Amanda Gevens, Potato & Vegetable Pathologist
I am receiving many questions on late blight symptoms associated with seed sources vs. aerial spore deposition (from external sources). I’ve included a nice review on this topic from Dennis Johnson, link below. In short, it is tough to identify, with certainty, the source of late blight inoculum in an epidemic. We know potential sources are multiple and include volunteers, cull piles, compost piles, seed, and infected transplants (tomato). We don’t have any evidence at this time that the pathogen is persisting in the soil outside of plant tissues.
Can we associate symptoms with inoculum source? Late blight is very dependent upon the environmental condition and thus, there have been conflicting past reports on the mechanism of disease transmission from seed to foliage. Seedborne late blight inoculum can result in poor stand/emergence. And, at times seedborne sources can create hot spots or disease foci in fields. However, seedborne sources can also infect shoots internally via mycelia and lesions may not be evident on lower stems. In this scenario, sporadic lower stem sporulation can occur ‘under the radar’ creating spores which then infect foliage giving the appearance of a ‘top down’ or ‘spore shower’ inoculation event. While the incidence of this happening is probably very low (as referenced in Johnson’s work below), just a few infected/sporulating plants are still very biologically relevant in initiating an epidemic. Past research has shown that variable symptoms and field signatures can result from infected seed depending upon timing of infection/inoculation, environmental conditions, and strain of the pathogen. Further complicating things are field factors like soil type, planting depth, whole vs. cut seed status, and pesticide inputs. In sum, we can try to piece together a story of how the late blight pathogen got into a production field – and this is a useful exercise to better manage the disease within that field and others – but we may not come to a certain conclusion. http://www.potatogrower.com/uploads/6244.pdf
We are working on characterizing the strain/genotype of the late blight pathogen in partnership with Dr. Bill Fry of Cornell Univ.. I will keep our growers informed of this information as we learn more.
Previously shared info on fungicide selections for conventional and organic systems can be found at my website at:
http://www.plantpath.wisc.edu/wivegdis/