APS Abstract of Presentation
Symptoms and signs of Stigmina lautii on spruce needles in North Dakota
J. A. WALLA (1), K. M. Kinzer (1)
(1) North Dakota State University, Fargo, ND, USA
Stigmina lautii Sutton has recently been associated with needlecast of spruce in ND, but has not been proven a pathogen. The objective of this research was to characterize symptoms and signs associated with S. lautii and to compare them with those of Rhizosphaera kalkhoffii Bubak. Spruce with needlecast were observed and fungal fruiting bodies on needles were examined microscopically to determine identity. Both fungi appear to cause greatest needlecast in the lower canopy and on the north side of a tree. Discoloration associated with both fungi may develop on second-year or older needles and may be grayish-green, yellow, tan, brown, reddish-brown or purple. Both Colorado and white spruce appear to be severely affected by S. lautii, while only Colorado spruce is commonly damaged by R. kalkhoffii in ND. S. lautii was generally present on second-year needles of all affected trees, while R. kalkhoffii varies from tree to tree in age of youngest needles affected. Fruiting bodies of both fungi can occur on green, discolored, and dead second-year and older needles, are similar in size and color, and cannot be distinguished without magnification of 20X or more. Microscopically, side-protruding conidia of S. lautii sporodochia give a feathery appearance to the fruiting bodies compared to the smooth R. kalkhoffii pycnidia. In ND samples, the brown S. lautii spores averaged 41.7 µm long, 3.2 µm wide at the narrow end, and 5.2 µm wide at the wide end. Most conidia (65%) had 3 to 5 septa. R. kalkhoffii spores are hyaline, smaller (4.5-8.6 × 2.5-4.6 µm), and aseptate. Because symptoms and signs associated with S. lautii and R. kalkhoffii on spruce appear superficially similar, microscopic confirmation is required to identify which fungus is present.