Question
Several varieties of agastache (cana, rupestris, etc.) have developed yellow leaves with green veins. Started last summer. Treated for iron deficiency-no change. Used Osmocote last spring. This spring plants came up all yellow w/green veins, no regular foilage.  Plants look OK otherwise.  Am sending a photo of foilage. Tried spraying with all-purpose fungicide, no change.

Answer
Hi Anne,

Thanks for the photos.  From the look of the interveinal chlorosis, this appears to be a nutrient deficiency.  The Osmocote provides nitrogen, potassium, and phosphorous (the macronutrients).  This appears like magnesium deficiency because of the mid-vein yellowing.  There are many products available that also provide micronutrients.  Magnesium deficiency sometimes results from too high a concentration of potassium in the soil.  

There are several ways to administer magnesium.  The first is to make a solution of Epsom salts (magnesium sulfate)(1 tablespoon/1 gallon water) and water the plants with the solution several weeks.  (If you have a large area, this is probably impractical.)  Second, you can look for micronutrient supplements at your local garden center.