Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/5/2007
Publication Date: 7/9/2007
Citation: Mengistu, A., Castlebury, L.A., Smith, J.R., Rossman, A.Y., Reddy, K.N. 2007. Isolates of Diaporthe - Phomopsis from Weeds and Their Effects on Soybeans. Canadian Journal of Plant Pathology. 29(3):283-289. Interpretive Summary: Phomopsis seed decay is an important soybean disease caused by fungi or molds that are members of the Diaporthe/Phomopsis complex, and the disease is commonly attributed to the mold named Phomopsis longicolla. Controlling this disease has been difficult which causes significant economic and seed quality loss in early-planted soybean grown in the southern United States. Some weeds are infected by the fungus. The fungus Phomopsis longicolla was recovered from weeds commonly found in soybean fields such as eclipta, pitted morningglory, nodding spurge and Illinois bundleflower, and the fungus caused significant disease on soybean leaves, pods and seed. There were significant differences among different soybean germplasm lines when seeds from diseased pods were evaluated. This research is important because it is the first demonstration that Phomopsis longicolla collected from diseased weeds also causes soybean seed decay in soybean.
Technical Abstract: Greenhouse and laboratory studies were conducted in 2004 to determine the identity and pathogenicity of Phomopsis longicolla and other Diaporthe/Phomopsis spp. recovered from weed species using DNA sequences, morphology and pathogenicity assays. Among the isolates recovered from eight weed species, P. longicolla isolates recovered from eclipta, pitted morningglory, nodding spurge and Illinois bundleflower caused a significant level of infection on soybean leaves, pods and seeds on soybeans. Recovery of P. longicolla from soybean seed of inoculated pods was lower than that recovered from inoculated pods. The same level of P. longicolla was recovered when threshed pods were assayed. The recovery of P. longicolla from soybean leaves was lower than from the hypocotyls, pods and seeds, indicating that the leaves may not be the preferred site of infection. There were significant differences among soybean accessions when seeds from inoculated pods were evaluated, suggesting that this inoculation technique may differentiate soybean genotypes for resistance. While wound inoculation on the hypocotyls resulted in infection, wounding did not provide evidence for natural ability of P. longicolla and Diaporthe isolates from weed hosts to cause soybean seed decay. The seed and pod inoculation techniques produced seed with high levels of P. longicolla recovery. In addition, Diaporthe phaseolorum was isolated from Illinois bundleflower, Texasweed and slender aster. An unidentified Phomopsis species was also isolated from prickly sida. The isolate from Illinois bundleflower had the lowest recovery of P. longicolla from hypocotyl (25 %), leaf (10 %), pod (45 %) and seed (55 %) compared to isolates from eclipta, morning-glory and nodding spurge. Isolates from eclipta and prostrate knotweed differed in their ITS sequence from the nodding spurge isolate by 3.6%. Results indicate that considerable variability exists in the DNA sequences of isolates of P. longicolla and Phomopsis/Diaporthe from weed hosts.