With the late start to planting, wet weather and late harvest, 2014 was a year many farmers are glad to have behind them. Despite this, many edible bean growers ended the season with good to excellent yields, but an increase in bacterial disease. Bacterial blights are worrisome when they strike as they typically can hurt both yields and seed quality.
There are three major bacterial blights of edible beans:
- common bacterial blight(CBB),
- halo blight (HB), and
- bacterial brown spot (BBS).
The adzuki bean crop was especially hit hard in 2014 by a serious outbreak of bacterial brown spot. Infections were first noticed in late July when hot spots spread quickly as wet, high humidity, windy weather prevailed. The disease continued to spread in fields until drier mid-August. By then, widespread infections of leaves, stems, and pods had occurred that resulted in poor yields and quality.
CBB is the most commonly occurring blight in dry beans. This was the first time BBS has been found in any edible bean type in Ontario. Other bean types can host the disease, but adzuki beans appear to be especially susceptible. Some Ontario snap bean growers also reported damage from BBS in 2014.
Where Does Blight Come From?
Infected seed is the initial source of bacterial infection in an area. Infected bean residue and weather play an important role in its spread. Serious outbreaks of CBB and HB develop when contaminated seed is planted. BBS can become established through infected seed, but can also survive on healthy plants for a long time without causing disease symptoms when conditions are not favourable. Unlike CBB and HB, these 'resident' BBS are considered the main source of infection. Other 'resident' host plants for BBS include corn, soybeans, other dry bean types and hairy vetch. Thus, it is possible for BBS to exist in an area before an outbreak occurs.
How Is Bacterial Disease Spread?
Long distance dispersal can occur on storms. The bacteria are thought to be able to travel in the atmosphere up to 100 miles. Spread from host plants can also occur by people, equipment, animals, insects and rain splash. When infected seed is planted and emerges, bacteria ooze from leaf surface and spread through rain splashed onto other plants. A severe epidemic can occur from only a few infected planted seeds and spread in a field with favourable weather conditions. Bacteria multiply rapidly and can more than double in as little as 30 minutes. Bacteria enter plants through natural openings (stomata) and through wounds caused by hail, driving rains, wind damage, blowing soil injury, insects, or equipment injury. Hard, intense rain storms have been shown to be especially conducive to the spread and multiplication. Localized thunder cells can be the source of infection in one field but not another just a few kilometers away.
Once introduced into a plant, the bacteria easily spread systemically though the leaves, stem, pod and into seed. HB is favoured by rainfall, high humidity and moderate temperatures (180 - 220 C), while CBB and BBS thrive under high temperatures (280 - 320 C).
Symptoms
Symptoms of CBB and HB can appear very similar. Initial leaf symptoms of all 3 blights appear as small (3-5 mm), water soaked spots that later turn pale green and eventually brown. With CBB and HB, the lesions enlarge, with the center becoming dry and may be surrounded by a yellow margin. Leaves can become ragged. Pod lesions start as water soaked spots that enlarge, merge and form sunken reddish-brown blotches. Cankers can appear greasy as bacteria ooze, and later dry to form a crust.
Leaf lesions of BBS often don't appear water soaked, and are much smaller than CBB and HB. When the disease becomes systemic, tan and sunken lesions with reddish brown border develop on stems and petioles. Pods may appear bent or have water soaked lesions with a reddish brown margin.
Figure 1: Common Bacterial Blight (CBB) mature lesions
Figure 2: Bacterial Brown Spot (BBS) - Adzuki beans
Figure 3: Bacterial Brown Spot (BBS) - Adzuki pod infection
How Effective Are Copper Bactericides?
Fungicides have no effect on bacteria blight diseases. Copper based bactericides can reduce growth of bacteria on foliage and limit spread to healthy foliage and pods. Applications are most successful when applied as an early preventative treatment during vegetative stage, because bacteria multiply and spread rapidly. Only poor to moderate control is achieved once an outbreak occurs because of the high level of bacteria inoculum usually already present in a field.
The first application should occur before symptoms appear and repeated every 7-10 days if conditions are favourable. Optimum application timing is following a rainstorm. In fields damaged by hail or violent storms, application of a bactericide can help protect plants if conditions are favourable for infection. Research has shown that copper bactericides are generally not as effective against CBB as BBS and HB.
What Else Can You Do?
- Plant certified seed from a known source! Seed that has been grown in semi-arid regions (i.e. western grown) has very low risk of being infected.
- Plant white beans resistant to CBB. White bean varieties are resistant to HB. Current white bean varieties with CBB resistance include OAC Rex, Lighthouse, Mist, Apex and Rexeter.
- Streptomycin seed treatment will reduce or eliminate surface contamination, but it does not control infections in cracks or under surface of seed.
- Do not grow beans (edible or soybeans) in the same field more than once every 3 years.
- Tillage of infected residue will speed breakdown and reduce pathogen survival. Bacterial pathogens survive longer on bean residue left on the surface after harvest.
- Sanitize cultivators, sprayers or other equipment between fields.
- Stay out of fields while foliage is wet.
- Avoid planting edible beans next to fields where beans were infected the previous year.
- Closely check bean fields beginning at the mid-vegetative stage, especially during periods of high humidity. Infections following violent storms with strong winds, or hail often begin to become visible 7-10 days later.
- Control volunteer adzuki beans.
Source: OMAFRA