Diplodia blight Q & A
By Nicholas J . Brazee , UMass Extension Plant Pathologist
Q : What can I do to protect two- and three-needle pines from Diplodia blight ?
A : Diplodia blight of two- and three-needle pines , caused by Diplodia sapinea , has been abundant this growing season . The disease primarily occurs on Scots ( Pinus sylvestris ), red ( P . resinosa ), mugo ( P . mugo ) and Austrian ( P . nigra ) pines . However , the pathogen has a very broad host range among the Pineaceae and can also attack five-needle pines ( e . g . P . strobus , P . flexis and P . koraiensis ), false-cypress ( Chamaecyparis ), spruce ( Picea ), true cedar ( Cedrus ), among other landscape conifers . Diplodia sapinea has even been found on oaks ( Quercus ), illustrating its dynamic nature . Symptoms of Diplodia blight appear as shoot tip blight in early summer , and browning needles / scattered dieback throughout the canopy during the summer . Infected needles often turn from brown to grey over the growing season . Drought stress from 2020 likely predisposed many landscape pines to infection , despite their natural drought resistance .
“ Drought stress from 2020 likely predisposed many landscape pines to infection , despite their natural drought resistance .”
Many two- and three-needle pines are adapted to dry and nutrient poor sites . By maintaining a naturally sparse canopy , these trees avoid the conditions that allow needle and stem blight pathogens to proliferate . Specifically , many conifer pathogens thrive within lower and interior portions of the canopy where dense branching and heavy shade allow moisture to linger . However , many dwarf and intermediate cultivars of two- and three-needle pines have very dense , mounded canopies that create ideal conditions for the disease ( i . e . P . resinosa ‘ Morel ’). These forms can create major problems should Diplodia blight establish .
If Diplodia blight is a concern or has been present in the past , consider pruning to improve sunlight penetration , increase air flow and accelerate drying through the canopy . This is especially true of lower canopy branches , especially if they receive little to no direct sunlight . Maintain a thick mulch
Needle and shoot blight caused by Diplodia sapinea on Ponderosa pine ( Pinus ponderosa ). Photo credit : https :// ag . umass . edu
layer around the base of the plants , remove underlying or surrounding plantings that cast shade , and provide supplemental irrigation during extended dry periods to manage drought stress . Care should be taken when fertilizing twoand three-needle pines with nitrogen . An abundance of succulent tissues in the spring can allow Diplodia blight to rapidly intensity when the pathogen is present .
Early detection is important when managing Diplodia blight . Carefully scout as new candles and needles are elongating . When Diplodia blight is present , the needles at the shoot tips will typically die when they are 1 / 3 to 1 / 2 elongated . Immediate pruning and removal of this diseased tissue when it appears can keep the infection from spreading throughout the canopy . Locally-systemic fungicides , such as azoxystrobin and thiophanate-methyl , when applied to the new growth can be helpful in keeping these new sensitive tissues protected . Systemic fungicides with a single mode of action should be rotated to avoid resistance development . Broad-spectrum protectants , such as copper hydroxide , metconaozle and mancozeb are effective chemicals as well .
For more information on Diplodia blight , see this UMass Extension fact sheet : https :// ag . umass . edu / landscape / fact-sheets / diplodia-blight
12 MAA NEWS WINTER 2021