Bacterial Leaf Streak: A New
Problem or an Old One?
Bill Meyer, Woodbury, Connecticut
summer, Chris Wilson of Hallson Gardens sent me the leaf photo at right.
He was curious about what I thought regarding the damage to it. At first
I thought nematodes, then I noticed near the tip of the leaf that the
damage crossed the vein. Nematodes can't do that, so this must be
something else but I didn't know what.
Chris is probably the most perceptive nurseryman in hostas,
and works as hard as anyone to keep a clean nursery. He
had found something interesting here. He cut a sample and put it under
the microscope, and it wasn't surprising that no nematodes appeared.
There was a milky substance oozing from the cut through the damaged
area. Going to a higher magnification, he saw that the milky ooze was
swarming with bacteria. This is known as "bacterial streaming"
- the first check to see if bacteria is the cause of the lesion.
Neither of us could find any information on bacterial
diseases in hostas other than bacterial soft rot which affects the
underground parts of the plants. Soon we were both wondering if this had
been with us all along tricking us into thinking we were seeing
Bacterial leaf diseases are
nothing new in other plants, but there has been little investigation into
them in hostas. In general they are thought of as causing small round
spots, and being insignificant in either frequency or in the amount of
damage they do, except in tissue culture labs. They are a common problem
in annual crops, particularly in peppers, and are caused by a number of
different bacteria species.
The foliar nematode plague unleashed by EPA nematicide bans
in the 1990's has become so widespread in the hosta world that it seems
most gardens and many nurseries have them these days. With no effective
nematicides available for home use, and only a few states allowing them
for nurseries, we have learned we have to live with them whether we like
it or not. Such is the way the world is now. So, when we all see the brown
stripes late in the season, we see nematode infection or think we do.
After seeing that photo and learning it was bacteria that caused
that damage, I began looking a little more carefully around the
garden here. What I thought was nematodes turned out to be
bacterial damage wherever I found the brown stripes in the leaves.
I couldn't even find anything that on closer examination was
clearly nematodes, but the bacterial damage was in quite a few
hostas. Have we all been mistaking this for nematodes for decades?
By late summer, nematode and bacterial damage can
look surprisingly similar, especially in a quick casual
inspection. They look much different earlier, though. When foliar
nematodes infect an area between the veins on a hosta leaf, the
tissue slowly turns yellow then brown as they multiply and kill
cell after cell with their feeding. After a month or so, the area
becomes uniformly brown with no living cells left.
watersoaked tissue. This is not the work of foliar nematodes.
| With the bacterial
infection, the area between the veins quickly takes on a
waterlogged appearance similar to the edema seen early in the
spring. The bacteria have destroyed the cells and released their
contents. The waterlogged area quickly turns brown and may not
completely fill the space between the veins, and may cross a vein.
Another common symptom is a fan pattern at the base of the leaf
with multiple veins involved. The brown stripes also can appear
much earlier than nematode-caused ones.
The photos in this article are all of
bacteria-infected hosta leaves - not foliar nematode symptoms. The
waterlogged areas are the main clue as to the source of the
The first sign of
bacterial leaf streak is waterlogged areas between the veins.
Within a week or so they start turning brown.
Identification and Treatment
There are a number of
bacteria species which could turn out to be the culprit - Erwinia sp.,
Pseudomonas sp., Xanthomonas sp., and Acidovorax sp. The latter would be
my guess, based on the similarity of symptoms seen in this article from
the University of North Carolina - HERE.
Hopefully we will be able to get the bacteria identified next summer.
Region One has committed to sponsoring the identification if funding is
While it would be nice to have it identified, it won't make
any difference in treatment as most bacterial leaf diseases are treated the
same way. The only commonly available antibiotic for plants is
agricultural streptomycin, and it is only effective on some strains of
some species. Once a leaf is infected there is no cure, but unless it
turns out to be an Erwinia species the infection is probably limited to the
The general advice for bacterial disease prevention is to
spray before the infection is seen with a copper-based fungicide,
although some report better results with mixing streptomycin with the
fungicide. This forms a barrier to keep the bacteria out of the leaves
rather than curing infected leaves. Leaves showing symptoms should be
immediately removed to keep them from becoming a source of infection for
In the fall, all dead foliage and flower stalks should be
cleaned up as thoroughly as possible. Not much is known at this time
about how bacteria like these overwinter, and we do not have an
It was quite a surprise to me to find out that much of what I
thought was nematodes was likely bacterial infection all along.
Photos shown on forums and Facebook last year by people thinking
they were of nematode infected leaves were in many cases clearly
the bacterial damage instead. This makes me wonder how widespread
this bacterial disease has become. Nematode articles at university
sites even used photos that look more like the bacterial leaf
streak. One photo (right) was particularly troubling as it was of
an infected plant just received from a wholesaler, indicating that
this bacteria was in the supply chain and being included as a free
"gift" to hosta buyers.
| From what I saw last
year after learning the difference in symptoms, I wonder now how
common this bacterial problem really is in hosta gardens and nurseries. It
seems possible that half or more of what we think are nematode
infections are just this instead. I expect this problem has been
with us all along, and just flying under the radar with its
similarity to nematodes. Thank you Chris, for seeing what the
rest of us have been missing.
Next season we all need to start looking more closely
at those brown stripes when the damage is in the early stages in
mid-summer and get a better understanding of how much this has
spread. Retail nurseries need to keep a close eye on new material
from wholesalers for signs of infection, especially the watery
stripes between veins, as this disease can spread fairly quickly
through their inventory. Any infected shipments should be returned
or destroyed, and the seller made aware of the problem with
their plants, and their state Agriculture department notified if
they continue selling infected plants.
|Note how bottom
stripe mimics nematodes perfectly
||This fan pattern is common with bacterial leaf
| Gardeners should keep
an eye out for the waterlogged early symptoms beginning in June
and immediately remove infected leaves, being careful to avoid
touching anything before washing hands after touching those
leaves. Dispose of the leaves immediately to prevent further
infection. Do not compost them or keep them around in a trash can.
It differs from edema in that the waterlogged areas are
mostly defined by the veins, where in edema the areas are random.
In the garden edema is common during cold wet days in early
spring, but not seen after that, but the bacterial infection can
show up throughout the season.
Spraying a copper-based fungicide as a preventative
is a good idea once symptoms have been found in any case, and it
should be understood that products sold for nematode control will
have no effect on bacterial infections and vice versa, so it
should be determined whether the problem is bacterial or nematodes
before treatment. Keep an eye out for early symptoms before
deciding on treatment.
At this point it seems that this bacterial
infection is less a problem than nematodes. It is likely that it
only affects the leaves and doesn't get into the rest of the
plant, so removing the leaves quickly will go a long way from
keeping it from spreading. One thing to keep in mind, though, when
handling infected leaves is that bacteria can be airborne. If it doesn't overwinter in the plants
or in the ground, as seems likely, fungicide, leaf removal,
and good thorough fall clean-up may be all that's necessary to
keep it out of the garden.