Raspberry crown borer can be hard to control
My well-established raspberry patch seems to be dying. Leaves are turning brown and have not flowered to produce fruit. It seems like the whole cane is drying up and dying!
The patch is watered heavily at least once per week from our irrigation ditch, with an occasional light watering mid-week. The dying canes do not seem to lack water because they are dying in the area which receives the most water while canes on the perimeter are OK.
This is the first time in five years the patch has shown any sign of distress. Is some kind of blight or fungus going around the area?
I think the culprit is an insect called the raspberry crown borer. This little critter bores down at the base of the cane and into the crown of the plant. The first symptom of it is that the cane starts to wilt and die back.
You can confirm this by cutting into the cane down near the base. You should see sawdust-like material in the center.
There’s a second borer called the raspberry cane borer that does much the same thing but it starts up toward the tip of the cane and tunnels down through the center. If there’s wilting just at the tip, then it’s probably cane borer.
Controlling cane borer takes a bit of work and persistence. The first and most important thing to do is to remove any canes that are infested. If this is the case with most of the canes, I strongly would consider replacing the plant with a new one.
Otherwise, use insecticide drenches at the base of the plant after you’ve picked the fall crop of berries in September and again a month later.
The best insecticide to use is Bifenthrin. The problem is finding an insecticide that is labeled for this pest. There are commercial products containing Bifenthrin that are labeled for it. The problem is that the homeowner products I’ve looked at that contain this chemical don’t list the borer on the label.
The way the law is, I can’t recommend something contrary to what the label says (sheesh!).
I also have seen Permethrin recommended for borer, but I can’t find the borer on any label I know.
Since this insect often has a two year life cycle, you’ll need to treat your plants again next year, and I’d recommend doing it another year after that just to be sure.
An alternative is to drench the base of the plant with a parasitic nematode (Steinemema species) in late April or early May. This nematode is a tiny round worm that preys on grubs. You probably have to find this online.
Be sure to water the bed well before applying them and again afterward. They need to stay moist to survive. This is a safe, organic way to get the job done.
The second possibility is that you have a fungal crown rot or root rot in your plants. I don’t think this is what’s going on, but it’s a possibility. These types of fungi usually need a high water environment in the soil or at the base of the plant to thrive. You mentioned that this spot gets the most water of the patch.
I really don’t have any fungicides to recommend to you. There are some commercial products out there, but they’re EXPENSIVE! The best way to go about it is to make the soil environment less favorable for the fungus and better for the raspberry.
Don’t let any standing water accumulate at the base of the plant, and allow the soil to dry a bit before watering deeply again.
If you lose any plants to a crown rot, you could replace them, just be sure to completely remove the old plants and do a thorough job amending the soil before replanting by mixing in a good amount of decomposed organic matter like Soil Pep or compost.