We have had a few questions about timing of Callisto with the heat conditions. From the manufacturer, if it is possible, they would prefer to see applications made in the morning. Weeds are less stressed due to the heat and will pick up the Callisto more readily verses day or evening applications. Realizing this is not always possible, they have not put restrictions about this on the label. When Syngenta test products, they test in all situations but for weeds that are listed as “suppression only” on the label, better control can be had when weeds are not stressed due to heat or etc. This stress factor is certainly applicable with other herbicides in other crops. Herbicides always work better when applied to non or less stressed plants.
Other Callisto thoughts: Since the rain fast time of Callisto is 4 hours, most of the ai is in the leaf by that time. On hard-to-kill weeds and weeds that have “suppression” on the label, Syngenta hopes that growers could do a full 24 hours of “water-free” to get the maximum amount of ai into the leaf. Normally, once Callisto is dry on the leaf, you will not get much more uptake by the plant. Similarly, try to wait 24 hours before irrigating. You can wash off the herbicide if enough rainfall or irrigation occurs shortly after an application even with a surfactant being used with Callisto.
Carolyn has been fielding a few calls about Yellow Vine Syndrome (YVS). It is likely showing up now due to the stress the vines are under with the heat and lack of rain. Since an imbalance of Potassium and Magnesium in the leaves have been associated with YV, one possible alleviation of symptoms can be the application of Magnesium sulfate (Epsom salts) at about 25 lb/A. Other products with Mg are out there and if you use one of those, just follow the rate recommendations on the label. The ion imbalance is only SECONDARY to the main issue of root insufficiency due to too much or too little water. Usually, root depth in areas of YV is shorter than in healthy, nonsymptomatic areas. If your tissue tests show high Mangenese (Mn), this is also an indication of poor drainage.