Onion growers in Kentucky should have their eye out for bacterial rots of onion this week, particularly center rot of onion. Bacterial rots are common diseases of onions in Kentucky, and homeowners or commercial growers not actively using preventative practices will likely experience at least some disease incidence. This disease can occur in transplants, and when those plants are set in the field, center rot can proliferate under wet, humid conditions. In addition, some of the pathogens can move into the crop from weeds or from the soil, depending on the pathogen. Prevention, early identification, and management will help reduce plant losses.
Here are the symptoms to look for, preventative tactics, and brief suggestions on how to treat crops once disease is confirmed.
Cause and Disease Development
As many as nine different species of bacteria can cause bacterial rots of onion. Center rot is caused by the bacterial pathogens, Pantoea ananatis and P. agglomerans. These pathogens may originate from transplants, weeds, or persist in contaminated soil. Cells of bacterial pathogens are spread via water, such as rain or irrigation, and infections occur most frequently through wounded leaves. In most cases, infections originate in onion foliage, then progress to bulbs over a period of a week or more. Center rot is favored by hail, wind-driven rain, warm temperatures, and high humidity. Once established, the disease can spread rapidly in dense plantings, and in severe cases, leaves dieback and bulbs rot.
Center rot begins as a foliar disease. The first symptoms are small, dry white lesions (Figure 1). These lesions expand, appear bleached, and spread down to the neck of the onion. Once the disease reaches the bulb, scales may become discolored and rot (Figure 2). Each infected leaf that eventually collapses represents a discolored scale within the bulb.
In high disease pressure situations, crop harvest may be expedited in order to prevent the bacteria from entering the bulb. Drying onion necks as quickly as possible (by placing on racks or in bins, in front of a high-power fan) can help prevent bacterial movement into bulbs from foliage.
- Grow or purchase disease-free transplants. Only set plants that are free of symptoms.
- Practice crop rotation.
- Remove plant debris and weeds from the growing area.
- Remove and destroy heavily symptomatic plants.
- Avoid overhead watering to reduce leaf wetness.
- Apply fertilizer at recommended rates.
- Use recommended plant spacing to facilitate air movement and leaf drying.
Bacterial diseases of onions are chemically managed using copper-mancozeb sprays. For highest efficacy, these should be applied on a weekly basis. As protectants, these chemicals are reliant on excellent coverage of plant tissue. They would be expected to slow spread of disease in the field, rather than slow advancement of disease in individual plants.
As always, all label recommendations must be followed when applying chemicals to crops. In particular, pay close attention to pre-harvest intervals.
- Newly Identified Bacterial Disease on Onions (Michigan State University Extension, July 22, 2014)
- Managing Bacterial Disease in Onions (PennState Extension Vegetable, Small Fruit, and Mushroom Production News, March 12, 2014)
- Home Vegetable Gardening (ID-128)
- Vegetable Production Guide for Commercial Growers (ID-36)
By Kim Leonberger, Extension Associate and Emily Pfeufer, Extension Plant Pathologist