Bacterial brown spot in snap beans
Bacterial brown spot is a serious disease of snap beans particularly some yellow varieties. The disease has caused damage in a few snap bean fields in Ontario this past month. It is caused by bacteria (Pseudomonas syringae pv. syringae) that overwinter in previously infected snap bean debris or on contaminated seed. Once introduced into a field the bacterial pathogen can infect and multiply on emerging volunteer snap bean seedlings. The bacteria have also been found to survive for a period of time on the surface of plants without causing disease symptoms.
Symptoms of bacterial brown spot on snap beans first appear as small water soaked lesions usually observed on the underside of leaves. As the lesions mature, they turn brown and the dead tissue in the center may fall out giving the infected leaf a "shot hole" appearance (Figure 1). Sometimes a small light yellow or pale green border may surround the lesions. The bacteria can also infect pods resulting in brown water soaked spots making them unmarketable (Figure 2).
Figure 1. Mature bacterial brown spot lesions become brown with dead grey tissue in the center.
Figure 2. Bacterial brown spot lesions on snap bean pods appear as brown water soaked spots.
Environmental factors have a significant effect on the incidents and severity of bacterial brown spot in snap beans fields. Rain splashing or overhead irrigation spreads the pathogen to other plants throughout the field or into neighbouring fields. Epidemics and severe localized disease outbreaks are favoured by high humidity with moderate to warm temperatures (26°C). Driving rains or hail which creates wounds for the pathogen to infect leaves often exacerbate infection and disease development and incidence. The bacteria can also infect through natural openings in leaves and pods but the disease is often most severe after a heavy rain or storm with damaging winds followed by warm days with high humidity. These ideal conditions have been experienced in many locations across southern Ontario earlier this season. The bacteria can be further spread on contaminated clothing or farm machinery when plants are wet.
Apply registered copper products with sufficient water to penetrate the canopy every 7 to 10 days particularly if the environmental conditions are conducive for infection and disease spread. Follow the directions on the label particularly the pre-harvest interval (PHI). Research has shown that applying registered copper products even after the disease becomes established in a field does not eliminate the disease but significantly reduces its severity and spread. If hail or a storm with damaging winds occurs in a field regardless if the disease is present or not, apply at least one preventative copper application immediately after the trauma event.
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