DISEASE IDENTIFICATION in the garden is sometimes a difficult thing to accomplish correctly as there can be many overlapping and similar symptoms. However, determining what is plaguing your plants is an important task to accomplish, not only for hopefully saving your garden, but for determining what to replace it with if saving it is not an option.
The following rundown is on a David Viburnum (Viburnum davidii) that went from healthy to dead in a matter of just a few weeks. While I am pretty certain of its ultimate diagnosis, I will say that the only real way for me to be sure is to take a sample to my local extension office and have them test it. However, if I were to go test every plant I came across that was sick, I would be at the extension office quite a bit. Ain’t nobody got time for that. With a little sleuthing, you can figure out what is going on by yourself. If it ends up stumping you, then give the extension office a whirl.
Start by gathering all the relevant information. Growing conditions (soil, sun exposure, time of year, temperatures for the period prior to illness (could be months back – pay attention), mulch layer, etc.), broken branches, cankers (and locations), mold/mildew/fungus presence, leaf conditions (not all “spots” on a leaf are the same – fungus spots are different than bacterial spots), interior wood appearance (cut a branch to see), and plant history/age are all some of the things to take note of.
Once the basic information is down, it is time to start looking for what the cause could be. A good disease ID book or website with their identification rubrics can help to run through the symptoms. It doesn’t hurt to simply look through the web and familiarize yourself with the different possibilities before trying to figure out what went wrong with yours. Phytophthora? Botryosphaeria? Verticillium Wilt? Old Age? Mildew? Root Rots? There are so many different causal agents out there it will make your head spin – at first. But like everything, a little practice goes a long ways.
So, here is the poor little shrub. It started out this year in great shape, minus the poor pruning job someone had subjected it to the previous year. Its home is here in the Willamette Valley of Oregon.
As you can
see, there is not a heck of a lot of green left (none, really). It went from
bright green and healthy to this in a time span of about 2-3 weeks.
Here is what
I observed:
- Died in mid-September
- Located on the NE side corner of the building between two houses
- No digging or other landscape construction work recently
- Hot summer sun in the early evening during the summer
- This past summer was HOT and DRY. Very little supplemental watering/irrigation and rainfall had been non-existent since April
- No mulch
- Leaves turned brown and leathery with large, black splotchy areas, but no real “spots”
- Previous poor pruning evidence, dead “stumps,” some broken (split) branches
- No good canker evidence
- No obvious fungal growths
- Still firmly held into the ground
- No obvious signs of insect infestation above or below ground
- No real ability to determine age, but could possibly be 70 years old, part of the original house landscaping.
- Dead tissue started with newly set berries, then one branch (which was pruned off) then the entire plant. From the branch pruning to whole plant death was in the neighborhood of 5-7 days. Whole process was 2-3 weeks.
- Vascular staining in the interior wood starting at the outermost (cambium/cork cambium) layer and in towards the sapwood.
 |
Brown, Leathery Leaves with Black Splotches
|
 |
Vascular Staining (the dark stuff)
|
I ruled out the Phytopthora spp. of fungal infections (which can cause various root rots and Fire Blight) due to the simple fact that most phytophthora attacks will happen following periods of wet weather. This is definitely NOT what this plant experienced. This viburnum experienced drought, not drowning. Although the speed at which the plant died could represent a Fire Blight attack, the other symptoms just weren’t adding up.
I thought Botryosphaeria might be a possibility, due to the way the leaves died leathery and the fact it was showing evidence of vascular staining in the wood. Also, Botryosphaeria attacks can often happen following periods of drought stress on a plant. Hmmm. Ok. Makes sense. The one thing I was not seeing was any evidence of cankers on the branches, which is a big indicator of Botryosphaeria attack.
I started leaning towards Verticillium Wilt due to:
1. It is rampant in this area.
2. Viburnums are susceptible to Verticillium
2. Viburnums are susceptible to Verticillium
3. Vascular staining
4. Leathery leaf death
5. No cankers
6. Fast death
However, I am not a disease specialist by any means. So, I ran it by my good friend Chris at the University of Wyoming. He seemed to agree that Verticillium was most likely the cause (Thanks Chris!!). He basically said that even though the plant had been well established, there may have been the perfect storm of disease presence, poor pruning, drought, and stress that it simply overwhelmed the plant and BOOM! Bye bye. Makes sense to me.
So that left me with the question of what to do. Obviously, the Viburnum is not going to make it. No amount of pruning is going to salvage this situation, so it needs to go. Replacing it, however, is not so straightforward. Verticillium Wilt is a soil-borne fungal infection and can remain in the soil for years and there is no curative treatment. Planting a shrub that is susceptible to Verticillium will more than likely result in another dead plant in the not-to-distant-future. The replacement plant needs to be Verticillium resistant.
As this is getting to be a long post, I will continue the discussion tomorrow. I have a couple of good resource sites I will share with you in the next post, including a list of Verticillium susceptible and resistant trees and shrubs.
Cheers!
