Wheat Leaf Topics and Physiological Leaf Spotting

Most of the wheat throughout MS can be considered to be “out of the woods” when it comes to yield loss as a result of foliar diseases.  However, in the past week I have observed a few wheat fields where flowering had just initiated, generally in the north Delta.  Given the strange environmental conditions this season I can not say that I am shocked by the delayed maturity in some situations.

Physiological leaf spotting of wheat will oftentimes appear like the early stages of many foliar diseases.

Physiological flecking, or genetic flecking as a result of specific resistance genes can appear as foliar disease.  However, oftentimes the spots that develop, although miniscule, will not translate into leaf rust, or other foliar diseases that could reduce yield.  Oftentimes I receive calls regarding “early developmental stages of leaf rust” appearing on leaves.  Determining whether or not the leaf spotting is a result of physiology or an actual fungus can be difficult especially since symptoms will vary depending on variety.  Keep in mind that in most situations where I have been alerted to a leaf spotting situation the malady is more often a result of physiological/genetic flecking rather than the early symptoms of rust.

In addition to physiological leaf flecking, specific genes in some varieties can produce nondescript symptoms that are regularly perceived to have been caused by many different issues.  Symptoms appearing as leaf tip necrosis (or leaf burn; LTN), or more commonly perceived to be a nutritional deficiency, are actually the result of a specific resistance gene for diseases such as leaf rust; otherwise known as the Lr34 gene.  In addition to the LTN that can result from the genes being present, and adult plant resistance to leaf rust being conferred, leaf flecking, or physiological flecking, can also occur.  In most cases the specific response of the variety that contains the gene can differ based on the environment that occurred over the season or the specific disease present in that part of the state.

Leaf tip necrosis (LTN) caused as a result of a specific wheat variety containing the Lr34 gene that codes for leaf rust resistance. Photo courtesy of B. Hunger, OK State Univ.

Numerous diseases can be confused with the occurrence of leaf flecking, physiological leaf spot, or LTN:

-leaf rust

-tan spot

-Septoria leaf blotch

-bacterial leaf streak

-nutritional deficiencies

-Barley yellow dwarf virus (in rare situations)

Proper disease diagnosis is one of the most important topics to consider prior to making a management decision.  At this point in the season we have passed the stages where a fungicide can be legally applied based on label restrictions.  However, this information may serve useful for next season’s wheat crop.