This month I wanted to take a look at a disease of Spruce shrubs and trees that rarely gets any attention ... Rhizosphaera. Like many other cool season diseases this pathogen rarely gets noticed until it is too late to treat and save the plant. This disease thrives in temperatures between 55-75 F and humidity is high during the spring months. This pathogen mainly attacks Blue Spruce trees and shrub cultivars. It seems to be most prevalent where air circulation is at a minimum, which is why it seems to love the dwarf compact varieties that are all rage right now in peoples' gardens. The tight compact heads of these plants provide the perfect environment for the pathogen to take hold.
As the disease progresses through the summer months it will first turn the needles yellow in color. Needlecast / blight caused by Rhizosphaera pathogen on dwarf Blue Spruce
Photo by Matthew Shultz
Shortly after this occurs the needles will start to drop off the plant, usually by mid to late summer. In severe infestations the pathogen can defoliate an entire plant, making them unsightly. If left unchecked the pathogen will slowly kill the host plant over the course of a couple of growing seasons.
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Reddish purple color in needles of Blue Spruce infected with Rhizosphaera / em>
Upon closer inspection of infected needles, little black pustules or zits have been seen on the needles which are the fruiting bodies of the fungi. Sanitation is key to controlling this disease, only healthy plant stock stock, clean pruners when trimming, and clean up any fallen debris. Spores of this disease are spread via splashing water, overhead irrigation and extended wet periods to help spur the disease on.
This month I wanted to take a look at a spruce tree and spruce tree disease that is rarely given attention ... Rhizosphaera. Like many other cold-weather diseases this pathogen is almost never noticed until it is too late to treat and save the plant. This disease thrives in temperatures between 55-75 F and when the wet is high during the spring months. These pathogens mainly attack blue spruce trees and shrubs. This seems to be more prevalent where air circulation is minimal, which apparently loves compact dwarf varieties which are the majority of what people have in their gardens. The tight compact heads of these plants offer the perfect environment to hold on to this pathogen.
As the disease progresses through the summer months the needles will turn yellow in color. Finally these needles will turn reddish violet.
Further research on infected needles, small black pustules or pimples can be seen on the needles which are the fungal fruiting bodies. Sanitation is key to controlling this disease so only healthy plants are stored, clean clippers when you can, and clean cuts and any other garbage. Spores of this disease spread through splashing water, so sprinkler irrigation and extended periods of wet can help spread the disease.