Cause Green algae. This elementary plant grows on leaves, twigs, and trunks in wet or humid weather. Algal growth on conifer needles is a response to high nitrogen deposition and may be the same for holly.
Symptoms Yellow-green powdery layer on leaf surfaces, twigs, and trunk, usually appearing first on the north side of the lower trunk and gradually spreading up and out to twigs and leaves.
Cultural control Adequate pruning to permit good air circulation.
Chemical control Use as preventive treatments.
- Mancozeb-based products used for leaf and twig blight also suppresses algae. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Junction at 1.5 to 3.5 lb/A. Spray solution pH should be above 6.5. Group M1 + M3 fungicide. 48-hr reentry.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- PERpose Plus at 0.3 to 1 fl oz/gal water. 1-hr reentry.
- ZeroTol 2.0 at 0.5 to 1.25 fl oz/gal water. 1-hr reentry.
Reference Pepin, H.S., and Orchard, W.R. 1960. Control of green algae on English holly. Canadian Plant Disease Survey 40:15-17.