
-red crown rot (but, only present in two counties: Chickasaw and Noxubee)
The last entry, phytotoxicity, will be the topic of this post. Over the past 10 days I’ve observed three fields, all geographically separate from one another (one in Hinds, two in Rankin, and one in Sunflower County), that have each received (by either air or ground) a labeled application of a pre-mix fungicide (strobilurin + triazole). The application was made correctly, containing a 0.25% (v/v) of a non-ionic surfactant in either 5 gallons of water by air or 15 gallons of water by ground. Yet, in the three specific situations leaf tissue has expressed symptoms of phytotoxicity following the application that is most likely due to the triazole component of the fungicide. Interveinal chlorosis with limited necrosis between veins has appeared in the most affected leaf tissue. However, one thing to note, roots that were removed from the soil were healthy, contained active nodules, and the internal vascular tissues were free from staining that would be indicative of a root issue such as SDS.
In the past three years I have observed similar phytotoxicity following an application of the same product to a large field in Clay County and observed phytotoxicity in trials conducted during the 2012 season with this particular product and the triazole compound included in the pre-mix in efficacy trials conducted in Stoneville on a buckshot soil. However, the fungicides were applied late morning on a hot day when the environment likely contributed to the increased risk of phytotoxicity.
An important point to note, the phytotoxicity may look bad but this does not mean that the plant is going to die. In most situations where phytotoxicity occurs the product was likely applied in an environmental situation (high temperature, high humidity, full sun) whereby the mixture of the fungicide and environmental components resulted in the phytotoxicity.
To prevent the occurrence of phytotoxicity from developing following the application of a fungicide that contains a triazole make sure the application is not made during the heat of the day. Waiting for the dew to burn off the plants is one thing, but making the fungicide application in full sun or the heat of the afternoon can increase the likelihood of foliar phytotoxicity.