Recent Bacterial Diseases of Beans
Co-workers,
Recently, I received a call from
some growers in the Sumter-Lee-Macon Co. area regarding a bean disease that is
primarily damaging the pods. We also
received some pole bean pods from Grady Co.
This disease has been identified as either common blight, caused by Xanthomonas campestris pv. phaseoli,
or brown spot of snap bean that is caused by the bacterium Pseudomonas syringae pv. syringae. Both
are bacterial diseases not commonly seen in GA.
With common bacterial blight, leaf symptoms initially appear as water soaked spots that become necrotic, light brown lesions of irregular shape with distinct, bright yellow halos. These lesions enlarge to 10mm or more and may kill the leaflet. Similar water-soaked spots form on the pods and enlarge into reddish brown lesions and are very similar to brown spot in appearance. In humid weather, a yellow bacterial ooze maybe present on the pod lesions. Infected developing seeds may abort or shrivel and discolor as they mature.
Brown spot symptoms on leaves start out as small, water-soaked spots
that gradually get larger, and become reddish-brown in appearance.
Once these lesions grow together they produce leaves that have a bronzed
and tattered appearance. Sunken, water-soaked spots can form on pods. Older
lesions can turn necrotic and brown. If
infection occurs early in pod development, the pod may become bent or twisted
at the infection site. See attached
pictures from Cornell.
Both diseases can be seedborne and seedborne inoculum is usually how it is introduced into an area. Both diseases can also survive and spread
from legume weeds. However, the disease
becomes a problem only when the bacteria build up and spread on leaves from one
crop (usually without causing symptoms), then another bean crop is planted
later and the correct environmental conditions allow the disease to infect and
spread throughout the field. In other
words, the disease can build up in bean fields undetected and can survive on
old plant debris until the right environmental situation causes an
outbreak. Environmental conditions that
lead to common blight outbreaks
include: 1)long periods of wet, humid weather; 2)warm
temperature between 82 and 90EF.
The environmental conditions that favor brown spot are: 1) long periods of wet weather or high humidity; 2)
plant damage from machinery or high winds; 3) sustained moderately cool
temperatures below 80EF.
Overhead irrigation or rain coupled with high wind cause much of the
field to field or in-field spread. Once either
disease has spread through a field, it can be spread to other fields by
machinery or by people walking (scouting) fields. Common
blight can also be spread by insects.
Controlling
either disease is done primarily through prevention. Do not save seed to plant the next year but
purchase seed that has been grown in dry regions where the disease has not been
known to occur. Rotate away from beans
(snap, pole, lima) for 2 years to allow bean crop residue to break down, thus
eliminating the inoculum bridge from one crop to the
next. Deep turning residue helps debris
break down faster and cuts down on pathogen survival. Do not work in fields when foliage is wet. Wet foliage allows the disease to be
picked-up more readily by equipment and people to be transferred to other
fields. If a field has been identified
with the disease, try to keep personnel and equipment from entering that field
and then going to other fields before being cleaned. A 10% sodium hypochlorite (Clorox) rinse will
disinfest most surfaces if used quickly after it is mixed. Copper sprayed on a #7 day schedule will help reduce
in-field disease spread. Below are
pictures of both diseases obtained from a Cornell website.
a. Brown spot on pods. b. Brown spot on leaves. c. Common blight on leaves.