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I've been told to follow up an infestation of apple scab by composting any leaf litter on the orchard floor to destroy spores. I plan to haul away most of the leaves to my garden composter but a 5% urea solution is indicated for the orchard floor. How much should I need per mature tree (10 ft diameter drip line) and are there any safety concerns with preparing the solution in a sprayer tank?

I figure 5% urea solution is about 2 lbs (pounds) urea in 5 gallons. It comes from my coop in 50 lb bags of prills.

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  • Where did you get your information from? I'm not questioning it, I just think being able to read through it (plus maybe other information that's there) might help people give you a better, more informed answer...
    – Mike Perry
    Oct 11, 2011 at 17:34
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    Does your compost pile get hot enough, 120°F (49°C) minimum for 2 weeks minimum (those are absolute minimums), to kill the spores? If no, you should dispose of those leaves via some other method (IMHO).
    – Mike Perry
    Oct 11, 2011 at 17:48
  • Straying off topic from the core question, but in response to Mike's temperature concern: In the article I linked in my answer, note that scab spores rarely travel more than 100'. If the garden composter is this far away from the orchard, it seems like there's a low risk of spreading scab from even a cold compost pile as long as you don't intentionally spread it in the orchard and follow reasonable hygiene procedures. Also: collect the leaves by mowing w/bagger, they will break down much faster (reducing spore count by 85% according to the article, and they assume you aren't hauling away).
    – bstpierre
    Oct 11, 2011 at 18:49
  • @bstpierre I would say it really comes down to personal (horticultural) viewpoint if "low risk" is acceptable or not...
    – Mike Perry
    Oct 11, 2011 at 19:14
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    Info comes from a class I took last March and a verbal reminder from a local farmer to use 5% urea at this stage.
    – Erik Olson
    Oct 11, 2011 at 19:38

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According to UMass, you should spray 100 gallons per acre, or .00229568 gallons per square foot. A 10' diameter circle is 78.5 square feet, thus requiring .18 gallons. If you make one gallon of solution, you can treat 5 mature trees.

If you have the trees planted in rows in an orchard, the orchard will cover more space than just adding up the trees. (E.g. 10 trees in two rows of five with a uniform 10' spacing will use more than 785 square feet of space.) This is because of the gaps between the circles -- and you'll want to treat those gaps as well.

A 50 lb bag of urea will make about 125 gallons of solution, or enough to treat 1.25 acres of orchard.

Note that the article linked above suggests the use of feed grade urea as it dissolves better.

I'm not qualified to comment on the safety aspect.

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