A 3-Point Plan for Avoiding Leaf Disease Losses

It’s that time of year where we transition from killing weeds to protecting our crops from diseases. Growing conditions across much of Western Canada have been good, with adequate moisture (to too much moisture!) and warm temperatures; this is excellent for the crops, but also creates a conducive environment for diseases to thrive.

Scouting: As the focus shifts to from weed control to cereal leaf diseases, it’s time to be out scouting for tan spot and septoria in wheat and scald, net blotch and spot blotch in barley. These diseases can be seen on many cereals as early as the 2- to 3-leaf stage, and tend to be worse in a cereal on cereal stubble rotation. Some aren’t concerned with leaf diseases, but some research suggests a high infestation of septoria can cause a yield loss of up to 20%. That’s 10 bushels/ac in a 50 bu/ac wheat crop.

Most leaf diseases we deal with have a life cycle of six to eight days from spore germination to production of spores, and even an early morning dew can provide enough moisture for spore germination. Some of these spores can be moved onto newer leaves by wind, but rain splash is the main mechanism by which these diseases progress up the plant. My general observations tend to be that if you have a 10% infection of tan spot for example on the penultimate leaf (second-from-the-top leaf), then in a weeks time, with good conditions and no fungicide, you will see roughly a 10% infection on the flag leaf.

Rusts have even become a concern across many parts of the Prairies in recent years. Rust tend to be more destructive and impact yield more than earlier listed leaf diseases. Rusts can be harder to manage for as the pathogen is carried in on winds — rotation isn’t going to help here.

Spray timing: The question becomes when do you spray? Do you protect the younger leaves to keep the disease low in the canopy (click here for more on that) , or target only the top two leaves? Generally, in a wheat crop your penultimate leaf and flag leaf will contribute 55% to 65% (35% to 45% from flag, 10-25% from penultimate) of the photosynthates to your yield, so these are the two leaves you want to protect most. Barley is similar, except that with a smaller flag leaf, the penultimate leaf actually contributes relatively more to yield.

Product choice: Now, what do you spray? There are many fungicides out there, with two groups of fungicides being the most common for leaf disease; The Group 3 triazoles (Tilt, Caramba, Folicur, Prosaro) and the Group 11 strobilurin (Acapela, Headline) fungicides. Both groups do an excellent job and give you around that 14 days of protection when it comes to leaf disease, but there is one key difference; triazoles do have some curative characteristics, meaning that if that spore has just germinated and is around 3 days into its life cycle a triazole fungicide will have some activity, where as a strobilurin will not. There are, however, products that allow you to get the best of both worlds such as Quilt and Twinline. At the end of the day all of our fungicides do a good job — the two most important things are getting them on at the right time (full flag leaf) and getting good coverage by using 10+ gallons of water an acre.

Significant losses from leaf disease can be avoided, you just need to be in your fields checking conditions, levels and timing. Get scouting!

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