What is the disease causing needle drop, and even killing in some cases, Cedrus trees in our area? Is there a fungicide recommended to help control the disease, and if so, what timing is recommended?
I can’t be absolutely certain what the cause might be, but taking a look at some Northwest integrated pest management resources could provide ideas on the likely culprits.
Oregon State University’s IPM site mentions Kabatina and Sirococcus conigenus.
Here is what the print companion to the website (2008 Pacific Northwest Plant Diseases Management Handbook) says about Cedar needle blight:
“The fungi Sirococcus conigenus and Kabatina sp. have been associated with blighted needles of Atlas and Deodara cedars in both Oregon and Washington; however, Sirococcus is found most often. More of a problem in years with prolonged wet, cool springs. Infection is on or adjacent to needle bases on new shoots. The disease cycle is completed in 1 year, although spore dispersal from dead parts may continue (…) 10 months. The fungus overwinters in dead shoots. Conidia are dispersed by splashing water in spring and summer. (…) Temperatures of 60 to 70 F are most favorable for disease development.(…) Cultural control: Remove and destroy blight plant material and debris that is found under trees or caught in limbs. Chemical control: No chemicals are specifically registered for this disease on cedar.” (The full information is available in the link above.)
Washington State University in Puyallup has a document indexing plant hosts of various pathogens, and for Cedrus they list Sirococcus species as well as Phomopsis.
There are discussions on the topic of Cedrus blights at University of British Columbia Botanical Garden’s online forum which mention tip blight sometimes appearing in tandem with borers or mites.