Cause The fungus Venturia populina is found on Lombardy poplar (P. nigra 'Italica') and eastern cottonwood (P. deltoides). The fungus Venturia inopina is found on black cottonwood (Populus trichocarpa) and on other Populus spp. in selections Tacamaha and Aigeiros and their hybrids. There may be some disagreement on the fungal names but isolates from Lombardy poplar did not infect black cottonwood or visa versa. The disease has caused extensive damage to older, established Lombardy poplars in areas of western Washington. Venturia macularis (formerly V. tremulae) is found on aspen (P. tremuloides).
These fungi overwinter in infected shoots but not in leaves. Spores are produced in wet spring weather, ascospores ejected from overwintered pseudothecia and overwintered conidia. Immature and young shoots are susceptible to infection but become resistant as tissues age.
Symptoms A variety of brown to black leaf spots and blights on leaf tips and margins. Necrotic areas expand rapidly; killing whole leaves and shoots. Shoot dieback is characterized by a "shepherd's crook" appearance of the blackened terminals. Lesions generally do not extend into woody tissues. Velvety mats of olive-green conidia occur on leaf spots and shepherd's crooks. Affected tissues dry out and become brittle. Disease impact is greatest on younger trees. Repeated blighting for several years results in stunted and malformed trees. Spring frost damage can be confused with shoot blight but frost damage is usually found more widespread and higher up on trees.
Cultural control
- Prune out infected shoots.
- Plant resistant clones.
- Avoid overhead irrigation that keeps trees wet for extended periods of time.
Chemical control Use during early shoot growth in spring.
- 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.
- Banner MAXX at 16 fl oz/100 gal water is registered for other pests and may be effective against this one. Group 3 fungicide. 12-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.
- Tebuject 16 is registered for tree injections. The number of capsules used is based on tree size. Group 3 fungicide.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Newcombe, G. 2003. Native Venturia inopina sp. nov., specific to Populus trichocarpa and its hybrids. Mycological research 107:108-116.