Article taken from Matt Montgomery, Sanagamon-Menard Ill. Extension.

Strobilurin fungicides were used in more Illinois row crop fields than ever before during the 2007 growing season. How did those applications fair for the producer?

In some cases, there were yield advantages; in some cases, those yield benefits were absent or marginal; and in a handful of cases, producers encountered yield penalties and accompanying symptoms that may be the result of “fungicide injury.”

Dr. Emerson Nafziger noted arrested ear development symptoms and hinted at some fungicide injury suspicions in the August 10th issue of the Pest Management and Crop Development Bulletin.

As agronomists began to explore these cases, some began to hypothesize that strobilurins could have influenced ethylene levels within the plant thus resulting in the poorly developed ears noted in some fields.

What is ethylene? What is the impact of ethylene on plants? Can strobilurins impact ethylene levels? Is this the explanation?

What is ethylene?

Ethylene is a colorless, odorless gas that acts as a plant growth regulator (PGR), or what some occasionally term a “plant hormone” (Dudek. 2005. MSU Field Crop Advisory Team Alert Newsletter No. 3). It is synthesized in all parts of the plant and is especially produced in response to stress or wounding. Ethylene is also markedly produced in those tissues undergoing ripening, and it is the explanation for why one piece of rotted fruit results in a cascade of additional rotting fruit in a barrel or bushel basket. In other words, as a piece of fruit begins to rot it produces ethylene gas that “signals” fruit in proximity to also rot.

What is the impact of ethylene upon plants?

As noted, this PGR is well known for signaling fruit to ripen. In addition to this, ethylene also plays a role in plant growth toward/away from light (phototropism), and it plays a primary role in signaling the plant to drop leaves, flowers, or fruit (reproductive abscission). Ethylene can promote irregular stem elongation (epinasty) and can also inhibit root elongation when present at high levels. This specific PGR is also believed the likely cause of irregular tissue development associated with injury via plant growth regulator herbicides such as 2,4-D (Gardner, Pearce, and Mitchell. 1985. Physiology of Crop Plants).

There is at least some evidence that ethylene can retard the onset of maturity or what is termed “senescence” (Grossman and Retlaff; Dunne. 2002. New Fungicide and Their Role in Disease Control Programs – Irish Agriculture and Fodd Development Authority). Some researchers also note that “those plants that bloom once” will “likely not bloom” when they are exposed to critical ethylene levels at just the right time (Dudek. 2005. MSU Field Crop Advisory Team Alert Newletter No. 3).

Plants are rather susceptible to small variations in ethylene. Some researchers have cited injury to greenhouse plants associated with ethylene at levels as low as 0.01 ppm (Dudek. 2005. MSU Field Crop Advisory Team Alert Newletter No. 3). In other words, ethylene does have the ability to impact reproductive growth and can do so following seemingly inconsequential plant exposure.

Can strobilurins impact ethylene levels?

The fact that strobilurins can impact ethylene synthesis seems both fairly well documented and fairly reasonable from a plant physiology standpoint. The literature is sprinkled with occasional citations of this fungicide’s ability to inhibit ethylene biosynthesis given the right circumstances (Dunne. 2002. Irish Agricultural and Food Development Authority). From a physiology standpoint, agronomists know that ethylene is derived from a chemical precursor termed ACC. An enzyme termed ACC synthase is needed to produce ACC and this enzyme is regulated by auxins in the plant. Researchers have noted that some strobilurins “mimic the effect of auxin” resulting in a “decrease in ACC…and a subsequent reduction in ethylene production” (Ypema and Gold. 1999. Plant Disease Vol 83. No. 1). In other words, strobilurins probably are able to influence ethylene production in some cases.

Are strobilurins impacting ethylene levels thus causing the symptoms recently termed “fungicide injury?” Is this the explanation for the symptoms observed?

As noted, ethylene exposure at just the right time can create reproductive issues in greenhouse plants and could thus be a theoretical cause behind reproductive issues in row crops. Dr. Bob Nielsen of Purdue noted in a September 4, 2007 article that “the symptomology of arrested ear development resembles that caused by plant growth regulators and thus might reflect the consequences of hormone-mediated responses to more than one type of stress.” Nielsen noted a couple possible explanations for this type of injury and probably could have mentioned more. One of those explanations cited the possibility of ethylene derived reproductive issues associated with the use of strobilurins.

All involved in agriculture must admit that the industry is on a learning curve related to fungicide use. The exact level of benefit/frequency of benefit associated with their use remains unclear, the exact reason for any observed benefits remains unclear, and the explanation for occasional incidents of “fungicide injury” also remain unclear. A final, definitive explanation for observed injury symptoms likely associated with fungicide in a few area fields will come in the future. However, that definitive explanation is probably not here yet. For now, strobilurin-derived reproductive issues associated with ethylene remain an interesting and intriguing theory.

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