Managing bacterial diseases in onions
Sweet onion production has soared in Pennsylvania in recent years and bacterial disease challenges have grown right along with production. Steve Bogash provided interview-type questions to Emily Pfeufer (pictured), Ph.D. candidate working with Beth Gugino, Vegetable Pathologist due to her four years of research into sweet onion bacterial diseases.
Emily has been doing research on several bacterial diseases on sweet onions for the past four years as she prepares her doctoral thesis. With so much interest in onion production, this seemed like a good time to get her take on managing bacterial diseases in onions prior to the 2014 planting season. The questions were provided by Steve Bogash and the responses by Emily Pfeufer.
What are the most significant bacterial diseases on sweet onions for PA and Mid-Atlantic growers?
Here in PA, we mostly see general soft rots and center rot of onion. Center rot begins as a foliar disease, and initial symptoms are typically small bleached, dry white lesions that expand and spread down into the neck and eventually into the bulb. The bleached leaf will end up collapsing and other leaves may develop symptoms; this bleached leaf corresponds to a discolored scale within the bulb when it is sliced open.
We also see soft rots, caused by a very general bacterial pathogen that can rot just about any vegetable, especially when damaged, and can particularly be a problem post-harvest. In NY, growers see more slippery and sour skin, which are caused by two soil-inhabiting bacterial pathogens, but in PA, we see those diseases infrequently due to our more diverse crop rotations.
Are these diseases indigenous to our area or were they brought in on infected plants?
As part of my research, we have screened a small subset of the imported and locally grown transplants for the most common bacterial pathogens and we have indeed isolated rot-inducing bacteria from them. However, we are continuing to test more individuals to see how commonly they occur and link these specific strains isolated from the transplants to those that we isolated from rotting bulbs at the end of the season.
The particularly tricky part is most common bacteria we find in onions are also very common in the environment, and we routinely identify two or three different bacterial species from rotting bulbs. Not only do we find these bacteria on transplants, but also on weeds and sometimes in soils. In testing selected species of bacteria we have isolated from the surfaces and tissue of weeds, we have found that over half of the isolates we test have the ability to rot an onion.