Agronomic News

Fungicides:  To Spray or Not to Spray?  That is the question that I have been getting.

An unusually dry early season coupled with March planted corn that is about to tassel, some already tasseling, has brought about the topic of fungicides earlier this year.  Some research has shown that the earlier we plant corn fewer problems with diseases arise, yet Purdue is already receiving reports of Holcus Leaf Spot this year.  Holcus tends to be a disease that is never wide spread and thought to have little yield effect but it does show that disease can happen even in not so ideal situations. I quickly think back to the winter that we had.  Very little freezing conditions could allow for disease that overwinter on corn to be more prevalent WHEN we do get the rainfall that we so desperately need in much of the Corn Belt.

Prioritize Your Fields

The more of these risk factors that you have the more likely a fungicide application will pay:

          Continuous corn
          History of disease
          High residue levels
          Maximum yield potential
          No-till, reduced tillage
          Low lying fields, especially areas prone to fog
          Susceptible hybrids
          Narrow Rows
 

Remember the disease triangle; all three are necessary for disease development.

 

Specific Diseases of Interest

Gray Leaf Spot

Temperatures have been favorable yet the humidity and dry conditions have not allowed this disease to take off yet.  If moisture becomes available, gray leaf spot can spread quickly especially in fields with heavy residue where it overwinters.

Anthracnose

Once again the lack of rains is playing against this disease as well but late season stress can cause the stack rot form of anthracnose to become a problem.  One more disease that overwinters on infected crop residue and top dieback was very evident last year.

Rust

Common rust should not be a huge issue with the hot temperatures that we have experienced but southern rust could be blown up from the south.

Stalk rots

Warm wet weather after pollination tends to favor the stalks rots that we tend to find troublesome.  Diplodia Stalk Rot, Gibberella Stalk Rot, and Fusarium Stalk Rot could be problems depending on the conditions later this summer.

Ear Rots

Though it is not conclusive how effective fungicide applications are on ear rots, I would still be watching out for these in hot dry conditions:

        Aspergillus Ear Rot
        Fusarium Ear Rot
        Diplodia Ear Rot
 

Benefits of Fungicides

Reduce Leaf Tissue Damage

As leaf surface area decreases from disease lesions, the amount of photosynthesis is reduced leaving the plant unable to produce enough food to fill the ear to its maximum potential.
Slows Down plant respiration

Strobilurin fungicides cause the disruption of cellular respiration of a fungus causing it to run out of energy and die.  It also helps the plant slow down its rate of respiration making it use the water that is available more efficiently.

Increases N content in tissue

Reducing the amount of energy necessary for the plant to make food increases the amount of Nitrogen in the plant tissue keeping the leaves alive and working longer to extend the grainfill.

Reduces Ethylene Production

Ethylene is produced by plants to ripen (mature).  The more the plant produces the quicker it will mature even if grain fill is not maximized.    Stress conditions such as hot temperatures, lack of rainfall, and disease presence lead to the plant increasing the production of ethylene.

Final Thoughts

Fungicides should be used to target fields with a high likelihood of disease presence within the growing season.  Fungicides show the best response when applied at tassel.  When applying fungicides during extreme heat, it can be a benefit to delay the timing of application until brown silk to ensure that you do not have any ill effects on pollination especially when surfactants are in the mix.

Source:  “Physiological Effects of Strobilurin Fungicides on Plants” – Venancio, Rodrigues, Begliomini, Souza