Raspberry Leaf Spot
Asked September 21, 2018, 1:13 PM EDT
Hello. I have an established raspberry patch of 4 years which has had fairly decent berry production in past 2 years. It is in a low-lying ravine area that frequently floods, which I now understand is the worst place for them. Soil is rich and drains ok, has a slight bit of clay. In May 2018 we had 50 year HISTORIC flooding in Southwestern Michigan which actually collapsed roads. This ravine and raspberry bushes had 10 feet of STANDING WATER FOR 24 - 48 HOURS AND RASPBERRIES WERE COMPLETELY SUBMERGED. The patch was covered in a thick mud that would not wash off, even after spraying and weeks. I removed many of the due to spotting and what appeared to be rot. Over the summer, new growth emerged lower, and appeared to be ok. I trimmed ALL of the branches with wilting and spot, and now -- even in September 2018 -- much of the new growth continues to develop spotting. I am wondering if the ground is contaminated. Even new plants which we got from another location, and planted in a new row 5 feet over, are showing spotted leaves. Can this be saved? I will start a new patch in higher, better draining ground with new plants- but would like to save this patch! Please tell me what to do. Do I need to treat the ground for contamination? I will contact the county and find out if the standing water contained sewage or not. Thank you so much.
Yes we got a lot of rain this year and as I read you post I thought you might have any of several root diseases, but when I saw the pictures I knew that you did not have phytophthora or verticillium so I think they will survive.
Your raspberries have Raspberry Leaf Spot.
To control the disease you will need to spray with a fungicide.
It is too late this year to stop the disease.
Next spring apply a dormant spray of either copper or lime sulfur to reduce the overwinter inoculum. This should be done before the leaves come out.
You also need to protect the young leaves and shoots as the appear. Only the young tissue are susceptible to the disease. Our wet spring was good for a lot of plant diseases.
The common homeowner fungicide Captan is not very effective on this disease. You need to use a material such as Spectracide Immunox Multipurpose Fungicide.
Be careful you get the fungicide and not one of the other Immunox products.
You should spray about once every 10 days early in the season. If it is rainy I would shorten the interval to 7 days and if it is dry you can slack off on the sprays.
Removing diseased canes and leaves and clean up weeds and thinning out the stand of primocanes will shorten the drying time and decease the disease.