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Managing Bacterial Soft Rot of Broccoli Heads

by Margaret Tuttle McGrath, Department of Plant Pathology, Long Island Horticultural Research and Extension Center, Cornell

and

Craig H. Canaday, West Tennessee Experiment Station, University of Tennessee, 605 Airways Blvd., Jackson, Tennessee

Several different bacteria can infect the head of broccoli and cause a soft rot (Figure 1). This disease affects production in many states. The first symptom is water-soaked areas involving only two to three florets (Figure 2). These tiny lesions can become large, brown, rotting, slightly sunken areas in just five to seven days when weather favors disease development (Figure 3). Long periods of wet weather and warm temperatures are favorable. Tiny lesions can make a head unmarketable, even if stored in a cooler, because the soft rot will continue to develop even at low temperatures. Growers have lost their entire broccoli crop to soft rot.

Several years of research in Tennessee have provided the information needed to design an integrated management program for bacterial soft rot. Control is based on cultural practices because chemical control has not been effective.

Select resistant varieties. Shogun, Arcadia, and Marathon have performed well in several evaluations. For example, in 1994, soft rot was observed on only 4.5 to 9 percent of the heads of these varieties while 34 to 64 percent of the heads were affected with Emperor, Green Comet, Green Valiant, Mariner, Packman, Paragon, Premium Crop, Southern Star, and Sultan. There were no significant differences among Shogun, Arcadia, and Marathon in bacterial soft rot incidence, severity, number of heads harvested, or head diameter. Arcadia has a very attractive head with a purplish hue; it is a few days earlier than Shogun (Figure 4). Arcadia has become a popular variety in Maine. Shogun has a very large stem.

The varieties that are resistant to bacterial soft rot have dome-shaped, tight heads with very small beads (Figure 4). Other varieties with these characteristics are also likely to be resistant. Resistant varieties have blue green foliage; however, some susceptible varieties also have blue green foliage. Varieties with a flat head are more susceptible to soft rot because water will tend to sit on the head, providing favorable conditions for soft rot. Water will flow off a dome-shaped head.

Warm, wet conditions favor soft rot development. Therefore, select fields with good drainage. It is best to irrigate when heads are already wet, such as at night when dew is present. The next best choice would be during the day when heads are dry at the start and conditions will favor rapid drying afterwards.

Avoid high nitrogen rates with susceptible varieties. Increasing nitrogen does not increase the amount of disease with resistant varieties. This is fortunate since 100 to 120 pounds of actual nitrogen per acre are needed to obtain large, attractive heads. In an experiment conducted in Tennessee in 1989, incidence of heads with soft rot for the susceptible variety Premium Crop was 53 percent when fertilized with only 100 pounds of ammonium nitrate (34-0-0) per acre but 84 percent when fertilized with 300 pounds per acre. In contrast, soft rot incidence for Shogun was 8 and 5 percent, with 100 and 300 pounds per acre, respectively.

Surfactants, which are in most insecticides, also have been shown to increase soft rot severity. Therefore, get insects under control before soft rot begins to develop. If an insecticide must be used when soft rot is present, applications should be made when rain is not forecasted.

Finally, cut heads such that the stem stump is angled to permit water run-off. Water pooling on a flat stump will provide favorable conditions for soft rot (Figure 5). Bacteria in rotting stem stumps can be dispersed to heads not yet harvested.

Click on each photo for Magnification

Figure 1 Figure 2 Figure 3
Figure 4 Figure 5

Figure 1. Advanced stage of bacterial soft rot. Many secondary soft rot bacteria are present on this broccoli head.

Figure 2. Early symptoms of soft rot are small water-soaked lesions.

Figure 3. Bacterial soft rot can reach this stage in just five to seven days after disease initiation when weather favors disease development.

Figure 4. Varieties such as Arcadia are resistant to soft rot because they have dome-shaped, tight heads with very small beads.

Figure 5. Bacterial soft rot developed following harvest on this stump because it was cut flat rather than angled to permit water run-off.

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