After winter floods were drawn down this spring, a handful of marshes in a similar geographic area found an unfamiliar residue clinging to their cranberry vines. The residue did not wash off with rainfall or with sprinkler irrigation, but rubbing it firmly with a fingertip was enough to remove it. The residue was present on cranberry vines as well as on perennial weeds, and vegetation at ditch edges.
Growers seeing this residue had one major concern: that the greyish-white residue would interfere with photosynthesis, and handicap the cranberry vines during a necessary carbohydrate production period. Growers wondered whether the residue was the result of an algae.
Three tests were conducted. The first was observing the vines through the spring stage—would new growth develop on the vines free of the residue? Thankfully, new growth developed at normal rates, and free of residue. This let growers feel comfortable that even if photosynthesis was compromised on the older leaves, the new leaves would be photosynthesizing effectively.
The second test was a windowsill assay. Residue-coated vines and residue-coated leaves were placed in an algae food media, and water was adjusted to a pH of 7.0. Sample jars were placed in a south-facing window, and no algae production was observed three weeks after the samples were placed. (Eventually after 4 weeks, three jars each developed unique algae growth, and one jar developed a pinkish mold. Because the timing was so delayed, and because each jar presented a different growth type, the expectation is that the biological species that eventually did grow are the result of trace contaminant cells. If the gray residue had been an algae species, the high concentration on all four samples would have displayed rapid growth, and the same species in all four jars.
The third test was the most formal: several samples were sent to the University of Wisconsin Plant Disease Diagnostics Clinic, where Brian Hudelson conducted a tape mount to determine whether the residue was algal or fungal in nature. Brian visually and microscopically examined the vines and leaves that I had submitted. The layer of material coating the leaves is a combination of irregular particulate matter and crystalline spine-like structures. None of the material appears to be biological in nature.
For growers who experienced this residue in 2023, there is not a specific management strategy if it reappears in 2024. If your marsh has multiple sufficient water sources available, it would be interesting to flood portions of the marsh from each water source individually without commingling, to see if some water sources will result in more residue than others.This article was posted in Cranberry and tagged after-flood residue, Allison Jonjak, Cranberries, Ethan Lehman, residue.