Location: Plant Science ResearchTitle: First report of Stemphylium globuliferum causing Stemphylium Leaf Spot on alfalfa (Medicago sativa) in the U.S.) Author
|Samac, Deborah - Debby|
Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/16/2014
Publication Date: 6/16/2014
Citation: Samac, D.A., Willbur, J.F., Behnken, L., Brietenbach, F., Blonde, G., Halfmann, B., Jensen, B., Sheaffer, C.C. 2014. First report of Stemphylium globuliferum causing Stemphylium Leaf Spot on alfalfa (Medicago sativa) in the U.S. Plant Disease. 98(7):993. Available: http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-08-13-0828-PDN. Interpretive Summary: A new pathogen causing a leaf spot disease of alfalfa was identified during the spring of 2012 in Minnesota and Wisconsin from alfalfa fields with significant leaf loss. Initial symptoms consisted of white to tan spots with a brown border, 2-3 mm in diameter, circular to oval in shape, enlarging to 5-8 mm in diameter. Lower killed leaves remained attached to the primary stem. Spore shape and size were used to identify the pathogen, which has not previously been found to cause disease on alfalfa. Diagnostic DNA sequences of the pathogen were obtained that can be used for rapid identification of the fungus causing the disease.
Technical Abstract: Stemphylium leaf spot occurs in most areas where alfalfa (Medicago sativa) is grown. In the U.S., Stemphylium botryosum is reported to be the predominant pathogen, although S. vesicarium and S. herbarum is also observed. In April and May of 2012 alfalfa plants with leaf spot symptoms were observed in Minnesota and Wisconsin. Initial symptoms consisted of white to tan spots with a brown border, 2-3 mm in diameter, circular to oval in shape, enlarging to 5-8 mm in diameter. Large lesions often coalesced. Small, narrow, brown lesions occurred on petioles. Lower killed leaves remained attached to the primary stem. Spots were larger than those caused by the cool temperature biotype of S. botryosum. Conidia on host material were borne singly on straight, unbranched, smooth conidiophores, medium brown at the apex. Conidia were medium to dark brown with small papillae, subspherical with 3-4 transverse and 3-4 complete or near complete longitudinal septa, with a distinct constriction at the median transverse septum. Conidia measured 27.5-32.5 µm in length x 20-22.5 µm in width with a length to width (L/W) ratio of 1.2-1.5. Conidia on V8 juice agar were smaller, 25-30 µm in length x 12.5-19 µm in width with a L/W of 1.6-1.8. Ascostromata 300 µm in diameter formed on leaves held at 4°C for 2 months and after 1 month of culture. Ascospores from host material were golden brown to reddish, 40-42.5 µm long x 20 µm wide, slightly broader in the upper half of the spore, with 7-8 transverse septa and one complete longitudinal septum with several incomplete septa. Ascospores from culture were smaller, 27.5-30 µm long x 12.5-15 µm wide. These morphological features are consistent with the description for S. globuliferum. The rDNA ITS, glyceraldehyde-3-phosphate dehydrogenase, and elongation factor sequences obtained were distinct from S. botryosum but could not distinguish the isolated strains from S. vesicarium, S. herbarum, and S. alfalfae. However, the vmaA spacer sequence was distinct from the other species. This is the first report to our knowledge of S. globuliferum causing disease on alfalfa.