Bacterial Leaf Streak: A New Problem or an Old One?
Bill Meyer, Woodbury, Connecticut
   Last 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 nematodes.
   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.

Telltale 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 damage.

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 necessary.
   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 leaves.
   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 surrounding plants.
   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 identification yet.

Conclusion

   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 streak
   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. 

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