Ribes-willow rust

Micro-habitat(s)

Distribution

North America

This fungus is widely distributed throughout the range of its hosts in B.C.

Damage, symptoms and biology

Damage to conifer hosts is minimal, infections are usually sparse and restricted to regeneration and the lower branches of saplings. Although the severity of the disease on willow is undocumented, it is likely that damage to willow plantations could be as high as that caused by M. albertensis on poplars.

Spermagonia and aecia appear on conifer hosts on new needles shortly after bud-break. Aecia are yellow-orange and differ from other conifer-needle rusts in that the whitish blister or tube-like covering (peridium) is rudimentary or lacking. Uredinia are readily observed throughout the summer on the underside of willow leaves as yellow-orange pustules. Since uredinial mycelium can overwinter on willow leaves, urediniospores produced in the spring can continue to infect willows from year-to-year; the rust does not need to alternate to the conifer host.

Spermagonia and aecia on current years needles, occasionally on cones. Spermagonia originating under the epidermis. Aeciospores 14-21 x 15-24 µm; wall thickened at the apex. Uredinia and telia hypophyllous. Urediniospores globoid to broadly ellipsoid, 12-17 x 14-20 µm; wall uniformly thick. Teliospores 6-14 x 16-30 µm, wall uniformly thick.

Other information

Willow rusts are often treated as one species complex, loosely called Melampsora epitea, since their uredinial and telial states on willow are indistinguishable. When aecial hosts are determined through inoculation tests, several species may be delimited from this complex. Using this approach, the following species are recognized:

• Melampsora abieti-capraearum - true firs
• Melampsora epitea f. sp. Tsugae - western and mountain hemlock
• Melampsora paradoxa - Tamarack, alpine and western larch
• Melampsora ribesii-purpureae - currant and gooseberry

Melampsora abieti-capraearum frequently appears together with Pucciniastrum epilobii on fir. The willow rust on conifers can be distinguished by naked aecia (lacking peridia). At the microscopic level, willow rusts can be distinguished from other Melampsora rusts on conifers. The other rusts exhibit bilateral thickening of the aeciospore walls, whereas willow rusts have no bilateral thickening.

Main host(s)

Amabilis fir, balsam fir, grand fir, mountain hemlock, shining willow, subalpine fir, subalpine larch, tamarack, western hemlock, western larch, white fir, willow