Field trials of copper fungicides for the control of potato blight; spray retention

Spray-retention estimations were made during four seasons at Dartington in south Devon, on maincrop potatoes, twice-sprayed, as in farm practice, with about 120 gal./acre for the first application and 160 gal. for the second.
Evidence is given concerning the limits of reliability of the disk method of leaf sampling; and an alternative method, batten.' washing, is described.
With from 2,1/2 to 3,1/2 in. of rain over test periods of 3-4 weeks, 1% Bordeaux mixture showed about 40 % spray retention. Cuprous oxide and copper oxychloride sprays at the same copper dosage but compounded with sulphite lye or other water-soluble dispersing agent showed less than 20% retention. Compounding of these materials with an insoluble sticker (bentonite) improved the adhesion.
The percentage retention of 1/2% Bordeaux mixture was found to be less than that of the usual 1 % mixture, but adequate spray deposits could be maintained with this and other low-copper fungicides by more frequent applications.
A rapid method for determining total expanse of foliage in the field is described, and the importance of such determinations in connexion with spray-retention trials, and practical spray timing, is stressed.
Experiments were made to determine whether any copper is absorbed by potato leaves from the spray deposits. Absorption, or acid-resistant adsorption, of the order of 0.02 mg. Cu/120 sq.cm. of leaf area was indicated.