Project objectives from proposal:
The focus of this project is to evaluate, demonstrate and encourage adoption of cultural tactics to reduce bacterial rots in sweet onions grown for fresh market. We will evaluate the effects of mulch type and planting configurations on incidence of bacterial rot of onions, yield, bulb size and economic return.
Center Rot, caused by Pantoea ananatis, is one of the bacterial rots that has been identified in sweet onions grown on plastic on small-scale vegetable farms in PA and NY. Losses of 35 to 45% occur regularly. This is a relatively new disease of onions, first identified in Georgia in 1997. P. ananatis survives in numerous weeds and crops, and in the gut of tobacco and onion thrips, and to some extent in soil and water. It enters the plant through natural openings and wounds, and by infected thrips. Infection can occur when contaminated rain and irrigation water collect in the leaf whorl and leaf axils, leading to congestion of the leaf tissues and eventual entry of the bacterium into the plant. Disease development is favored by moderate to high temperatures (82.4 – 95 ˚F) that coincide with rainfall during or after bulb initiation. Generally, all cultivars are susceptible to center rot, although there is some variability in susceptibility. Sour skin, caused by Burkholderia cepacia, is another common bacterial disease of onion, similar to center rot, but more prevalent in soil and water, and favored by slightly cooler temperatures.
Bacterial rots first appear as leaf blight symptoms on the young center leaves of the plant, resulting in yellowing or bleaching and wilting of these leaves. The infection progresses down the leaves and the neck, and eventually into the bulb. Affected bulb scales become soft and watery with rot and are yellowish-brown in appearance. It is not uncommon for bulbs infected with center rot or sour skin to be invaded secondarily by other bacterial organisms. At harvest, the foliage often tears away from the bulb when pulled. At first, the rot can be hard to detect, because the outer unaffected scales remain firm, but bulbs continue to break down in storage.
Black plastic absorbs sunlight thus increasing soil temperature, which in turn, promotes early crop development of onions. However, during the heat of June and July, the warmer soil temperatures provided by the black plastic may actually be creating a more favorable environment for bacterial diseases to develop and spread. Preliminary temperature data collected by Reid in NY in 2007 showed that the daily high soil line temperature during July was approximately 10 ˚F cooler under reflective metallic silver mulch than it was under black plastic. This could be the difference between optimum and below optimum temperatures for bacteria to grow. In this project we will determine whether alternative mulch types, specifically, reflective metallic silver mulch or none (i.e. bare ground) result in lower incidence of bacterial rots compared to black plastic.
Onion thrips are known to spread the center rot bacterium, thus, improved control of onion thrips may reduce incidence of bacterial rots. In a previous Northeast SARE Partnership grant (ONE06-62, Reflective mulch to repel thrips in onions), Reid and Hoepting demonstrated that fewer onion thrips colonized onions grown on reflective metallic silver mulch compared to those grown on black plastic. This project also demonstrated higher yields and larger bulb sizes in the onions grown on reflective metallic silver mulch, supposedly a result of lower soil line temperatures that are more favorable for bulbing. In this proposed project, we will study the interaction among bacterial rots, onion thrips and mulch type.
Small-scale fresh market onion growers use wide plant spacing (i.e. 6 to 12 inches) to produce onions with large bulbs. When plant spacing is too wide, this encourages the onion plant to produce leaves at the expense of bulbing. This growth habit results in plants with very large, bushy foliage and thick necks. It also can delay or inhibit normal maturity. Such growth habit could be favorable for development of bacterial diseases for the following reasons:
• Large bushy leaves are more conducive to holding water in the leaf axils and whorls, which can favor initial bacterial infections.
• Thick necks take longer to dry and remain succulent and greener for a prolonged period of time, which provides ideal conditions for bacterial diseases to spread from the leaves into the bulb.
• Delayed maturity interferes with proper lodging and curing of the neck and bulbs, allowing for increased risk of bacterial infections in the leaves to spread into the bulbs, and reduced storability in general.
It is possible for a bacterial infection spreading down a leaf to be stopped in its tracks before it makes its way into the bulb, if the neck tissue is no longer conducive to its spread (i.e. the tissue is dry and not green and succulent). Our proposed solution is to narrow plant spacing, which will produce plants with thinner, tighter necks that mature on time, which are less conducive to development of bulb rots and have improved storability.
High planting density has been questioned by small-scale fresh market onion growers as a contributing factor to bacterial rots, because they wonder if planting 4 rows per bed (= 6 to 8 inch row spacing) is too dense for adequate air circulation through the canopy. It is a well known fact that leaf wetness, and water congested tissue is conducive to development and spread of bacterial diseases. Our proposed solution is to trial onions grown with 3 instead of 4 rows per bed with different plant spacings, which will increase between row aeration, minimize periods of water pooling in leaf axils and whorls, and make conditions less favorable for development and spread of bacterial diseases.
Never before has such a comprehensive study been conducted to investigate the effects of cultural practices on bacterial rots in intensively managed fresh market onions. It is also the first study to be designed to investigate planting configurations on onions grown on plastic. This project will allow us to identify, demonstrate and promote simple cultural changes that will reduce incidence of bacterial rots and, improve storability and profitability of fresh market sweet onions. These techniques will be readily adopted by fresh market onion growers, acreage of intensively managed sweet onions will increase, more locally grown produce will be available to the consumer and the small-scale sweet onion industry will be sustained.