Managing bacterial diseases in onions

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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.

In sourcing plants, what should a grower look for / ask their transplant producer in order to reduce the chance for bringing diseases into their fields?

A grower should look for plants that are dry to the touch (but not dried out) and ideally have relatively little soil on them (unless they are plug plants). When a grower receives their transplants, it's best to plant in the field as soon as possible - if you can't, I would suggest keeping them cool, opening the box or crate and spreading them out to prevent condensation from developing, which can encourage fungal and bacterial growth.

If you can start your own transplants, or buy them locally, that might be preferable simply because the plants seem to be in better condition. With bacterial diseases, any wound is a potential infection point, so the less injured the plants, the fewer chances for bacterial infection.

Is there variation among sweet onion varieties in their susceptibility to bacterial diseases? If there is, what varieties have tolerance / resistance?

As far as I know, there is no tolerance or resistance to bacterial diseases in onion, and especially not in sweet onion varieties. The high sugar content and low pungency of these bulbs likely make them perfect hosts for these pathogens, since bacteria grow more quickly in the presence of sugars and pungency compounds can sometimes restrict microbial growth. Plans are underway to conduct some variety trials to look more closely at varietal susceptibility.

How does plant fertility impact the incidence / seriousness of onion bacterial diseases?

Work that had been done previously in Georgia indicated that very high levels of nitrogen fertilization resulted in high levels of bacterial decay in onion. From our work here in PA, we saw a strong relationship between the levels of foliar N determined from tissue tests and the incidence of bacterial disease - actually that the growers with low foliar N at midseason had higher bacterial disease at harvest.

We did not see any close relationships between soil N and bacterial disease OR bulb size, so what this has begun to suggest is that N fertility is important early in the season, to get the foliar N up, but later in the season, N fertility may be less important. Additional trials are being planned to look more closely at nutrient relationships and develop research-based management recommendations.

How does plant density impact the incidence / seriousness of onion bacterial diseases?

Work that my advisor, Beth Gugino, has completed on onion spacing has indicated that spacing onions closer together results in lower bacterial disease incidence. The more closely the onions are spaced, the smaller the onion necks, and (we think) the more quickly the necks dry down, stopping the movement of bacteria into the bulbs.

Unfortunately, close spacing of onions also results in smaller onion bulbs, probably as a result of root competition for nutrients, so this works well for disease management, but not in terms of overall marketable yield. Thus, only consider close spacing if you can tolerate mostly medium-sized bulbs (2.25 to 3-in. diameter).

However, a good take-home message is that drying down the onion necks quickly and thoroughly will reduce the ability of the bacteria to move from the leaves into the bulb and potentially reduce losses. We are currently working on a scouting protocol to help growers time their harvest based on bacterial disease symptoms in order to minimize bacterial movement from the leaves into the bulb.

If a field experienced high infection rates last season, how long should that field stay out of Alliums?

If that was my field, I would stay out of Alliums (including onions, garlic, leeks, and shallots) for two or three years to allow the crop residue to thoroughly decompose. I would also avoid following with beans, corn, alfalfa, or tomatoes immediately, because there have been reports of those crops being able to support some of our bacterial pathogens.

Through our grower survey, we identified more bacterial pathogens in soil after a mild winter, compared to a more severe winter, so the harshness of the off-season may play a role in how the pathogens survive from year to year. Hopefully that is a silver lining for onion growers, judging by the record cold temperatures of this past winter!

How does plastic mulch impact these diseases? Does mulch color matter?

We have observed a relationship between plastic mulch color, its effect on soil temperatures, and the incidence of disease in the field. Based on a number of replicated field trials using different mulch types, we have begun to recommend black biodegradable plastic mulch for several reasons.

Early in the season, the plants get the same amount of soil warming and weed suppression as traditional black plastic mulch, but in June, when air temperatures (and therefore soil temperatures) begin to really heat up, the mulch has started breaking down and soils will stay cooler. The mulch also stretches away from the rapidly growing onions, which prevents moist conditions around the bulb, which along with high temperatures, really favor bacterial growth.

There is a price premium associated with the biodegradable mulch and its reduced shelf life can make it challenging to work with, so anecdotally, some growers have told us they have had decent results when they slash the standard plastic on the side of their raised beds in the middle of the season to help lower soil temperatures by increasing air circulation.

Does straw mulch have an impact on these diseases?

We included straw mulch in a couple of trials early on and found it very challenging to work with. Not only did we have trouble keeping it in place when the plants were small, but we also were unsuccessfully managing weeds. In addition, we saw an increase in purple blotch, a common foliar fungal disease, because of reduced air circulation and higher humidity in the onion canopy.

The PA Vegetable Production Guide only lists copper and Mancozeb as bactericides for onions. How about other products that bolster a plants' defenses such as Regalia, and Actinovate AG, do they have a place in managing bacterial diseases in onions?

We completed some studies on plant-defense inducing products at our research farms, and we saw no differences in disease management using plant defense-inducing products compared to the copper/mancozeb standard, that is, the plant defense inducers fared no better and no worse than copper/mancozeb. In our trials, we had included Actigard, Employ, and Companion and some combinations of those products with copper/mancozeb as well.

It may be worth re-evaluating the efficacy of these products now that we have developed a more reliable method for attaining uniform disease pressure within our research plots.


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