The scattered showers over the weekend and the higher relative humidity are causing the risk models to indicate favorable conditions for tan spot and leaf rust. Over the last four mornings, favorable conditions for both diseases have occurred across much of northwest Minnesota and eastern North Dakota. Meanwhile, the risk for Fusarium head blight remains low.
Most of the barley and spring wheat reached anthesis last week and the window to apply a fungicide has closed. You may now have second thoughts. Should you have sprayed a fungicide to control those leaf diseases even if the risk for FHB was and continues to be low. After all, nearly all the products labeled for suppression of FHB also provide good to excellent control of tan spot, Septoria leaf blotch, and leaf rust.
Unlike FHB, tan spot, Septoria leaf blotch, and leaf rust are polycyclic (multiple cycle) diseases. This means that all three can have (and most often do have) multiple cycles, or generations, per growing season. Most often these three diseases start in the lower or middle part of the canopy and it takes two or three diseases cycle before the penultimate leaf and flag leaf are infected and reach severities that are high enough to cause economic damages. Each disease cycle takes anywhere from 5 to 14 days to complete, depending on temperature.
This means that if some initial infections of any of these diseases took hold over the weekend, that it will be another 10 days at a minimum before I would expect the diseases to reach the penultimate leaf or flag leaf if favorable conditions continue. At that point, most of the barley and spring wheat will be near the end of the grainfill period.
Bottomline - I doubt the severities of any of these three diseases will get high enough to cause economic losses on susceptible varieties, even if some initial infections are now starting to develop. Although environmental conditions favored initial infections, the forecast looks hot and dry, which will slow progression and disrupt additional infection events.
Meanwhile, I did see some rare and curious pest and disease symptoms today in one of my trials. The first was the telltale 'ticker tape perforations (Photo 1) of billbug (Sphenophorus parvulus). Prevalence of this telltale sign of the adult females laying eggs when the flag leaf is still furled up inside the elongating wheat stem appears to be higher than most years. It is still more a curiosity than an economic pest.
The second rare and curious symptom of a disease I found today was the stunted growth and development of a single spring wheat plant that was likely caused by early infection of Barley Yellow Dwarf Virus. We seldom, if ever, see the D in BYDV. The transmission of BYDV by aphids has to occur prior to jointing for the D in BYDV to occur. In the past decade, it was confirmed twice in the most southern part of the state, but never this far north.
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| Photo 1 - Perforations of the flag leaf of spring wheat caused by billbug (Sphenophorus parvulus) |
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| Photo 2 - Stunted growth and development of a spring wheat plant likely caused by an early infection of Barley Yellow Dwarf virus |
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