treatment for needlecast - Ask Extension
I have two spruce trees (each over 100 years old) being treated for needlecast. Each has received: 3 treatments for needlecast, the soil turned over a...
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treatment for needlecast #874084
Asked June 21, 2024, 1:34 PM EDT
I have two spruce trees (each over 100 years old) being treated for needlecast. Each has received: 3 treatments for needlecast, the soil turned over around them and 3" of mulch to cover. Both Forthiphite and Boost liquid have also been applied over the past year. Both trees now look good, with ample new growth. Do I need to continue with the treatments and fertilizers? (I leave fallen debris under the tree to compost.)
Baltimore County Maryland
Expert Response
The branches pictured look good, and we don't see signs of needle cast infection, either because it might not have been present in prior seasons (symptoms can be easy to confuse with other ailments), or because the treatments might be working well. Since fungicides have a finite lifespan after application, they usually need repeat use every year when the spores of the disease in question are dispersing and causing new infections. It's not uncommon for applications to be needed several times per year, in fact. Once treatments begin, if the disease is prevalent in the area, then they would have to continue for the life of the tree if it doesn't outgrow vulnerability. (Which wouldn't apply in this case, since spruces tend to struggle in our summer heat, humidity, and more compacted suburban soils in areas with clay.)
The hassle and expense of treating a large mature tree, other than the potential for treatments to not work fully in years with uncooperative weather or high disease pressure, is one reason why we discourage fungicide use for diseases like needle cast. Plus, if an infection does set in, fungicides can't cure existing disease; they only help to suppress infection of new parts of the plant that are still healthy at the time of treatment. A delay between infection and the development of disease symptoms can make it hard to tell at first if fungicides are providing benefit.
While plant disease spores sometimes do splash-up onto foliage and branches from the soil surface, in this case, needle cast spores produced by already-infected branches serve as the source of new infections. This means that disturbing the soil to overturn it won't provide much benefit, and in the process risks stressing or damaging a lot of fine feeder roots, increasing the tree's vulnerability to pests or disease.
The Forti-Phite brand you mention is actually a collection of different products, so we don't know which type was used here (same for the Boost). Even so, unless a laboratory soil test indicated a nutrient deficiency in the spruce's root zone, supplemental nutrients are probably not needed and might be going to waste, since adding more when the soil already has sufficient levels will not necessarily stimulate more growth. Plus, fertilizing plants under any type of stress can make the situation worse; sometimes forced growth can be more vulnerable to pests and infections. We don't recommend continuing the use of fertilizer unless testing has shown that certain key nutrient levels are still low in the soil in that area of the garden.
Miri
The hassle and expense of treating a large mature tree, other than the potential for treatments to not work fully in years with uncooperative weather or high disease pressure, is one reason why we discourage fungicide use for diseases like needle cast. Plus, if an infection does set in, fungicides can't cure existing disease; they only help to suppress infection of new parts of the plant that are still healthy at the time of treatment. A delay between infection and the development of disease symptoms can make it hard to tell at first if fungicides are providing benefit.
While plant disease spores sometimes do splash-up onto foliage and branches from the soil surface, in this case, needle cast spores produced by already-infected branches serve as the source of new infections. This means that disturbing the soil to overturn it won't provide much benefit, and in the process risks stressing or damaging a lot of fine feeder roots, increasing the tree's vulnerability to pests or disease.
The Forti-Phite brand you mention is actually a collection of different products, so we don't know which type was used here (same for the Boost). Even so, unless a laboratory soil test indicated a nutrient deficiency in the spruce's root zone, supplemental nutrients are probably not needed and might be going to waste, since adding more when the soil already has sufficient levels will not necessarily stimulate more growth. Plus, fertilizing plants under any type of stress can make the situation worse; sometimes forced growth can be more vulnerable to pests and infections. We don't recommend continuing the use of fertilizer unless testing has shown that certain key nutrient levels are still low in the soil in that area of the garden.
Miri
Thank you, this is very helpful. I am confused however, because in the first paragraph you say that the fungicide needs to be continued, but in the second paragraph you don't recommend them. Would you mind clarifying this for me please?
I appreciate your help,
Amy
Hello Amy,
Certainly -- once a fungicide routine has begun, if you want to maintain its protective benefits, it will have to be reapplied yearly in order to keep the target fungus at bay. Given that hassle, especially since it can become costly for a large tree needing one or more sprays per year, and the fact that fungicides don't always work well (a fungus might become resistant to the chemical, or rains interrupt spraying), those are among the reasons we discourage fungicide use overall. While not as applicable to spruce in this case, another reason we shy away from recommending fungicides is that they can impact pollinators or other organisms. If you are okay with continuing the treatments in terms of the logistics of spraying and the cost, then you can plan to have them treated every year. Needle cast will always be a threat to spruces in the mid-Atlantic, so the risk of infection will never really go away.
Miri
Certainly -- once a fungicide routine has begun, if you want to maintain its protective benefits, it will have to be reapplied yearly in order to keep the target fungus at bay. Given that hassle, especially since it can become costly for a large tree needing one or more sprays per year, and the fact that fungicides don't always work well (a fungus might become resistant to the chemical, or rains interrupt spraying), those are among the reasons we discourage fungicide use overall. While not as applicable to spruce in this case, another reason we shy away from recommending fungicides is that they can impact pollinators or other organisms. If you are okay with continuing the treatments in terms of the logistics of spraying and the cost, then you can plan to have them treated every year. Needle cast will always be a threat to spruces in the mid-Atlantic, so the risk of infection will never really go away.
Miri
Thank you. This has been extremely helpful. One last question please. For now, I will continue the fungicide treatments. The company doing them recommends 4x/year. Is it necessary to treat that often?
Amy
It's hard to say, unfortunately, because application frequency will greatly depend on the product used. Each pesticide can have different application instructions based on how long residues last and how long the infection/re-infection period lasts in a given area for a given pathogen. Overall, more than one application per year is likely needed, but whether it's four or not depends on the exact fungicide formulation.
Miri
Miri
