2Plant Pathology Department, Rural Development Center

The University of Georgia

Tifton, Georgia 31793

There are about 5 million units (50-pound bags) of low pungency, sweet onions produced in the Vidalia onion growing area annually. Approximately half of them, 2.4 million units, are put in controlled atmosphere (CA) storage to extend the marketing period. Onions coming out of CA storage command a higher price, usually $20 to $24 per unit compared to $10 to $14 (or less) per unit on the fresh market. Losses during storage range from 10 to 20 percent in good years to 50 to 70 percent in poor years. The primary cause of storage losses is rots from infection by the fungus, Botrytis allii. Thus annual losses caused by Botrytis allii in storage can cost Vidalia onion growers from $5 to $25 million. The question most frequently asked is: Does Botrytis allii spreads from infected onions to sound onions in storage? We conducted studies in the Vidalia Onion Research Laboratory at the Coastal Plain Experiment Station during the past onion growing season to determine if Botrytis allii spreads from inoculated onions to sound onions during storage in refrigerated air and CA (3% O₂ and 5% CO₂).

Mechanically harvested, cured and graded Jumbo Savannah Sweet onions were carefully examined to insure that only sound onions free from external blemishes were stored. Storage treatments consisted of: sound onions, sound onions plus five onions inoculated with Botrytis allii, bruised onions, bruised onions plus five onions inoculated with Botrytis allii, cut onions, and cut onions plus five onions inoculated with Botrytis allii. The onions were bruised by dropping them individually onto a concrete floor from a height of 30 inches. The cut onions had two one and one-half inch gashes, one-fourth inch deep, at the equator on opposite sides of the onions. Each treatment was replicated four times with 84 onions per replicate. The onions were stored in either CA (3% O₂ and 5% CO₂) or refrigerated air at 34EF and 65% RH for five months. At the end of storage, the temperature of the storage rooms was increased gradually until it was the same as the outside temperature to prevent the onions from sweating. The onions were graded immediately upon removal from storage and again after two weeks at room temperature.

Petri plates were inoculated with pure cultures of Botrytis allii, Botrytis cinerea and Botrytis tulipae and stored in eight gallon sealed containers continually flushed with air or 3% O₂ and 5% CO₂ at a rate of 340 milliliters per minute. The containers were kept at temperatures of 68EF, 50EF and 32EF until the fungal colonies had grown several millimeters, at which time they were measured and the amount of sporulation determined. Sporulation was rated on a scale of 0 to 5 and growth expressed as mm/day. Each treatment was replicated five times.

Results indicate that the primary infection of Vidalia onions by Botrytis allii most likely occurs in the field prior to or during the harvesting, curing and handling operations prior to storage. The fungus grew in inoculated onions in CA and refrigerated air storage. Inoculated onions stored with sound onions did not infect the sound onions, but did infect bruised onions stored in air and onions with cuts in CA and air. Thus, Botrytis allii does spread from externally infected onions to bruised onions and onions with cuts, but apparently not to sound onions.

All three species of Botrytis (B. allii, B. cinerea and B. tulipae) grew in culture at about the same rates in both CA and in air. The rate of growth was much greater at 68EF than at 50EF and 32EF. Nevertheless, all three species grew at a rate of about one millimeter/day at 32EF, which means that during storage of onions for three to four months considerable growth can occur.

Botrytis allii produced more spores than the other two species in culture, but produced spores only in air and not in CA. None of the three species produced spores at 32EF and only Botrytis allii produced spores at 50EF. Botrytis cinerea was the only species that produced spores in both CA and air storage.

Sound, bruised and mechanically injured onions can be inoculated with Botrytis allii spores during harvest and from contaminated pallet bins and packing line equipment. The spores, however, will germinate only when free moisture is present, such as in bruised and mechanically injured onions or when onions sweat during the warm up period after low temperature storage. Therefore, maintaining a low humidity around individual onions during storage is extremely important in reducing infection by Botrytis allii. Once the spore has germinated Botrytis allii grows mycelia during storage of the onion at low temperatures. Since spread of the fungus is by spores, it is important to reduce the inoculum by sanitizing pallet bins, equipment and storage rooms and then store only sound onions.

Fig. 1. Effect of mechanical damage and storage with onions inoculated with Botrytis allii on number of marketable onions after five months of storage in CA (3% O₂ and 5% CO₂) and refrigerated air at 34EF and 65% RH. Eighty four onions were stored with or without five inoculated onions. Means and standard deviations of four replications. Means with the same letter are not significantly different (P = 0.05).

Fig. 2. Effect of mechanical damage and storage with onions inoculated with Botrytis allii on spread of Botrytis allii during five months of storage in CA (3% O₂ and 5% CO₂) and refrigerated air at 34EF and 65% RH. Eighty four onions were stored with or without five inoculated onions. Means and standard deviations of four replications. Means with the same letter are not significantly different (P = 0.05).

Fig. 3. Effect of temperature and atmosphere on the growth of Botrytis spp. in culture. Eight-gallon sealed buckets were flushed with air or 3% O₂ and 5% CO₂ at a rate of 340 ml/minute. Means and standard deviations of five replications.

Fig. 4. Effect of temperature and storage atmosphere on the sporulation of Botrytis spp. in culture. Eight-gallon sealed buckets were flushed with air or 3% O₂ and 5% CO₂ at a rate of 340 ml/minute. Means and standard deviations of five replications.