Cause The fungus Sphaerulina populicola (formerly Septoria populicola) has been reported in the PNW following comprehensive surveys of native Populus trichocarpa (black cottonwood) and commercial plantations. This species only causes a leaf spot disease on native and hybrid poplars and rarely causes significant adverse impact on tree growth and survival. Two closely related species also cause a leaf-spot disease and were recently reported on P. angustifolia and P. fremontii east of the Cascade and Sierra Nevada ranges in the US.
Sphaerulina musiva (formerly Septoria musiva) was first detected west of the Rocky Mountains in 2006 in a hybrid poplar plantation located in the Fraser Valley in British Columbia. This species causes cankers in addition to leaf spots in susceptible hybrid poplar plantations resulting in tree death and reduced biomass. The disease is more common in northeastern and north-central regions of North America. In natural stands of P. deltoides (eastern cottonwood), the fungus only causes leaf spots that have a relatively minor impact on tree growth and survival.
These fungi can overwinter on leaves with pseudothecia production in the spring. (Cankers can also be a source of inoculum depending on the disease.) Rain or irrigation can spread spores to healthy tissue and leaf wetness encourages spores to germinate. The fungus penetrates through stomata or has direct penetration followed by colonization of palisade mesophyll cells. Lesions have been observed within 2 weeks of inoculation of stems and from 7 to 21 days of inoculation of leaves. New spores may be formed on leaves 2 to 4 weeks after infection. Multiple infection cycles can occur during the growing season.
Symptoms
Necrotic spots on leaves may develop within 3-4 weeks of leaf expansion and may appear first on the foliage of branches close to the ground. Spots enlarge and become more generally distributed as the growing season proceeds. Spots may be circular to angular, appear grey to silver or become yellow to brown, and coalesce to include large portions of the leaf including the petiole. Pycnidia are often produced within spots on either leaf surface. Affected leaves may turn yellow and drop prematurely.
For canker - Lesions enlarge to form elongate, often depressed, cankers that are dark brown or black but may include lighter brown or tan areas. Irregular concentric rings may appear in the bark of rapidly expanding cankers on highly susceptible hosts. Stems may be constricted at the canker or appear swollen due to the production of callus. As cankers enlarge, they often become more irregular and the vascular cambium often becomes discolored. The stems of highly susceptible clones may be girdled in their first season of growth.
Cultural control
- Plant resistant clones.
- Collect and burn leaves in fall.
- Nursery trees with cankers should be destroyed.
- Avoid overhead irrigation that keeps trees wet for extended periods of time.
- Plant at wide spacing to improve air circulation.
Chemical control Although this disease may not be on the label, these are legal to use and effective if used prior to infection.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide. H
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 oz/100 gal water for nurseries or landscapes. Group 3 fungicides. 24-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Ortho MAX Garden Disease Control at 2 teaspoons/gal water. H
- Propiconazole-based products. Group 3 fungicides.
- Banner MAXX at 16 fl oz/100 gal water. 12-hr reentry.
- Infuse Systemic Disease Control at 0.5 Tbsp/gal water. H
- ProCon-Z at 16 oz/100 gal water. 24-hr reentry.
- Rally 40 WSP at 4 to 6 oz/A. Nursery or forest-area use only. Group 3 fungicides. 24-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
References Herath, P., Beauseigle, S., Dhillon, B., Ojeda, D.I., Bilodeau, G., Isabel, N., Gros-Louis, M.C., Kope, H., Zeglen, S., Hamelin, R.C., and Feau, N. 2016. Anthropogenic signature in the incidence and distribution of an emerging pathogen of poplars. Biological invasions 18:1147-1161.
Abraham, N., Chitrampalam, P., Nelson Jr, B., Sharma Poudel, R., Chittem, K., Borowicz, P., Brueggeman, R., Jain, S., and LeBoldus, J.M. 2019. Microscopic, Biochemical, and Molecular Comparisons of Moderately Resistant and Susceptible Populus Genotypes Inoculated with Sphaerulina musiva. Phytopathology 109:2074-2086.